Add reserve/confirm for usernames

service/src/main/java/org/whispersystems/textsecuregcm/WhisperServerService.java

@@ -195,7 +195,7 @@ import org.whispersystems.textsecuregcm.storage.RemoteConfigs;
 import org.whispersystems.textsecuregcm.storage.RemoteConfigsManager;
 import org.whispersystems.textsecuregcm.storage.ReportMessageDynamoDb;
 import org.whispersystems.textsecuregcm.storage.ReportMessageManager;
-import org.whispersystems.textsecuregcm.storage.ReservedUsernames;
+import org.whispersystems.textsecuregcm.storage.ProhibitedUsernames;
 import org.whispersystems.textsecuregcm.storage.StoredVerificationCodeManager;
 import org.whispersystems.textsecuregcm.storage.SubscriptionManager;
 import org.whispersystems.textsecuregcm.storage.VerificationCodeStore;
@@ -337,7 +337,7 @@ public class WhisperServerService extends Application<WhisperServerConfiguration
         config.getDynamoDbTables().getAccounts().getScanPageSize());
     PhoneNumberIdentifiers phoneNumberIdentifiers = new PhoneNumberIdentifiers(dynamoDbClient,
         config.getDynamoDbTables().getPhoneNumberIdentifiers().getTableName());
-    ReservedUsernames reservedUsernames = new ReservedUsernames(dynamoDbClient,
+    ProhibitedUsernames prohibitedUsernames = new ProhibitedUsernames(dynamoDbClient,
         config.getDynamoDbTables().getReservedUsernames().getTableName());
     Profiles profiles = new Profiles(dynamoDbClient, dynamoDbAsyncClient,
         config.getDynamoDbTables().getProfiles().getTableName());
@@ -447,7 +447,7 @@ public class WhisperServerService extends Application<WhisperServerConfiguration
     DeletedAccountsManager deletedAccountsManager = new DeletedAccountsManager(deletedAccounts,
         deletedAccountsLockDynamoDbClient, config.getDynamoDbTables().getDeletedAccountsLock().getTableName());
     AccountsManager accountsManager = new AccountsManager(accounts, phoneNumberIdentifiers, cacheCluster,
-        deletedAccountsManager, directoryQueue, keys, messagesManager, reservedUsernames, profilesManager,
+        deletedAccountsManager, directoryQueue, keys, messagesManager, prohibitedUsernames, profilesManager,
         pendingAccountsManager, secureStorageClient, secureBackupClient, clientPresenceManager, usernameGenerator,
         experimentEnrollmentManager, clock);
     RemoteConfigsManager       remoteConfigsManager       = new RemoteConfigsManager(remoteConfigs);

service/src/main/java/org/whispersystems/textsecuregcm/configuration/RateLimitsConfiguration.java

@@ -62,6 +62,9 @@ public class RateLimitsConfiguration {
   @JsonProperty
   private RateLimitConfiguration usernameSet = new RateLimitConfiguration(100, 100 / (24.0 * 60.0));
 
+  @JsonProperty
+  private RateLimitConfiguration usernameReserve = new RateLimitConfiguration(100, 100 / (24.0 * 60.0));
+
   @JsonProperty
   private RateLimitConfiguration checkAccountExistence = new RateLimitConfiguration(1_000, 1_000 / 60.0);
 
@@ -137,6 +140,10 @@ public class RateLimitsConfiguration {
     return usernameSet;
   }
 
+  public RateLimitConfiguration getUsernameReserve() {
+    return usernameReserve;
+  }
+
   public RateLimitConfiguration getCheckAccountExistence() {
     return checkAccountExistence;
   }

service/src/main/java/org/whispersystems/textsecuregcm/configuration/UsernameConfiguration.java

@@ -7,6 +7,7 @@ package org.whispersystems.textsecuregcm.configuration;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
 import javax.validation.constraints.Min;
+import java.time.Duration;
 
 public class UsernameConfiguration {
 
@@ -22,6 +23,9 @@ public class UsernameConfiguration {
   @Min(1)
   private int attemptsPerWidth = 10;
 
+  @JsonProperty
+  private Duration reservationTtl = Duration.ofMinutes(5);
+
   public int getDiscriminatorInitialWidth() {
     return discriminatorInitialWidth;
   }
@@ -33,4 +37,8 @@ public class UsernameConfiguration {
   public int getAttemptsPerWidth() {
     return attemptsPerWidth;
   }
+
+  public Duration getReservationTtl() {
+    return reservationTtl;
+  }
 }

service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java

@@ -68,11 +68,14 @@ import org.whispersystems.textsecuregcm.entities.AccountIdentifierResponse;
 import org.whispersystems.textsecuregcm.entities.AccountIdentityResponse;
 import org.whispersystems.textsecuregcm.entities.ApnRegistrationId;
 import org.whispersystems.textsecuregcm.entities.ChangePhoneNumberRequest;
+import org.whispersystems.textsecuregcm.entities.ConfirmUsernameRequest;
 import org.whispersystems.textsecuregcm.entities.DeviceName;
 import org.whispersystems.textsecuregcm.entities.GcmRegistrationId;
 import org.whispersystems.textsecuregcm.entities.MismatchedDevices;
 import org.whispersystems.textsecuregcm.entities.RegistrationLock;
 import org.whispersystems.textsecuregcm.entities.RegistrationLockFailure;
+import org.whispersystems.textsecuregcm.entities.ReserveUsernameRequest;
+import org.whispersystems.textsecuregcm.entities.ReserveUsernameResponse;
 import org.whispersystems.textsecuregcm.entities.StaleDevices;
 import org.whispersystems.textsecuregcm.entities.UsernameRequest;
 import org.whispersystems.textsecuregcm.entities.UsernameResponse;
@@ -92,6 +95,7 @@ import org.whispersystems.textsecuregcm.storage.Device;
 import org.whispersystems.textsecuregcm.storage.DynamicConfigurationManager;
 import org.whispersystems.textsecuregcm.storage.StoredVerificationCodeManager;
 import org.whispersystems.textsecuregcm.storage.UsernameNotAvailableException;
+import org.whispersystems.textsecuregcm.storage.UsernameReservationNotFoundException;
 import org.whispersystems.textsecuregcm.util.Constants;
 import org.whispersystems.textsecuregcm.util.ForwardedIpUtil;
 import org.whispersystems.textsecuregcm.util.Hex;
@@ -642,6 +646,52 @@ public class AccountController {
     accounts.clearUsername(auth.getAccount());
   }
 
+
+  @Timed
+  @PUT
+  @Path("/username/reserved")
+  @Produces(MediaType.APPLICATION_JSON)
+  @Consumes(MediaType.APPLICATION_JSON)
+  public ReserveUsernameResponse reserveUsername(@Auth AuthenticatedAccount auth,
+      @HeaderParam("X-Signal-Agent") String userAgent,
+      @NotNull @Valid ReserveUsernameRequest usernameRequest) throws RateLimitExceededException {
+
+    rateLimiters.getUsernameReserveLimiter().validate(auth.getAccount().getUuid());
+
+    try {
+      final AccountsManager.UsernameReservation reservation = accounts.reserveUsername(
+          auth.getAccount(),
+          usernameRequest.nickname()
+      );
+      return new ReserveUsernameResponse(reservation.reservedUsername(), reservation.reservationToken());
+    } catch (final UsernameNotAvailableException e) {
+      throw new WebApplicationException(Status.CONFLICT);
+    }
+  }
+
+  @Timed
+  @PUT
+  @Path("/username/confirm")
+  @Produces(MediaType.APPLICATION_JSON)
+  @Consumes(MediaType.APPLICATION_JSON)
+  public UsernameResponse confirmUsername(@Auth AuthenticatedAccount auth,
+      @HeaderParam("X-Signal-Agent") String userAgent,
+      @NotNull @Valid ConfirmUsernameRequest confirmRequest) throws RateLimitExceededException {
+    rateLimiters.getUsernameSetLimiter().validate(auth.getAccount().getUuid());
+
+    try {
+      final Account account = accounts.confirmReservedUsername(auth.getAccount(), confirmRequest.usernameToConfirm(), confirmRequest.reservationToken());
+      return account
+          .getUsername()
+          .map(UsernameResponse::new)
+          .orElseThrow(() -> new IllegalStateException("Could not get username after setting"));
+    } catch (final UsernameReservationNotFoundException e) {
+      throw new WebApplicationException(Status.CONFLICT);
+    } catch (final UsernameNotAvailableException e) {
+      throw new WebApplicationException(Status.GONE);
+    }
+  }
+
   @Timed
   @PUT
   @Path("/username")
@@ -652,14 +702,7 @@ public class AccountController {
       @HeaderParam("X-Signal-Agent") String userAgent,
       @NotNull @Valid UsernameRequest usernameRequest) throws RateLimitExceededException {
     rateLimiters.getUsernameSetLimiter().validate(auth.getAccount().getUuid());
-
+    checkUsername(usernameRequest.existingUsername(), userAgent);
-    if (StringUtils.isNotBlank(usernameRequest.existingUsername()) &&
-        !UsernameGenerator.isStandardFormat(usernameRequest.existingUsername())) {
-      // Technically, a username may not be in the nickname#discriminator format
-      // if created through some out-of-band mechanism, but it is atypical.
-      Metrics.counter(NONSTANDARD_USERNAME_COUNTER_NAME, Tags.of(UserAgentTagUtil.getPlatformTag(userAgent)))
-          .increment();
-    }
 
     try {
       final Account account = accounts.setUsername(auth.getAccount(), usernameRequest.nickname(),
@@ -688,15 +731,10 @@ public class AccountController {
       throw new BadRequestException();
     }
 
-    if (!UsernameGenerator.isStandardFormat(username)) {
-      // Technically, a username may not be in the nickname#discriminator format
-      // if created through some out-of-band mechanism, but it is atypical.
-      Metrics.counter(NONSTANDARD_USERNAME_COUNTER_NAME, Tags.of(UserAgentTagUtil.getPlatformTag(userAgent)))
-          .increment();
-    }
-
     rateLimitByClientIp(rateLimiters.getUsernameLookupLimiter(), forwardedFor);
 
+    checkUsername(username, userAgent);
+
     return accounts
         .getByUsername(username)
         .map(Account::getUuid)
@@ -866,6 +904,15 @@ public class AccountController {
     accounts.delete(auth.getAccount(), AccountsManager.DeletionReason.USER_REQUEST);
   }
 
+  private void checkUsername(final String username, final String userAgent) {
+    if (StringUtils.isNotBlank(username) && !UsernameGenerator.isStandardFormat(username)) {
+      // Technically, a username may not be in the nickname#discriminator format
+      // if created through some out-of-band mechanism, but it is atypical.
+      Metrics.counter(NONSTANDARD_USERNAME_COUNTER_NAME, Tags.of(UserAgentTagUtil.getPlatformTag(userAgent)))
+          .increment();
+    }
+  }
+
   private boolean shouldAutoBlock(String sourceHost) {
     try {
       rateLimiters.getAutoBlockLimiter().validate(sourceHost);

service/src/main/java/org/whispersystems/textsecuregcm/entities/ConfirmUsernameRequest.java

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2022 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.entities;
+
+import javax.validation.constraints.NotBlank;
+import javax.validation.constraints.NotNull;
+import java.util.UUID;
+
+public record ConfirmUsernameRequest(@NotBlank String usernameToConfirm, @NotNull UUID reservationToken) {}

service/src/main/java/org/whispersystems/textsecuregcm/entities/ReserveUsernameRequest.java

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2022 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.entities;
+
+import org.whispersystems.textsecuregcm.util.Nickname;
+
+import javax.validation.Valid;
+
+public record ReserveUsernameRequest(@Valid @Nickname String nickname) {}

service/src/main/java/org/whispersystems/textsecuregcm/entities/ReserveUsernameResponse.java

@@ -0,0 +1,10 @@
+/*
+ * Copyright 2022 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.entities;
+
+import java.util.UUID;
+
+public record ReserveUsernameResponse(String username, UUID reservationToken) {}

service/src/main/java/org/whispersystems/textsecuregcm/limits/RateLimiters.java

@@ -33,6 +33,8 @@ public class RateLimiters {
   private final RateLimiter usernameLookupLimiter;
   private final RateLimiter usernameSetLimiter;
 
+  private final RateLimiter usernameReserveLimiter;
+
   private final RateLimiter checkAccountExistenceLimiter;
 
   public RateLimiters(RateLimitsConfiguration config, FaultTolerantRedisCluster cacheCluster) {
@@ -108,6 +110,11 @@ public class RateLimiters {
                                               config.getUsernameSet().getBucketSize(),
                                               config.getUsernameSet().getLeakRatePerMinute());
 
+    this.usernameReserveLimiter = new RateLimiter(cacheCluster, "usernameReserve",
+        config.getUsernameReserve().getBucketSize(),
+        config.getUsernameReserve().getLeakRatePerMinute());
+
+
     this.checkAccountExistenceLimiter = new RateLimiter(cacheCluster, "checkAccountExistence",
         config.getCheckAccountExistence().getBucketSize(),
         config.getCheckAccountExistence().getLeakRatePerMinute());
@@ -185,6 +192,10 @@ public class RateLimiters {
     return usernameSetLimiter;
   }
 
+  public RateLimiter getUsernameReserveLimiter() {
+    return usernameReserveLimiter;
+  }
+
   public RateLimiter getCheckAccountExistenceLimiter() {
     return checkAccountExistenceLimiter;
   }

service/src/main/java/org/whispersystems/textsecuregcm/storage/Account.java

@@ -42,6 +42,10 @@ public class Account {
   @Nullable
   private String username;
 
+  @JsonProperty
+  @Nullable
+  private byte[] reservedUsernameHash;
+
   @JsonProperty
   private List<Device> devices = new ArrayList<>();
 
@@ -133,6 +137,18 @@ public class Account {
     this.username = username;
   }
 
+  public Optional<byte[]> getReservedUsernameHash() {
+    requireNotStale();
+
+    return Optional.ofNullable(reservedUsernameHash);
+  }
+
+  public void setReservedUsernameHash(final byte[] reservedUsernameHash) {
+    requireNotStale();
+
+    this.reservedUsernameHash = reservedUsernameHash;
+  }
+
   public void addDevice(Device device) {
     requireNotStale();
 

service/src/main/java/org/whispersystems/textsecuregcm/storage/Accounts.java

@@ -13,6 +13,10 @@ import io.micrometer.core.instrument.Metrics;
 import io.micrometer.core.instrument.Timer;
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.time.Clock;
 import java.time.Duration;
 import java.time.Instant;
 import java.util.ArrayList;
@@ -70,7 +74,10 @@ public class Accounts extends AbstractDynamoDbStore {
   static final String ATTR_USERNAME = "N";
   // unidentified access key; byte[] or null
   static final String ATTR_UAK = "UAK";
+  // time to live; number
+  static final String ATTR_TTL = "TTL";
 
+  private final Clock clock;
   private final DynamoDbClient client;
   private final DynamoDbAsyncClient asyncClient;
 
@@ -81,9 +88,12 @@ public class Accounts extends AbstractDynamoDbStore {
 
   private final int scanPageSize;
 
+  private static final byte RESERVED_USERNAME_HASH_VERSION = 1;
+
   private static final Timer CREATE_TIMER = Metrics.timer(name(Accounts.class, "create"));
   private static final Timer CHANGE_NUMBER_TIMER = Metrics.timer(name(Accounts.class, "changeNumber"));
   private static final Timer SET_USERNAME_TIMER = Metrics.timer(name(Accounts.class, "setUsername"));
+  private static final Timer RESERVE_USERNAME_TIMER = Metrics.timer(name(Accounts.class, "reserveUsername"));
   private static final Timer CLEAR_USERNAME_TIMER = Metrics.timer(name(Accounts.class, "clearUsername"));
   private static final Timer UPDATE_TIMER = Metrics.timer(name(Accounts.class, "update"));
   private static final Timer GET_BY_NUMBER_TIMER = Metrics.timer(name(Accounts.class, "getByNumber"));
@@ -96,13 +106,16 @@ public class Accounts extends AbstractDynamoDbStore {
 
   private static final Logger log = LoggerFactory.getLogger(Accounts.class);
 
-  public Accounts(final DynamicConfigurationManager<DynamicConfiguration> dynamicConfigurationManager,
+  @VisibleForTesting
+  public Accounts(
+      final Clock clock,
+      final DynamicConfigurationManager<DynamicConfiguration> dynamicConfigurationManager,
       DynamoDbClient client, DynamoDbAsyncClient asyncClient,
       String accountsTableName, String phoneNumberConstraintTableName,
       String phoneNumberIdentifierConstraintTableName, final String usernamesConstraintTableName,
       final int scanPageSize) {
-
     super(client);
+    this.clock = clock;
     this.client = client;
     this.asyncClient = asyncClient;
     this.phoneNumberConstraintTableName = phoneNumberConstraintTableName;
@@ -112,6 +125,16 @@ public class Accounts extends AbstractDynamoDbStore {
     this.scanPageSize = scanPageSize;
   }
 
+  public Accounts(final DynamicConfigurationManager<DynamicConfiguration> dynamicConfigurationManager,
+      DynamoDbClient client, DynamoDbAsyncClient asyncClient,
+      String accountsTableName, String phoneNumberConstraintTableName,
+      String phoneNumberIdentifierConstraintTableName, final String usernamesConstraintTableName,
+      final int scanPageSize) {
+    this(Clock.systemUTC(), dynamicConfigurationManager, client, asyncClient, accountsTableName,
+        phoneNumberConstraintTableName, phoneNumberIdentifierConstraintTableName, usernamesConstraintTableName,
+        scanPageSize);
+  }
+
   public boolean create(Account account) {
     return CREATE_TIMER.record(() -> {
 
@@ -331,33 +354,150 @@ public class Accounts extends AbstractDynamoDbStore {
     });
   }
 
+  public static byte[] reservedUsernameHash(final UUID accountId, final String reservedUsername) {
+    final MessageDigest sha256;
+    try {
+      sha256 = MessageDigest.getInstance("SHA-256");
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    }
+    final ByteBuffer byteBuffer = ByteBuffer.allocate(32 + 1);
+    sha256.update(reservedUsername.getBytes(StandardCharsets.UTF_8));
+    sha256.update(UUIDUtil.toBytes(accountId));
+    byteBuffer.put(RESERVED_USERNAME_HASH_VERSION);
+    byteBuffer.put(sha256.digest());
+    return byteBuffer.array();
+  }
+
   /**
-   * Set the account username
+   * Reserve a username under a token
    *
-   * @param account to update
+   * @return a reservation token that must be provided when {@link #confirmUsername(Account, String, UUID)} is called
-   * @param username believed to be available
-   * @throws ContestedOptimisticLockException if the account has been updated or the username taken by someone else
    */
-  public void setUsername(final Account account, final String username)
+  public UUID reserveUsername(
-      throws ContestedOptimisticLockException {
+      final Account account,
+      final String reservedUsername,
+      final Duration ttl) {
     final long startNanos = System.nanoTime();
 
-    final Optional<String> maybeOriginalUsername = account.getUsername();
+    // if there is an existing old reservation it will be cleaned up via ttl
-    account.setUsername(username);
+    final Optional<byte[]> maybeOriginalReservation = account.getReservedUsernameHash();
+    account.setReservedUsernameHash(reservedUsernameHash(account.getUuid(), reservedUsername));
 
     boolean succeeded = false;
 
+    long expirationTime = clock.instant().plus(ttl).getEpochSecond();
+
+    final UUID reservationToken = UUID.randomUUID();
     try {
       final List<TransactWriteItem> writeItems = new ArrayList<>();
 
+      writeItems.add(TransactWriteItem.builder()
+          .put(Put.builder()
+              .tableName(usernamesConstraintTableName)
+              .item(Map.of(
+                  KEY_ACCOUNT_UUID, AttributeValues.fromUUID(reservationToken),
+                  ATTR_USERNAME, AttributeValues.fromString(reservedUsername),
+                  ATTR_TTL, AttributeValues.fromLong(expirationTime)))
+              .conditionExpression("attribute_not_exists(#username) OR (#ttl < :now)")
+              .expressionAttributeNames(Map.of("#username", ATTR_USERNAME, "#ttl", ATTR_TTL))
+              .expressionAttributeValues(Map.of(":now", AttributeValues.fromLong(clock.instant().getEpochSecond())))
+              .returnValuesOnConditionCheckFailure(ReturnValuesOnConditionCheckFailure.ALL_OLD)
+              .build())
+          .build());
+
+      writeItems.add(
+          TransactWriteItem.builder()
+              .update(Update.builder()
+                  .tableName(accountsTableName)
+                  .key(Map.of(KEY_ACCOUNT_UUID, AttributeValues.fromUUID(account.getUuid())))
+                  .updateExpression("SET #data = :data ADD #version :version_increment")
+                  .conditionExpression("#version = :version")
+                  .expressionAttributeNames(Map.of("#data", ATTR_ACCOUNT_DATA, "#version", ATTR_VERSION))
+                  .expressionAttributeValues(Map.of(
+                      ":data", AttributeValues.fromByteArray(SystemMapper.getMapper().writeValueAsBytes(account)),
+                      ":version", AttributeValues.fromInt(account.getVersion()),
+                      ":version_increment", AttributeValues.fromInt(1)))
+                  .build())
+              .build());
+
+      final TransactWriteItemsRequest request = TransactWriteItemsRequest.builder()
+          .transactItems(writeItems)
+          .build();
+
+      client.transactWriteItems(request);
+
+      account.setVersion(account.getVersion() + 1);
+      succeeded = true;
+    } catch (final JsonProcessingException e) {
+      throw new IllegalArgumentException(e);
+    } catch (final TransactionCanceledException e) {
+      if (e.cancellationReasons().stream().map(CancellationReason::code).anyMatch("ConditionalCheckFailed"::equals)) {
+        throw new ContestedOptimisticLockException();
+      }
+      throw e;
+    } finally {
+      if (!succeeded) {
+        account.setReservedUsernameHash(maybeOriginalReservation.orElse(null));
+      }
+      RESERVE_USERNAME_TIMER.record(System.nanoTime() - startNanos, TimeUnit.NANOSECONDS);
+    }
+    return reservationToken;
+  }
+
+  /**
+   * Confirm (set) a previously reserved username
+   *
+   * @param account to update
+   * @param username believed to be available
+   * @param reservationToken a token returned by the call to {@link #reserveUsername(Account, String, Duration)},
+   *                         only required if setting a reserved username
+   * @throws ContestedOptimisticLockException if the account has been updated or the username taken by someone else
+   */
+  public void confirmUsername(final Account account, final String username, final UUID reservationToken)
+      throws ContestedOptimisticLockException {
+    setUsername(account, username, Optional.of(reservationToken));
+  }
+
+  /**
+   * Set the account username
+   *
+   * @param account to update
+   * @param username believed to be available
+   * @throws ContestedOptimisticLockException if the account has been updated or the username taken by someone else
+   */
+  public void setUsername(final Account account, final String username) throws ContestedOptimisticLockException {
+    setUsername(account, username, Optional.empty());
+  }
+
+  private void setUsername(final Account account, final String username, final Optional<UUID> reservationToken)
+      throws ContestedOptimisticLockException {
+    final long startNanos = System.nanoTime();
+
+    final Optional<String> maybeOriginalUsername = account.getUsername();
+    final Optional<byte[]> maybeOriginalReservation = account.getReservedUsernameHash();
+
+    account.setUsername(username);
+    account.setReservedUsernameHash(null);
+
+    boolean succeeded = false;
+
+    try {
+      final List<TransactWriteItem> writeItems = new ArrayList<>();
+
+      // add the username to the constraint table, wiping out the ttl if we had already reserved the name
       writeItems.add(TransactWriteItem.builder()
           .put(Put.builder()
               .tableName(usernamesConstraintTableName)
               .item(Map.of(
                   KEY_ACCOUNT_UUID, AttributeValues.fromUUID(account.getUuid()),
                   ATTR_USERNAME, AttributeValues.fromString(username)))
-              .conditionExpression("attribute_not_exists(#username)")
+              // it's not in the constraint table OR it's expired OR it was reserved by us
-              .expressionAttributeNames(Map.of("#username", ATTR_USERNAME))
+              .conditionExpression("attribute_not_exists(#username) OR #ttl < :now OR #aci = :reservation ")
+              .expressionAttributeNames(Map.of("#username", ATTR_USERNAME, "#ttl", ATTR_TTL, "#aci", KEY_ACCOUNT_UUID))
+              .expressionAttributeValues(Map.of(
+                  ":now", AttributeValues.fromLong(clock.instant().getEpochSecond()),
+                  ":reservation", AttributeValues.fromUUID(reservationToken.orElseGet(UUID::randomUUID))))
               .returnValuesOnConditionCheckFailure(ReturnValuesOnConditionCheckFailure.ALL_OLD)
               .build())
           .build());
@@ -405,6 +545,7 @@ public class Accounts extends AbstractDynamoDbStore {
     } finally {
       if (!succeeded) {
         account.setUsername(maybeOriginalUsername.orElse(null));
+        account.setReservedUsernameHash(maybeOriginalReservation.orElse(null));
       }
       SET_USERNAME_TIMER.record(System.nanoTime() - startNanos, TimeUnit.NANOSECONDS);
     }
@@ -553,11 +694,29 @@ public class Accounts extends AbstractDynamoDbStore {
   }
 
   public boolean usernameAvailable(final String username) {
+    return usernameAvailable(Optional.empty(), username);
+  }
+
+  public boolean usernameAvailable(final Optional<UUID> reservationToken, final String username) {
     final GetItemResponse response = client.getItem(GetItemRequest.builder()
         .tableName(usernamesConstraintTableName)
         .key(Map.of(ATTR_USERNAME, AttributeValues.fromString(username)))
         .build());
-    return !response.hasItem();
+    if (!response.hasItem()) {
+      // username is free
+      return true;
+    }
+    final Map<String, AttributeValue> item = response.item();
+
+    if (AttributeValues.getLong(item, ATTR_TTL, Long.MAX_VALUE) < clock.instant().getEpochSecond()) {
+      // username was reserved, but has expired
+      return true;
+    }
+
+    // username is reserved by us
+    return reservationToken
+        .map(AttributeValues.getUUID(item, KEY_ACCOUNT_UUID, new UUID(0, 0))::equals)
+        .orElse(false);
   }
 
   public Optional<Account> getByE164(String number) {
@@ -583,7 +742,10 @@ public class Accounts extends AbstractDynamoDbStore {
           .key(Map.of(ATTR_USERNAME, AttributeValues.fromString(username)))
           .build());
 
+
       return Optional.ofNullable(response.item())
+          // ignore items with a ttl (reservations)
+          .filter(item -> !item.containsKey(ATTR_TTL))
           .map(item -> item.get(KEY_ACCOUNT_UUID))
           .map(this::accountByUuid)
           .map(Accounts::fromItem);

service/src/main/java/org/whispersystems/textsecuregcm/storage/AccountsManager.java

@@ -21,6 +21,7 @@ import io.micrometer.core.instrument.Tags;
 import java.io.IOException;
 import java.time.Clock;
 import java.time.Duration;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
@@ -86,7 +87,7 @@ public class AccountsManager {
   private final DirectoryQueue directoryQueue;
   private final Keys keys;
   private final MessagesManager messagesManager;
-  private final ReservedUsernames reservedUsernames;
+  private final ProhibitedUsernames prohibitedUsernames;
   private final ProfilesManager profilesManager;
   private final StoredVerificationCodeManager pendingAccounts;
   private final SecureStorageClient secureStorageClient;
@@ -128,7 +129,7 @@ public class AccountsManager {
       final DirectoryQueue directoryQueue,
       final Keys keys,
       final MessagesManager messagesManager,
-      final ReservedUsernames reservedUsernames,
+      final ProhibitedUsernames prohibitedUsernames,
       final ProfilesManager profilesManager,
       final StoredVerificationCodeManager pendingAccounts,
       final SecureStorageClient secureStorageClient,
@@ -149,7 +150,7 @@ public class AccountsManager {
     this.secureStorageClient = secureStorageClient;
     this.secureBackupClient  = secureBackupClient;
     this.clientPresenceManager = clientPresenceManager;
-    this.reservedUsernames = reservedUsernames;
+    this.prohibitedUsernames = prohibitedUsernames;
     this.usernameGenerator = usernameGenerator;
     this.experimentEnrollmentManager = experimentEnrollmentManager;
     this.clock = Objects.requireNonNull(clock);
@@ -322,12 +323,112 @@ public class AccountsManager {
     return updatedAccount.get();
   }
 
+  public record UsernameReservation(Account account, String reservedUsername, UUID reservationToken){}
+
+  /**
+   * Generate a username from a nickname, and reserve it so no other accounts may take it.
+   *
+   * The reserved username can later be set with {@link #confirmReservedUsername(Account, String, UUID)}. The reservation
+   * will eventually expire, after which point confirmReservedUsername may fail if another account has taken the
+   * username.
+   *
+   * @param account the account to update
+   * @param requestedNickname the nickname to reserve a username for
+   * @return the reserved username and an updated Account object
+   * @throws UsernameNotAvailableException no username is available for the requested nickname
+   */
+  public UsernameReservation reserveUsername(final Account account, final String requestedNickname) throws UsernameNotAvailableException {
+    if (!experimentEnrollmentManager.isEnrolled(account.getUuid(), USERNAME_EXPERIMENT_NAME)) {
+      throw new UsernameNotAvailableException();
+    }
+
+    if (prohibitedUsernames.isProhibited(requestedNickname, account.getUuid())) {
+      throw new UsernameNotAvailableException();
+    }
+    redisDelete(account);
+
+    class Reserver implements AccountPersister {
+      UUID reservationToken;
+      String reservedUsername;
+
+      @Override
+      public void persistAccount(final Account account) throws UsernameNotAvailableException {
+        // In the future, this may also check for any forbidden discriminators
+        reservedUsername = usernameGenerator.generateAvailableUsername(requestedNickname, accounts::usernameAvailable);
+        reservationToken = accounts.reserveUsername(
+            account,
+            reservedUsername,
+            usernameGenerator.getReservationTtl());
+      }
+    }
+    final Reserver reserver = new Reserver();
+    final Account updatedAccount = failableUpdateWithRetries(
+        account,
+        a -> true,
+        reserver,
+        () -> accounts.getByAccountIdentifier(account.getUuid()).orElseThrow(),
+        AccountChangeValidator.USERNAME_CHANGE_VALIDATOR);
+    return new UsernameReservation(updatedAccount, reserver.reservedUsername, reserver.reservationToken);
+  }
+
+  /**
+   * Set a username previously reserved with {@link #reserveUsername(Account, String)}
+   *
+   * @param account the account to update
+   * @param reservedUsername the previously reserved username
+   * @param reservationToken the UUID returned from the reservation
+   * @return the updated account with the username field set
+   * @throws UsernameNotAvailableException if the reserved username has been taken (because the reservation expired)
+   * @throws UsernameReservationNotFoundException if `reservedUsername` was not reserved in the account
+   */
+  public Account confirmReservedUsername(final Account account, final String reservedUsername, final UUID reservationToken) throws UsernameNotAvailableException, UsernameReservationNotFoundException {
+    if (!experimentEnrollmentManager.isEnrolled(account.getUuid(), USERNAME_EXPERIMENT_NAME)) {
+      throw new UsernameNotAvailableException();
+    }
+
+    if (account.getUsername().map(reservedUsername::equals).orElse(false)) {
+      // the client likely already succeeded and is retrying
+      return account;
+    }
+
+    final byte[] newHash = Accounts.reservedUsernameHash(account.getUuid(), reservedUsername);
+    if (!account.getReservedUsernameHash().map(oldHash -> Arrays.equals(oldHash, newHash)).orElse(false)) {
+      // no such reservation existed, either there was no previous call to reserveUsername
+      // or the reservation changed
+      throw new UsernameReservationNotFoundException();
+    }
+
+    redisDelete(account);
+
+    return failableUpdateWithRetries(
+        account,
+        a -> true,
+        a -> {
+          // though we know this username was reserved, the reservation could have lapsed
+          if (!accounts.usernameAvailable(Optional.of(reservationToken), reservedUsername)) {
+            throw new UsernameNotAvailableException();
+          }
+          accounts.confirmUsername(a, reservedUsername, reservationToken);
+        },
+        () -> accounts.getByAccountIdentifier(account.getUuid()).orElseThrow(),
+        AccountChangeValidator.USERNAME_CHANGE_VALIDATOR);
+  }
+
+  /**
+   * Sets a username generated from `requestedNickname` with no prior reservation
+   *
+   * @param account the account to update
+   * @param requestedNickname the nickname to generate a username from
+   * @param expectedOldUsername the expected existing username of the account (for replay detection)
+   * @return the updated account with the username field set
+   * @throws UsernameNotAvailableException if no free username could be set for `requestedNickname`
+   */
   public Account setUsername(final Account account, final String requestedNickname, final @Nullable String expectedOldUsername) throws UsernameNotAvailableException {
     if (!experimentEnrollmentManager.isEnrolled(account.getUuid(), USERNAME_EXPERIMENT_NAME)) {
       throw new UsernameNotAvailableException();
     }
 
-    if (reservedUsernames.isReserved(requestedNickname, account.getUuid())) {
+    if (prohibitedUsernames.isProhibited(requestedNickname, account.getUuid())) {
       throw new UsernameNotAvailableException();
     }
 
@@ -346,7 +447,9 @@ public class AccountsManager {
         account,
         a -> true,
         // In the future, this may also check for any forbidden discriminators
-        a -> accounts.setUsername(a, usernameGenerator.generateAvailableUsername(requestedNickname, accounts::usernameAvailable)),
+        a -> accounts.setUsername(
+            a,
+            usernameGenerator.generateAvailableUsername(requestedNickname, accounts::usernameAvailable)),
         () -> accounts.getByAccountIdentifier(account.getUuid()).orElseThrow(),
         AccountChangeValidator.USERNAME_CHANGE_VALIDATOR);
   }
@@ -539,7 +642,6 @@ public class AccountsManager {
   public Optional<Account> getByUsername(final String username) {
     try (final Timer.Context ignored = getByUsernameTimer.time()) {
       Optional<Account> account = redisGetByUsername(username);
-
       if (account.isEmpty()) {
         account = accounts.getByUsername(username);
         account.ifPresent(this::redisSet);

service/src/main/java/org/whispersystems/textsecuregcm/storage/ProhibitedUsernames.java

@@ -26,7 +26,7 @@ import software.amazon.awssdk.services.dynamodb.model.ScanRequest;
 import software.amazon.awssdk.services.dynamodb.model.ScanResponse;
 import software.amazon.awssdk.services.dynamodb.paginators.ScanIterable;
 
-public class ReservedUsernames {
+public class ProhibitedUsernames {
 
   private final DynamoDbClient dynamoDbClient;
   private final String tableName;
@@ -44,17 +44,17 @@ public class ReservedUsernames {
   static final String KEY_PATTERN = "P";
   private static final String ATTR_RESERVED_FOR_UUID = "U";
 
-  private static final Timer IS_RESERVED_TIMER = Metrics.timer(name(ReservedUsernames.class, "isReserved"));
+  private static final Timer IS_PROHIBITED_TIMER = Metrics.timer(name(ProhibitedUsernames.class, "isProhibited"));
 
-  private static final Logger log = LoggerFactory.getLogger(ReservedUsernames.class);
+  private static final Logger log = LoggerFactory.getLogger(ProhibitedUsernames.class);
 
-  public ReservedUsernames(final DynamoDbClient dynamoDbClient, final String tableName) {
+  public ProhibitedUsernames(final DynamoDbClient dynamoDbClient, final String tableName) {
     this.dynamoDbClient = dynamoDbClient;
     this.tableName = tableName;
   }
 
-  public boolean isReserved(final String nickname, final UUID accountIdentifier) {
+  public boolean isProhibited(final String nickname, final UUID accountIdentifier) {
-    return IS_RESERVED_TIMER.record(() -> {
+    return IS_PROHIBITED_TIMER.record(() -> {
       final ScanIterable scanIterable = dynamoDbClient.scanPaginator(ScanRequest.builder()
           .tableName(tableName)
           .build());
@@ -80,7 +80,13 @@ public class ReservedUsernames {
     });
   }
 
-  public void reserveUsername(final String pattern, final UUID reservedFor) {
+  /**
+   * Prohibits username except for all accounts except `reservedFor`
+   *
+   * @param pattern pattern to prohibit
+   * @param reservedFor an account that is allowed to use names in the pattern
+   */
+  public void prohibitUsername(final String pattern, final UUID reservedFor) {
     dynamoDbClient.putItem(PutItemRequest.builder()
         .tableName(tableName)
         .item(Map.of(

service/src/main/java/org/whispersystems/textsecuregcm/storage/UsernameReservationNotFoundException.java

@@ -0,0 +1,10 @@
+/*
+ * Copyright 2022 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.storage;
+
+public class UsernameReservationNotFoundException extends Exception {
+
+}

service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameGenerator.java

@@ -13,6 +13,7 @@ import io.micrometer.core.instrument.Metrics;
 import org.apache.commons.lang3.StringUtils;
 import org.whispersystems.textsecuregcm.configuration.UsernameConfiguration;
 import org.whispersystems.textsecuregcm.storage.UsernameNotAvailableException;
+import java.time.Duration;
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.function.Predicate;
 import java.util.regex.Pattern;
@@ -41,16 +42,21 @@ public class UsernameGenerator {
   private final int initialWidth;
   private final int discriminatorMaxWidth;
   private final int attemptsPerWidth;
+  private final Duration reservationTtl;
 
   public UsernameGenerator(UsernameConfiguration configuration) {
-    this(configuration.getDiscriminatorInitialWidth(), configuration.getDiscriminatorMaxWidth(), configuration.getAttemptsPerWidth());
+    this(configuration.getDiscriminatorInitialWidth(),
+        configuration.getDiscriminatorMaxWidth(),
+        configuration.getAttemptsPerWidth(),
+        configuration.getReservationTtl());
   }
 
   @VisibleForTesting
-  public UsernameGenerator(int initialWidth, int discriminatorMaxWidth, int attemptsPerWidth) {
+  public UsernameGenerator(int initialWidth, int discriminatorMaxWidth, int attemptsPerWidth, final Duration reservationTtl) {
     this.initialWidth = initialWidth;
     this.discriminatorMaxWidth = discriminatorMaxWidth;
     this.attemptsPerWidth = attemptsPerWidth;
+    this.reservationTtl = reservationTtl;
   }
 
   /**
@@ -109,6 +115,10 @@ public class UsernameGenerator {
     return String.format("%s#%0" + initialWidth + "d", nickname, discriminator);
   }
 
+  public Duration getReservationTtl() {
+    return reservationTtl;
+  }
+
   public static boolean isValidNickname(final String nickname) {
     return StringUtils.isNotBlank(nickname) && NICKNAME_PATTERN.matcher(nickname).matches();
   }

service/src/main/java/org/whispersystems/textsecuregcm/workers/AssignUsernameCommand.java

@@ -47,7 +47,7 @@ import org.whispersystems.textsecuregcm.storage.Profiles;
 import org.whispersystems.textsecuregcm.storage.ProfilesManager;
 import org.whispersystems.textsecuregcm.storage.ReportMessageDynamoDb;
 import org.whispersystems.textsecuregcm.storage.ReportMessageManager;
-import org.whispersystems.textsecuregcm.storage.ReservedUsernames;
+import org.whispersystems.textsecuregcm.storage.ProhibitedUsernames;
 import org.whispersystems.textsecuregcm.storage.StoredVerificationCodeManager;
 import org.whispersystems.textsecuregcm.storage.UsernameNotAvailableException;
 import org.whispersystems.textsecuregcm.storage.VerificationCodeStore;
@@ -152,7 +152,7 @@ public class AssignUsernameCommand extends EnvironmentCommand<WhisperServerConfi
         configuration.getDynamoDbTables().getPhoneNumberIdentifiers().getTableName());
     Profiles profiles = new Profiles(dynamoDbClient, dynamoDbAsyncClient,
         configuration.getDynamoDbTables().getProfiles().getTableName());
-    ReservedUsernames reservedUsernames = new ReservedUsernames(dynamoDbClient,
+    ProhibitedUsernames prohibitedUsernames = new ProhibitedUsernames(dynamoDbClient,
         configuration.getDynamoDbTables().getReservedUsernames().getTableName());
     Keys keys = new Keys(dynamoDbClient,
         configuration.getDynamoDbTables().getKeys().getTableName());
@@ -194,7 +194,7 @@ public class AssignUsernameCommand extends EnvironmentCommand<WhisperServerConfi
     UsernameGenerator usernameGenerator = new UsernameGenerator(configuration.getUsername());
     StoredVerificationCodeManager pendingAccountsManager = new StoredVerificationCodeManager(pendingAccounts);
     AccountsManager accountsManager = new AccountsManager(accounts, phoneNumberIdentifiers, cacheCluster,
-        deletedAccountsManager, directoryQueue, keys, messagesManager, reservedUsernames, profilesManager,
+        deletedAccountsManager, directoryQueue, keys, messagesManager, prohibitedUsernames, profilesManager,
         pendingAccountsManager, secureStorageClient, secureBackupClient, clientPresenceManager, usernameGenerator,
         experimentEnrollmentManager, Clock.systemUTC());
 

service/src/main/java/org/whispersystems/textsecuregcm/workers/DeleteUserCommand.java

@@ -50,7 +50,7 @@ import org.whispersystems.textsecuregcm.storage.Profiles;
 import org.whispersystems.textsecuregcm.storage.ProfilesManager;
 import org.whispersystems.textsecuregcm.storage.ReportMessageDynamoDb;
 import org.whispersystems.textsecuregcm.storage.ReportMessageManager;
-import org.whispersystems.textsecuregcm.storage.ReservedUsernames;
+import org.whispersystems.textsecuregcm.storage.ProhibitedUsernames;
 import org.whispersystems.textsecuregcm.storage.StoredVerificationCodeManager;
 import org.whispersystems.textsecuregcm.storage.VerificationCodeStore;
 import org.whispersystems.textsecuregcm.util.DynamoDbFromConfig;
@@ -154,7 +154,7 @@ public class DeleteUserCommand extends EnvironmentCommand<WhisperServerConfigura
           configuration.getDynamoDbTables().getPhoneNumberIdentifiers().getTableName());
       Profiles profiles = new Profiles(dynamoDbClient, dynamoDbAsyncClient,
           configuration.getDynamoDbTables().getProfiles().getTableName());
-      ReservedUsernames reservedUsernames = new ReservedUsernames(dynamoDbClient,
+      ProhibitedUsernames prohibitedUsernames = new ProhibitedUsernames(dynamoDbClient,
           configuration.getDynamoDbTables().getReservedUsernames().getTableName());
       Keys keys = new Keys(dynamoDbClient,
           configuration.getDynamoDbTables().getKeys().getTableName());
@@ -196,7 +196,7 @@ public class DeleteUserCommand extends EnvironmentCommand<WhisperServerConfigura
       StoredVerificationCodeManager pendingAccountsManager = new StoredVerificationCodeManager(pendingAccounts);
       UsernameGenerator usernameGenerator = new UsernameGenerator(configuration.getUsername());
       AccountsManager accountsManager = new AccountsManager(accounts, phoneNumberIdentifiers, cacheCluster,
-          deletedAccountsManager, directoryQueue, keys, messagesManager, reservedUsernames, profilesManager,
+          deletedAccountsManager, directoryQueue, keys, messagesManager, prohibitedUsernames, profilesManager,
           pendingAccountsManager, secureStorageClient, secureBackupClient, clientPresenceManager, usernameGenerator,
           experimentEnrollmentManager, clock);
 

service/src/main/java/org/whispersystems/textsecuregcm/workers/ReserveUsernameCommand.java

@@ -10,7 +10,7 @@ import io.dropwizard.setup.Bootstrap;
 import net.sourceforge.argparse4j.inf.Namespace;
 import net.sourceforge.argparse4j.inf.Subparser;
 import org.whispersystems.textsecuregcm.WhisperServerConfiguration;
-import org.whispersystems.textsecuregcm.storage.ReservedUsernames;
+import org.whispersystems.textsecuregcm.storage.ProhibitedUsernames;
 import org.whispersystems.textsecuregcm.util.DynamoDbFromConfig;
 import software.amazon.awssdk.services.dynamodb.DynamoDbClient;
 import java.util.UUID;
@@ -44,7 +44,7 @@ public class ReserveUsernameCommand extends ConfiguredCommand<WhisperServerConfi
     final DynamoDbClient dynamoDbClient = DynamoDbFromConfig.client(config.getDynamoDbClientConfiguration(),
         software.amazon.awssdk.auth.credentials.InstanceProfileCredentialsProvider.create());
 
-    final ReservedUsernames reservedUsernames = new ReservedUsernames(dynamoDbClient,
+    final ProhibitedUsernames prohibitedUsernames = new ProhibitedUsernames(dynamoDbClient,
         config.getDynamoDbTables().getReservedUsernames().getTableName());
 
     final String pattern = namespace.getString("pattern").trim();
@@ -57,7 +57,7 @@ public class ReserveUsernameCommand extends ConfiguredCommand<WhisperServerConfi
 
     final UUID aci = UUID.fromString(namespace.getString("uuid").trim());
 
-    reservedUsernames.reserveUsername(pattern, aci);
+    prohibitedUsernames.prohibitUsername(pattern, aci);
 
     System.out.format("Reserved %s for account %s\n", pattern, aci);
   }

service/src/main/java/org/whispersystems/textsecuregcm/workers/SetUserDiscoverabilityCommand.java

@@ -48,7 +48,7 @@ import org.whispersystems.textsecuregcm.storage.Profiles;
 import org.whispersystems.textsecuregcm.storage.ProfilesManager;
 import org.whispersystems.textsecuregcm.storage.ReportMessageDynamoDb;
 import org.whispersystems.textsecuregcm.storage.ReportMessageManager;
-import org.whispersystems.textsecuregcm.storage.ReservedUsernames;
+import org.whispersystems.textsecuregcm.storage.ProhibitedUsernames;
 import org.whispersystems.textsecuregcm.storage.StoredVerificationCodeManager;
 import org.whispersystems.textsecuregcm.storage.VerificationCodeStore;
 import org.whispersystems.textsecuregcm.util.DynamoDbFromConfig;
@@ -157,7 +157,7 @@ public class SetUserDiscoverabilityCommand extends EnvironmentCommand<WhisperSer
           configuration.getDynamoDbTables().getPhoneNumberIdentifiers().getTableName());
       Profiles profiles = new Profiles(dynamoDbClient, dynamoDbAsyncClient,
           configuration.getDynamoDbTables().getProfiles().getTableName());
-      ReservedUsernames reservedUsernames = new ReservedUsernames(dynamoDbClient,
+      ProhibitedUsernames prohibitedUsernames = new ProhibitedUsernames(dynamoDbClient,
           configuration.getDynamoDbTables().getReservedUsernames().getTableName());
       Keys keys = new Keys(dynamoDbClient,
           configuration.getDynamoDbTables().getKeys().getTableName());
@@ -197,7 +197,7 @@ public class SetUserDiscoverabilityCommand extends EnvironmentCommand<WhisperSer
       StoredVerificationCodeManager pendingAccountsManager = new StoredVerificationCodeManager(pendingAccounts);
       UsernameGenerator usernameGenerator = new UsernameGenerator(configuration.getUsername());
       AccountsManager accountsManager = new AccountsManager(accounts, phoneNumberIdentifiers, cacheCluster,
-          deletedAccountsManager, directoryQueue, keys, messagesManager, reservedUsernames, profilesManager,
+          deletedAccountsManager, directoryQueue, keys, messagesManager, prohibitedUsernames, profilesManager,
           pendingAccountsManager, secureStorageClient, secureBackupClient, clientPresenceManager, usernameGenerator,
           experimentEnrollmentManager, clock);
 

service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerChangeNumberIntegrationTest.java

@@ -193,7 +193,7 @@ class AccountsManagerChangeNumberIntegrationTest {
           mock(DirectoryQueue.class),
           mock(Keys.class),
           mock(MessagesManager.class),
-          mock(ReservedUsernames.class),
+          mock(ProhibitedUsernames.class),
           mock(ProfilesManager.class),
           mock(StoredVerificationCodeManager.class),
           secureStorageClient,

service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerConcurrentModificationIntegrationTest.java

@@ -160,7 +160,7 @@ class AccountsManagerConcurrentModificationIntegrationTest {
           mock(DirectoryQueue.class),
           mock(Keys.class),
           mock(MessagesManager.class),
-          mock(ReservedUsernames.class),
+          mock(ProhibitedUsernames.class),
           mock(ProfilesManager.class),
           mock(StoredVerificationCodeManager.class),
           mock(SecureStorageClient.class),

service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerTest.java

@@ -71,7 +71,7 @@ class AccountsManagerTest {
   private Keys keys;
   private MessagesManager messagesManager;
   private ProfilesManager profilesManager;
-  private ReservedUsernames reservedUsernames;
+  private ProhibitedUsernames prohibitedUsernames;
   private ExperimentEnrollmentManager enrollmentManager;
 
   private Map<String, UUID> phoneNumberIdentifiersByE164;
@@ -87,6 +87,8 @@ class AccountsManagerTest {
     return null;
   };
 
+  private static final UUID RESERVATION_TOKEN = UUID.randomUUID();
+
   @BeforeEach
   void setup() throws InterruptedException {
     accounts = mock(Accounts.class);
@@ -95,7 +97,7 @@ class AccountsManagerTest {
     keys = mock(Keys.class);
     messagesManager = mock(MessagesManager.class);
     profilesManager = mock(ProfilesManager.class);
-    reservedUsernames = mock(ReservedUsernames.class);
+    prohibitedUsernames = mock(ProhibitedUsernames.class);
 
     //noinspection unchecked
     commands = mock(RedisAdvancedClusterCommands.class);
@@ -149,7 +151,7 @@ class AccountsManagerTest {
         directoryQueue,
         keys,
         messagesManager,
-        reservedUsernames,
+        prohibitedUsernames,
         profilesManager,
         mock(StoredVerificationCodeManager.class),
         storageClient,
@@ -737,6 +739,65 @@ class AccountsManagerTest {
     verify(accounts).setUsername(eq(account),  startsWith(nickname));
   }
 
+  @Test
+  void testReserveUsername() throws UsernameNotAvailableException {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    final String nickname = "beethoven";
+    accountsManager.reserveUsername(account, nickname);
+    verify(accounts).reserveUsername(eq(account),  startsWith(nickname), any());
+  }
+
+  @Test
+  void testSetReservedUsername() throws UsernameNotAvailableException, UsernameReservationNotFoundException {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    final String reserved = "scooby#1234";
+    setReservationHash(account, reserved);
+    when(accounts.usernameAvailable(eq(Optional.of(RESERVATION_TOKEN)), eq(reserved))).thenReturn(true);
+    accountsManager.confirmReservedUsername(account, reserved, RESERVATION_TOKEN);
+    verify(accounts).confirmUsername(eq(account), eq(reserved), eq(RESERVATION_TOKEN));
+  }
+
+  @Test
+  void testSetReservedHashNameMismatch() {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    setReservationHash(account, "pluto#1234");
+    when(accounts.usernameAvailable(eq(Optional.of(RESERVATION_TOKEN)), eq("pluto#1234"))).thenReturn(true);
+    assertThrows(UsernameReservationNotFoundException.class,
+        () -> accountsManager.confirmReservedUsername(account, "goofy#1234", RESERVATION_TOKEN));
+  }
+
+  @Test
+  void testSetReservedHashAciMismatch() {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    final String reserved = "toto#1234";
+    account.setReservedUsernameHash(Accounts.reservedUsernameHash(UUID.randomUUID(), reserved));
+    when(accounts.usernameAvailable(eq(Optional.of(RESERVATION_TOKEN)), eq(reserved))).thenReturn(true);
+    assertThrows(UsernameReservationNotFoundException.class,
+        () -> accountsManager.confirmReservedUsername(account, reserved, RESERVATION_TOKEN));
+  }
+
+  @Test
+  void testSetReservedLapsed() {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    final String reserved = "porkchop#1234";
+    // name was reserved, but the reservation lapsed and another account took it
+    setReservationHash(account, reserved);
+    when(accounts.usernameAvailable(eq(Optional.of(RESERVATION_TOKEN)), eq(reserved))).thenReturn(false);
+    assertThrows(UsernameNotAvailableException.class, () -> accountsManager.confirmReservedUsername(account, reserved, RESERVATION_TOKEN));
+    verify(accounts, never()).confirmUsername(any(), any(), any());
+  }
+
+  @Test
+  void testSetReservedRetry() throws UsernameNotAvailableException, UsernameReservationNotFoundException {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    final String username = "santaslittlehelper#1234";
+    account.setUsername(username);
+
+    // reserved username already set, should be treated as a replay
+    accountsManager.confirmReservedUsername(account, username, RESERVATION_TOKEN);
+    verifyNoInteractions(accounts);
+  }
+
   @Test
   void testSetUsernameSameUsername() {
     final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
@@ -761,7 +822,29 @@ class AccountsManagerTest {
   }
 
   @Test
-  void testSetUsernameExpandDiscriminator() throws UsernameNotAvailableException {
+  void testReserveUsernameReroll() throws UsernameNotAvailableException {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
+    final String nickname = "clifford";
+    final String username = nickname + "#ZZZ";
+    account.setUsername(username);
+
+    // given the correct old username, should reroll discriminator even if the nick matches
+    accountsManager.reserveUsername(account, nickname);
+    verify(accounts).reserveUsername(eq(account), and(startsWith(nickname), not(eq(username))), any());
+  }
+
+  @Test
+  void testSetReservedUsernameWithNoReservation() {
+    final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(),
+        new ArrayList<>(), new byte[16]);
+    assertThrows(UsernameReservationNotFoundException.class,
+        () -> accountsManager.confirmReservedUsername(account, "laika#1234", RESERVATION_TOKEN));
+    verify(accounts, never()).confirmUsername(any(), any(), any());
+  }
+
+  @ParameterizedTest
+  @ValueSource(booleans = {false, true})
+  void testUsernameExpandDiscriminator(boolean reserve) throws UsernameNotAvailableException {
     final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
     final String nickname = "test";
 
@@ -775,9 +858,15 @@ class AccountsManagerTest {
     when(accounts.usernameAvailable(any())).thenReturn(false);
     when(accounts.usernameAvailable(argThat(isWide))).thenReturn(true);
 
+    if (reserve) {
+      accountsManager.reserveUsername(account, nickname);
+      verify(accounts).reserveUsername(eq(account), and(startsWith(nickname), argThat(isWide)), any());
+
+    } else {
       accountsManager.setUsername(account, nickname, null);
       verify(accounts).setUsername(eq(account), and(startsWith(nickname), argThat(isWide)));
     }
+  }
 
   @Test
   void testChangeUsername() throws UsernameNotAvailableException {
@@ -801,7 +890,7 @@ class AccountsManagerTest {
   @Test
   void testSetUsernameReserved() {
     final String nickname = "reserved";
-    when(reservedUsernames.isReserved(eq(nickname), any())).thenReturn(true);
+    when(prohibitedUsernames.isProhibited(eq(nickname), any())).thenReturn(true);
 
     final Account account = AccountsHelper.generateTestAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID(), new ArrayList<>(), new byte[16]);
 
@@ -823,6 +912,10 @@ class AccountsManagerTest {
     assertThrows(UsernameNotAvailableException.class, () -> accountsManager.setUsername(account, "n00bkiller", null));
   }
 
+  private void setReservationHash(final Account account, final String reservedUsername) {
+    account.setReservedUsernameHash(Accounts.reservedUsernameHash(account.getUuid(), reservedUsername));
+  }
+
   private static Device generateTestDevice(final long lastSeen) {
     final Device device = new Device();
     device.setId(Device.MASTER_ID);

service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerUsernameIntegrationTest.java

@@ -8,6 +8,8 @@ package org.whispersystems.textsecuregcm.storage;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.mockito.Mockito;
 import org.mockito.invocation.InvocationOnMock;
 import org.whispersystems.textsecuregcm.configuration.dynamic.DynamicConfiguration;
@@ -22,6 +24,8 @@ import org.whispersystems.textsecuregcm.util.AttributeValues;
 import org.whispersystems.textsecuregcm.util.UsernameGenerator;
 import software.amazon.awssdk.services.dynamodb.model.*;
 import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
 import java.util.*;
 import java.util.function.Consumer;
 
@@ -110,7 +114,11 @@ class AccountsManagerUsernameIntegrationTest {
         .build();
 
     ACCOUNTS_DYNAMO_EXTENSION.getDynamoDbClient().createTable(createPhoneNumberIdentifierTableRequest);
+    buildAccountsManager(1, 2, 10);
+  }
 
+  private void buildAccountsManager(final int initialWidth, int discriminatorMaxWidth, int attemptsPerWidth)
+      throws InterruptedException {
     @SuppressWarnings("unchecked") final DynamicConfigurationManager<DynamicConfiguration> dynamicConfigurationManager =
         mock(DynamicConfigurationManager.class);
 
@@ -127,6 +135,8 @@ class AccountsManagerUsernameIntegrationTest {
         USERNAMES_TABLE_NAME,
         SCAN_PAGE_SIZE));
 
+    usernameGenerator = new UsernameGenerator(initialWidth, discriminatorMaxWidth, attemptsPerWidth,
+        Duration.ofDays(1));
     final DeletedAccountsManager deletedAccountsManager = mock(DeletedAccountsManager.class);
     doAnswer((final InvocationOnMock invocationOnMock) -> {
       @SuppressWarnings("unchecked")
@@ -141,8 +151,6 @@ class AccountsManagerUsernameIntegrationTest {
     final ExperimentEnrollmentManager experimentEnrollmentManager = mock(ExperimentEnrollmentManager.class);
     when(experimentEnrollmentManager.isEnrolled(any(UUID.class), eq(AccountsManager.USERNAME_EXPERIMENT_NAME)))
         .thenReturn(true);
-
-    usernameGenerator = new UsernameGenerator(1, 2, 10);
     accountsManager = new AccountsManager(
         accounts,
         phoneNumberIdentifiers,
@@ -151,7 +159,7 @@ class AccountsManagerUsernameIntegrationTest {
         mock(DirectoryQueue.class),
         mock(Keys.class),
         mock(MessagesManager.class),
-        mock(ReservedUsernames.class),
+        mock(ProhibitedUsernames.class),
         mock(ProfilesManager.class),
         mock(StoredVerificationCodeManager.class),
         mock(SecureStorageClient.class),
@@ -190,19 +198,32 @@ class AccountsManagerUsernameIntegrationTest {
     assertThat(accountsManager.getByAccountIdentifier(account.getUuid()).orElseThrow().getUsername()).isEmpty();
   }
 
-  @Test
+  @ParameterizedTest
-  void testNoUsernames() throws InterruptedException {
+  @ValueSource(booleans = {false, true})
+  void testNoUsernames(boolean reserve) throws InterruptedException {
     Account account = accountsManager.create("+18005551111", "password", null, new AccountAttributes(),
         new ArrayList<>());
     for (int i = 1; i <= 99; i++) {
+      final Map<String, AttributeValue> item = new HashMap<>(Map.of(
+          Accounts.KEY_ACCOUNT_UUID, AttributeValues.fromUUID(UUID.randomUUID()),
+          Accounts.ATTR_USERNAME, AttributeValues.fromString(usernameGenerator.fromParts("n00bkiller", i))));
+      // half of these are taken usernames, half are only reservations (have a TTL)
+      if (i % 2 == 0) {
+        item.put(Accounts.ATTR_TTL,
+            AttributeValues.fromLong(Instant.now().plus(Duration.ofMinutes(1)).getEpochSecond()));
+      }
       ACCOUNTS_DYNAMO_EXTENSION.getDynamoDbClient().putItem(PutItemRequest.builder()
           .tableName(USERNAMES_TABLE_NAME)
-          .item(Map.of(
+          .item(item)
-              Accounts.KEY_ACCOUNT_UUID, AttributeValues.fromUUID(UUID.randomUUID()),
-              Accounts.ATTR_USERNAME, AttributeValues.fromString(usernameGenerator.fromParts("n00bkiller", i))))
           .build());
     }
-    assertThrows(UsernameNotAvailableException.class, () -> accountsManager.setUsername(account, "n00bkiller", null));
+    assertThrows(UsernameNotAvailableException.class, () -> {
+      if (reserve) {
+        accountsManager.reserveUsername(account, "n00bkiller");
+      } else {
+        accountsManager.setUsername(account, "n00bkiller", null);
+      }
+    });
     assertThat(accountsManager.getByAccountIdentifier(account.getUuid()).orElseThrow().getUsername()).isEmpty();
   }
 
@@ -237,4 +258,112 @@ class AccountsManagerUsernameIntegrationTest {
     verify(accounts, times(1)).usernameAvailable(argThat(un -> discriminator(un) >= 10));
   }
 
+  @Test
+  public void testReserveSetClear()
+      throws InterruptedException, UsernameNotAvailableException, UsernameReservationNotFoundException {
+    Account account = accountsManager.create("+18005551111", "password", null, new AccountAttributes(),
+        new ArrayList<>());
+    AccountsManager.UsernameReservation reservation = accountsManager.reserveUsername(account, "n00bkiller");
+    account = reservation.account();
+    assertThat(account.getReservedUsernameHash()).isPresent();
+    assertThat(reservation.reservedUsername()).startsWith("n00bkiller");
+    int discriminator = discriminator(reservation.reservedUsername());
+    assertThat(discriminator).isGreaterThan(0).isLessThan(10);
+    assertThat(accountsManager.getByUsername(reservation.reservedUsername())).isEmpty();
+
+    account = accountsManager.confirmReservedUsername(
+        account,
+        reservation.reservedUsername(),
+        reservation.reservationToken());
+
+    assertThat(account.getUsername().get()).startsWith("n00bkiller");
+    assertThat(accountsManager.getByUsername(account.getUsername().get()).orElseThrow().getUuid()).isEqualTo(
+        account.getUuid());
+
+    // reroll
+    reservation = accountsManager.reserveUsername(account, "n00bkiller");
+    account = reservation.account();
+    account = accountsManager.confirmReservedUsername(
+        account,
+        reservation.reservedUsername(),
+        reservation.reservationToken());
+
+    final String newUsername = account.getUsername().orElseThrow();
+    assertThat(discriminator(account.getUsername().orElseThrow())).isNotEqualTo(discriminator);
+
+    // clear
+    account = accountsManager.clearUsername(account);
+    assertThat(accountsManager.getByUsername(newUsername)).isEmpty();
+    assertThat(accountsManager.getByAccountIdentifier(account.getUuid()).orElseThrow().getUsername()).isEmpty();
+
+  }
+
+  @Test
+  public void testReservationLapsed()
+      throws InterruptedException, UsernameNotAvailableException, UsernameReservationNotFoundException {
+    // use a username generator that can retry a lot
+    buildAccountsManager(1, 1, 1000000);
+
+    final Account account = accountsManager.create("+18005551111", "password", null, new AccountAttributes(),
+        new ArrayList<>());
+    AccountsManager.UsernameReservation reservation1 = accountsManager.reserveUsername(account, "n00bkiller");
+    final String reservedUsername = reservation1.reservedUsername();
+
+    long past = Instant.now().minus(Duration.ofMinutes(1)).getEpochSecond();
+    // force expiration
+    ACCOUNTS_DYNAMO_EXTENSION.getDynamoDbClient().updateItem(UpdateItemRequest.builder()
+        .tableName(USERNAMES_TABLE_NAME)
+        .key(Map.of(Accounts.ATTR_USERNAME, AttributeValues.fromString(reservedUsername)))
+        .updateExpression("SET #ttl = :ttl")
+        .expressionAttributeNames(Map.of("#ttl", Accounts.ATTR_TTL))
+        .expressionAttributeValues(Map.of(":ttl", AttributeValues.fromLong(past)))
+        .build());
+
+    int discriminator = discriminator(reservedUsername);
+
+    // use up all names except the reserved one
+    for (int i = 1; i <= 9; i++) {
+      if (i == discriminator) {
+        continue;
+      }
+
+      ACCOUNTS_DYNAMO_EXTENSION.getDynamoDbClient().putItem(PutItemRequest.builder()
+          .tableName(USERNAMES_TABLE_NAME)
+          .item(Map.of(
+              Accounts.KEY_ACCOUNT_UUID, AttributeValues.fromUUID(UUID.randomUUID()),
+              Accounts.ATTR_USERNAME, AttributeValues.fromString(usernameGenerator.fromParts("n00bkiller", i))))
+          .build());
+    }
+
+    // a different account should be able to reserve it
+    Account account2 = accountsManager.create("+18005552222", "password", null, new AccountAttributes(),
+        new ArrayList<>());
+    final AccountsManager.UsernameReservation reservation2 = accountsManager.reserveUsername(account2, "n00bkiller"
+    );
+    assertThat(reservation2.reservedUsername()).isEqualTo(reservedUsername);
+
+    assertThrows(UsernameNotAvailableException.class,
+        () -> accountsManager.confirmReservedUsername(reservation1.account(), reservedUsername, reservation1.reservationToken()));
+    accountsManager.confirmReservedUsername(reservation2.account(), reservedUsername, reservation2.reservationToken());
+  }
+
+  @Test
+  void testUsernameReserveClearSetReserved()
+      throws InterruptedException, UsernameNotAvailableException, UsernameReservationNotFoundException {
+    Account account = accountsManager.create("+18005551111", "password", null, new AccountAttributes(),
+        new ArrayList<>());
+    account = accountsManager.setUsername(account, "n00bkiller", null);
+    final AccountsManager.UsernameReservation reservation = accountsManager.reserveUsername(account, "other");
+    account = reservation.account();
+
+    assertThat(reservation.reservedUsername()).startsWith("other");
+    assertThat(account.getUsername()).hasValueSatisfying(s -> s.startsWith("n00bkiller"));
+
+    account = accountsManager.clearUsername(account);
+    assertThat(account.getReservedUsernameHash()).isPresent();
+    assertThat(account.getUsername()).isEmpty();
+
+    account = accountsManager.confirmReservedUsername(account, reservation.reservedUsername(), reservation.reservationToken());
+    assertThat(account.getUsername()).hasValueSatisfying(s -> s.startsWith("other"));
+  }
 }

service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsTest.java

@@ -17,6 +17,9 @@ import static org.mockito.Mockito.when;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.uuid.UUIDComparator;
 import java.nio.charset.StandardCharsets;
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -26,6 +29,7 @@ import java.util.Random;
 import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionException;
+import java.util.function.Supplier;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -74,6 +78,7 @@ class AccountsTest {
           .build())
       .build();
 
+  private Clock mockClock;
   private DynamicConfigurationManager<DynamicConfiguration> mockDynamicConfigManager;
   private Accounts accounts;
 
@@ -130,7 +135,11 @@ class AccountsTest {
     when(mockDynamicConfigManager.getConfiguration())
         .thenReturn(new DynamicConfiguration());
 
+    mockClock = mock(Clock.class);
+    when(mockClock.instant()).thenReturn(Instant.EPOCH);
+
     this.accounts = new Accounts(
+        mockClock,
         mockDynamicConfigManager,
         dynamoDbExtension.getDynamoDbClient(),
         dynamoDbExtension.getDynamoDbAsyncClient(),
@@ -608,7 +617,7 @@ class AccountsTest {
   }
 
   @Test
-  void testSetUsername() throws UsernameNotAvailableException {
+  void testSetUsername() {
     final Account account = generateAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID());
     accounts.create(account);
 
@@ -679,7 +688,7 @@ class AccountsTest {
   }
 
   @Test
-  void testClearUsername() throws UsernameNotAvailableException {
+  void testClearUsername() {
     final Account account = generateAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID());
     accounts.create(account);
 
@@ -704,7 +713,7 @@ class AccountsTest {
   }
 
   @Test
-  void testClearUsernameVersionMismatch() throws UsernameNotAvailableException {
+  void testClearUsernameVersionMismatch() {
     final Account account = generateAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID());
     accounts.create(account);
 
@@ -719,6 +728,154 @@ class AccountsTest {
     assertThat(account.getUsername()).hasValueSatisfying(u -> assertThat(u).isEqualTo(username));
   }
 
+  @Test
+  void testReservedUsername() {
+    final Account account1 = generateAccount("+18005551111", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account1);
+    final Account account2 = generateAccount("+18005552222", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account2);
+
+    final UUID token = accounts.reserveUsername(account1, "garfield", Duration.ofDays(1));
+    assertThat(account1.getReservedUsernameHash()).get().isEqualTo(Accounts.reservedUsernameHash(account1.getUuid(), "garfield"));
+    assertThat(account1.getUsername()).isEmpty();
+
+    // account 2 shouldn't be able to reserve the username
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.reserveUsername(account2, "garfield", Duration.ofDays(1)));
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.confirmUsername(account2, "garfield", UUID.randomUUID()));
+    assertThat(accounts.getByUsername("garfield")).isEmpty();
+
+    accounts.confirmUsername(account1, "garfield", token);
+    assertThat(account1.getReservedUsernameHash()).isEmpty();
+    assertThat(account1.getUsername()).get().isEqualTo("garfield");
+    assertThat(accounts.getByUsername("garfield").get().getUuid()).isEqualTo(account1.getUuid());
+
+    assertThat(dynamoDbExtension.getDynamoDbClient()
+        .getItem(GetItemRequest.builder()
+            .tableName(USERNAME_CONSTRAINT_TABLE_NAME)
+            .key(Map.of(Accounts.ATTR_USERNAME, AttributeValues.fromString("garfield")))
+            .build())
+        .item())
+        .doesNotContainKey(Accounts.ATTR_TTL);
+  }
+
+  @Test
+  void testUsernameAvailable() {
+    final Account account1 = generateAccount("+18005551111", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account1);
+
+    final String username = "unsinkablesam";
+
+    final UUID token = accounts.reserveUsername(account1, username, Duration.ofDays(1));
+    assertThat(accounts.usernameAvailable(username)).isFalse();
+    assertThat(accounts.usernameAvailable(Optional.empty(), username)).isFalse();
+    assertThat(accounts.usernameAvailable(Optional.of(UUID.randomUUID()), username)).isFalse();
+    assertThat(accounts.usernameAvailable(Optional.of(token), username)).isTrue();
+
+    accounts.confirmUsername(account1, username, token);
+    assertThat(accounts.usernameAvailable(username)).isFalse();
+    assertThat(accounts.usernameAvailable(Optional.empty(), username)).isFalse();
+    assertThat(accounts.usernameAvailable(Optional.of(UUID.randomUUID()), username)).isFalse();
+    assertThat(accounts.usernameAvailable(Optional.of(token), username)).isFalse();
+  }
+
+
+    @Test
+  void testReservedUsernameWrongToken() {
+    final Account account = generateAccount("+18005551111", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account);
+    accounts.reserveUsername(account, "grumpy", Duration.ofDays(1));
+    assertThat(account.getReservedUsernameHash())
+        .get()
+        .isEqualTo(Accounts.reservedUsernameHash(account.getUuid(), "grumpy"));
+    assertThat(account.getUsername()).isEmpty();
+
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.confirmUsername(account, "grumpy", UUID.randomUUID()));
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.setUsername(account, "grumpy"));
+  }
+
+  @Test
+  void testReserveExpiredReservedUsername() {
+    final Account account1 = generateAccount("+18005551111", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account1);
+    final Account account2 = generateAccount("+18005552222", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account2);
+
+    accounts.reserveUsername(account1, "snowball#0002", Duration.ofDays(2));
+
+    Supplier<UUID> take = () -> accounts.reserveUsername(account2, "snowball#0002", Duration.ofDays(2));
+
+    for (int i = 0; i <= 2; i++) {
+      when(mockClock.instant()).thenReturn(Instant.EPOCH.plus(Duration.ofDays(i)));
+      assertThrows(ContestedOptimisticLockException.class, take::get);
+    }
+
+    // after 2 days, can take the name
+    when(mockClock.instant()).thenReturn(Instant.EPOCH.plus(Duration.ofDays(2)).plus(Duration.ofSeconds(1)));
+    final UUID token = take.get();
+
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.reserveUsername(account1, "snowball#0002", Duration.ofDays(2)));
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.setUsername(account1, "snowball#0002"));
+
+    accounts.confirmUsername(account2, "snowball#0002", token);
+    assertThat(accounts.getByUsername("snowball#0002").get().getUuid()).isEqualTo(account2.getUuid());
+  }
+
+  @Test
+  void testTakeExpiredReservedUsername() {
+    final Account account1 = generateAccount("+18005551111", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account1);
+    final Account account2 = generateAccount("+18005552222", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account2);
+
+    accounts.reserveUsername(account1, "snowball#0002", Duration.ofDays(2));
+
+    Runnable take = () -> accounts.setUsername(account2, "snowball#0002");
+
+    for (int i = 0; i <= 2; i++) {
+      when(mockClock.instant()).thenReturn(Instant.EPOCH.plus(Duration.ofDays(i)));
+      assertThrows(ContestedOptimisticLockException.class, take::run);
+    }
+
+    // after 2 days, can take the name
+    when(mockClock.instant()).thenReturn(Instant.EPOCH.plus(Duration.ofDays(2)).plus(Duration.ofSeconds(1)));
+    take.run();
+
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.reserveUsername(account1, "snowball#0002", Duration.ofDays(2)));
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.setUsername(account1, "snowball#0002"));
+    assertThat(accounts.getByUsername("snowball#0002").get().getUuid()).isEqualTo(account2.getUuid());
+  }
+
+  @Test
+  void testRetryReserveUsername() {
+    final Account account = generateAccount("+18005551111", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account);
+    accounts.reserveUsername(account, "jorts", Duration.ofDays(2));
+
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.reserveUsername(account, "jorts", Duration.ofDays(2)),
+        "Shouldn't be able to re-reserve same username (would extend ttl)");
+  }
+
+  @Test
+  void testReserveUsernameVersionConflict() {
+    final Account account = generateAccount("+18005551234", UUID.randomUUID(), UUID.randomUUID());
+    accounts.create(account);
+    account.setVersion(account.getVersion() + 12);
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.reserveUsername(account, "salem", Duration.ofDays(1)));
+    assertThrows(ContestedOptimisticLockException.class,
+        () -> accounts.setUsername(account, "salem"));
+
+  }
+
   private Device generateDevice(long id) {
     return DevicesHelper.createDevice(id);
   }

service/src/test/java/org/whispersystems/textsecuregcm/storage/ProhibitedUsernamesTest.java

@@ -17,37 +17,37 @@ import org.junit.jupiter.params.provider.MethodSource;
 import software.amazon.awssdk.services.dynamodb.model.AttributeDefinition;
 import software.amazon.awssdk.services.dynamodb.model.ScalarAttributeType;
 
-class ReservedUsernamesTest {
+class ProhibitedUsernamesTest {
 
   private static final String RESERVED_USERNAMES_TABLE_NAME = "reserved_usernames_test";
 
   @RegisterExtension
   static DynamoDbExtension dynamoDbExtension = DynamoDbExtension.builder()
       .tableName(RESERVED_USERNAMES_TABLE_NAME)
-      .hashKey(ReservedUsernames.KEY_PATTERN)
+      .hashKey(ProhibitedUsernames.KEY_PATTERN)
       .attributeDefinition(AttributeDefinition.builder()
-          .attributeName(ReservedUsernames.KEY_PATTERN)
+          .attributeName(ProhibitedUsernames.KEY_PATTERN)
           .attributeType(ScalarAttributeType.S)
           .build())
       .build();
 
   private static final UUID RESERVED_FOR_UUID = UUID.randomUUID();
 
-  private ReservedUsernames reservedUsernames;
+  private ProhibitedUsernames prohibitedUsernames;
 
   @BeforeEach
   void setUp() {
-    reservedUsernames =
+    prohibitedUsernames =
-        new ReservedUsernames(dynamoDbExtension.getDynamoDbClient(), RESERVED_USERNAMES_TABLE_NAME);
+        new ProhibitedUsernames(dynamoDbExtension.getDynamoDbClient(), RESERVED_USERNAMES_TABLE_NAME);
   }
 
   @ParameterizedTest
   @MethodSource
   void isReserved(final String username, final UUID uuid, final boolean expectReserved) {
-    reservedUsernames.reserveUsername(".*myusername.*", RESERVED_FOR_UUID);
+    prohibitedUsernames.prohibitUsername(".*myusername.*", RESERVED_FOR_UUID);
-    reservedUsernames.reserveUsername("^foobar$", RESERVED_FOR_UUID);
+    prohibitedUsernames.prohibitUsername("^foobar$", RESERVED_FOR_UUID);
 
-    assertEquals(expectReserved, reservedUsernames.isReserved(username, uuid));
+    assertEquals(expectReserved, prohibitedUsernames.isProhibited(username, uuid));
   }
 
   private static Stream<Arguments> isReserved() {

service/src/test/java/org/whispersystems/textsecuregcm/tests/controllers/AccountControllerTest.java

@@ -73,10 +73,13 @@ import org.whispersystems.textsecuregcm.entities.AccountIdentifierResponse;
 import org.whispersystems.textsecuregcm.entities.AccountIdentityResponse;
 import org.whispersystems.textsecuregcm.entities.ApnRegistrationId;
 import org.whispersystems.textsecuregcm.entities.ChangePhoneNumberRequest;
+import org.whispersystems.textsecuregcm.entities.ConfirmUsernameRequest;
 import org.whispersystems.textsecuregcm.entities.GcmRegistrationId;
 import org.whispersystems.textsecuregcm.entities.IncomingMessage;
 import org.whispersystems.textsecuregcm.entities.RegistrationLock;
 import org.whispersystems.textsecuregcm.entities.RegistrationLockFailure;
+import org.whispersystems.textsecuregcm.entities.ReserveUsernameRequest;
+import org.whispersystems.textsecuregcm.entities.ReserveUsernameResponse;
 import org.whispersystems.textsecuregcm.entities.SignedPreKey;
 import org.whispersystems.textsecuregcm.entities.UsernameRequest;
 import org.whispersystems.textsecuregcm.entities.UsernameResponse;
@@ -99,6 +102,7 @@ import org.whispersystems.textsecuregcm.storage.Device;
 import org.whispersystems.textsecuregcm.storage.DynamicConfigurationManager;
 import org.whispersystems.textsecuregcm.storage.StoredVerificationCodeManager;
 import org.whispersystems.textsecuregcm.storage.UsernameNotAvailableException;
+import org.whispersystems.textsecuregcm.storage.UsernameReservationNotFoundException;
 import org.whispersystems.textsecuregcm.tests.util.AccountsHelper;
 import org.whispersystems.textsecuregcm.tests.util.AuthHelper;
 import org.whispersystems.textsecuregcm.util.Hex;
@@ -121,6 +125,7 @@ class AccountControllerTest {
 
   private static final UUID   SENDER_REG_LOCK_UUID = UUID.randomUUID();
   private static final UUID   SENDER_TRANSFER_UUID = UUID.randomUUID();
+  private static final UUID   RESERVATION_TOKEN    = UUID.randomUUID();
 
   private static final String ABUSIVE_HOST             = "192.168.1.1";
   private static final String NICE_HOST                = "127.0.0.1";
@@ -144,6 +149,7 @@ class AccountControllerTest {
   private static RateLimiter            smsVoicePrefixLimiter  = mock(RateLimiter.class);
   private static RateLimiter            autoBlockLimiter       = mock(RateLimiter.class);
   private static RateLimiter            usernameSetLimiter     = mock(RateLimiter.class);
+  private static RateLimiter            usernameReserveLimiter = mock(RateLimiter.class);
   private static RateLimiter            usernameLookupLimiter  = mock(RateLimiter.class);
   private static SmsSender              smsSender              = mock(SmsSender.class);
   private static TurnTokenGenerator     turnTokenGenerator     = mock(TurnTokenGenerator.class);
@@ -208,6 +214,7 @@ class AccountControllerTest {
     when(rateLimiters.getSmsVoicePrefixLimiter()).thenReturn(smsVoicePrefixLimiter);
     when(rateLimiters.getAutoBlockLimiter()).thenReturn(autoBlockLimiter);
     when(rateLimiters.getUsernameSetLimiter()).thenReturn(usernameSetLimiter);
+    when(rateLimiters.getUsernameReserveLimiter()).thenReturn(usernameReserveLimiter);
     when(rateLimiters.getUsernameLookupLimiter()).thenReturn(usernameLookupLimiter);
 
     when(senderPinAccount.getLastSeen()).thenReturn(System.currentTimeMillis());
@@ -319,6 +326,7 @@ class AccountControllerTest {
         smsVoicePrefixLimiter,
         autoBlockLimiter,
         usernameSetLimiter,
+        usernameReserveLimiter,
         usernameLookupLimiter,
         smsSender,
         turnTokenGenerator,
@@ -1733,6 +1741,63 @@ class AccountControllerTest {
     assertThat(response.readEntity(UsernameResponse.class).username()).isEqualTo("n00bkiller#1234");
   }
 
+  @Test
+  void testReserveUsername() throws UsernameNotAvailableException {
+    when(accountsManager.reserveUsername(any(), eq("n00bkiller")))
+        .thenReturn(new AccountsManager.UsernameReservation(null, "n00bkiller#1234", RESERVATION_TOKEN));
+    Response response =
+        resources.getJerseyTest()
+            .target("/v1/accounts/username/reserved")
+            .request()
+            .header("Authorization", AuthHelper.getAuthHeader(AuthHelper.VALID_UUID, AuthHelper.VALID_PASSWORD))
+            .put(Entity.json(new ReserveUsernameRequest("n00bkiller")));
+    assertThat(response.getStatus()).isEqualTo(200);
+    assertThat(response.readEntity(ReserveUsernameResponse.class))
+        .satisfies(r -> r.username().equals("n00bkiller#1234"))
+        .satisfies(r -> r.reservationToken().equals(RESERVATION_TOKEN));
+  }
+
+  @Test
+  void testCommitUsername() throws UsernameNotAvailableException, UsernameReservationNotFoundException {
+    Account account = mock(Account.class);
+    when(account.getUsername()).thenReturn(Optional.of("n00bkiller#1234"));
+    when(accountsManager.confirmReservedUsername(any(), eq("n00bkiller#1234"), eq(RESERVATION_TOKEN))).thenReturn(account);
+    Response response =
+        resources.getJerseyTest()
+            .target("/v1/accounts/username/confirm")
+            .request()
+            .header("Authorization", AuthHelper.getAuthHeader(AuthHelper.VALID_UUID, AuthHelper.VALID_PASSWORD))
+            .put(Entity.json(new ConfirmUsernameRequest("n00bkiller#1234", RESERVATION_TOKEN)));
+    assertThat(response.getStatus()).isEqualTo(200);
+    assertThat(response.readEntity(UsernameResponse.class).username()).isEqualTo("n00bkiller#1234");
+  }
+
+  @Test
+  void testCommitUnreservedUsername() throws UsernameNotAvailableException, UsernameReservationNotFoundException {
+    when(accountsManager.confirmReservedUsername(any(), eq("n00bkiller#1234"), eq(RESERVATION_TOKEN)))
+        .thenThrow(new UsernameReservationNotFoundException());
+    Response response =
+        resources.getJerseyTest()
+            .target("/v1/accounts/username/confirm")
+            .request()
+            .header("Authorization", AuthHelper.getAuthHeader(AuthHelper.VALID_UUID, AuthHelper.VALID_PASSWORD))
+            .put(Entity.json(new ConfirmUsernameRequest("n00bkiller#1234", RESERVATION_TOKEN)));
+    assertThat(response.getStatus()).isEqualTo(409);
+  }
+
+  @Test
+  void testCommitLapsedUsername() throws UsernameNotAvailableException, UsernameReservationNotFoundException {
+    when(accountsManager.confirmReservedUsername(any(), eq("n00bkiller#1234"), eq(RESERVATION_TOKEN)))
+        .thenThrow(new UsernameNotAvailableException());
+    Response response =
+        resources.getJerseyTest()
+            .target("/v1/accounts/username/confirm")
+            .request()
+            .header("Authorization", AuthHelper.getAuthHeader(AuthHelper.VALID_UUID, AuthHelper.VALID_PASSWORD))
+            .put(Entity.json(new ConfirmUsernameRequest("n00bkiller#1234", RESERVATION_TOKEN)));
+    assertThat(response.getStatus()).isEqualTo(410);
+  }
+
   @Test
   void testSetTakenUsername() {
     Response response =

service/src/test/java/org/whispersystems/textsecuregcm/tests/util/UsernameGeneratorTest.java

@@ -8,6 +8,7 @@ import org.junit.jupiter.params.provider.MethodSource;
 import org.whispersystems.textsecuregcm.storage.UsernameNotAvailableException;
 import org.whispersystems.textsecuregcm.util.UsernameGenerator;
 
+import java.time.Duration;
 import java.util.HashSet;
 import java.util.Set;
 import java.util.function.Predicate;
@@ -17,6 +18,8 @@ import static org.assertj.core.api.Assertions.assertThat;
 
 public class UsernameGeneratorTest {
 
+  private static final Duration TTL = Duration.ofMinutes(5);
+
   @ParameterizedTest(name = "[{index}]:{0} ({2})")
   @MethodSource
   public void nicknameValidation(String nickname, boolean valid, String testCaseName) {
@@ -64,7 +67,7 @@ public class UsernameGeneratorTest {
 
   @Test
   public void zeroPadDiscriminators() {
-    final UsernameGenerator generator = new UsernameGenerator(4, 5, 1);
+    final UsernameGenerator generator = new UsernameGenerator(4, 5, 1, TTL);
     assertThat(generator.fromParts("test", 1)).isEqualTo("test#0001");
     assertThat(generator.fromParts("test", 123)).isEqualTo("test#0123");
     assertThat(generator.fromParts("test", 9999)).isEqualTo("test#9999");
@@ -73,16 +76,16 @@ public class UsernameGeneratorTest {
 
   @Test
   public void expectedWidth() throws UsernameNotAvailableException {
-    String username = new UsernameGenerator(1, 6, 1).generateAvailableUsername("test", t -> true);
+    String username = new UsernameGenerator(1, 6, 1, TTL).generateAvailableUsername("test", t -> true);
     assertThat(extractDiscriminator(username)).isGreaterThan(0).isLessThan(10);
 
-    username = new UsernameGenerator(2, 6, 1).generateAvailableUsername("test", t -> true);
+    username = new UsernameGenerator(2, 6, 1, TTL).generateAvailableUsername("test", t -> true);
     assertThat(extractDiscriminator(username)).isGreaterThan(0).isLessThan(100);
   }
 
   @Test
   public void expandDiscriminator() throws UsernameNotAvailableException {
-    UsernameGenerator ug = new UsernameGenerator(1, 6, 10);
+    UsernameGenerator ug = new UsernameGenerator(1, 6, 10, TTL);
     final String username = ug.generateAvailableUsername("test", allowDiscriminator(d -> d >= 10000));
     int discriminator = extractDiscriminator(username);
     assertThat(discriminator).isGreaterThanOrEqualTo(10000).isLessThan(100000);
@@ -90,7 +93,7 @@ public class UsernameGeneratorTest {
 
   @Test
   public void expandDiscriminatorToMax() throws UsernameNotAvailableException {
-    UsernameGenerator ug = new UsernameGenerator(1, 6, 10);
+    UsernameGenerator ug = new UsernameGenerator(1, 6, 10, TTL);
     final String username = ug.generateAvailableUsername("test", allowDiscriminator(d -> d >= 100000));
     int discriminator = extractDiscriminator(username);
     assertThat(discriminator).isGreaterThanOrEqualTo(100000).isLessThan(1000000);
@@ -98,7 +101,7 @@ public class UsernameGeneratorTest {
 
   @Test
   public void exhaustDiscriminator() {
-    UsernameGenerator ug = new UsernameGenerator(1, 6, 10);
+    UsernameGenerator ug = new UsernameGenerator(1, 6, 10, TTL);
     Assertions.assertThrows(UsernameNotAvailableException.class, () -> {
       // allow greater than our max width
       ug.generateAvailableUsername("test", allowDiscriminator(d -> d >= 1000000));
@@ -107,7 +110,7 @@ public class UsernameGeneratorTest {
 
   @Test
   public void randomCoverageMinWidth() throws UsernameNotAvailableException {
-    UsernameGenerator ug = new UsernameGenerator(1, 6, 10);
+    UsernameGenerator ug = new UsernameGenerator(1, 6, 10, TTL);
     final Set<Integer> seen = new HashSet<>();
     for (int i = 0; i < 1000 && seen.size() < 9; i++) {
       seen.add(extractDiscriminator(ug.generateAvailableUsername("test", ignored -> true)));
@@ -120,7 +123,7 @@ public class UsernameGeneratorTest {
 
   @Test
   public void randomCoverageMidWidth() throws UsernameNotAvailableException {
-    UsernameGenerator ug = new UsernameGenerator(1, 6, 10);
+    UsernameGenerator ug = new UsernameGenerator(1, 6, 10, TTL);
     final Set<Integer> seen = new HashSet<>();
     for (int i = 0; i < 100000 && seen.size() < 90; i++) {
       seen.add(extractDiscriminator(ug.generateAvailableUsername("test", allowDiscriminator(d -> d >= 10))));