Throw a `MismatchedDevicesException` for empty message lists to support iOS clients

service/src/main/java/org/whispersystems/textsecuregcm/push/MessageSender.java

@@ -13,6 +13,7 @@ import io.micrometer.core.instrument.DistributionSummary;
 import io.micrometer.core.instrument.Metrics;
 import io.micrometer.core.instrument.Tag;
 import io.micrometer.core.instrument.Tags;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
@@ -99,7 +100,11 @@ public class MessageSender {
       @Nullable final String userAgent) throws MismatchedDevicesException, MessageTooLargeException {
 
     if (messagesByDeviceId.isEmpty()) {
-      return;
+      // TODO Simply return and don't throw an exception when iOS clients no longer depend on this behavior
+      throw new MismatchedDevicesException(new MismatchedDevices(
+          destination.getDevices().stream().map(Device::getId).collect(Collectors.toSet()),
+          Collections.emptySet(),
+          Collections.emptySet()));
     }
 
     if (!destination.isIdentifiedBy(destinationIdentifier)) {

service/src/test/java/org/whispersystems/textsecuregcm/push/MessageSenderTest.java

@@ -344,4 +344,19 @@ class MessageSenderTest {
             Optional.of(new MismatchedDevices(Set.of(primaryDeviceId), Set.of(extraDeviceId), Set.of(linkedDeviceId))))
     );
   }
+
+  @Test
+  void sendMessageEmptyMessageList() {
+    final Device device = mock(Device.class);
+    when(device.getId()).thenReturn(Device.PRIMARY_ID);
+
+    final Account account = mock(Account.class);
+    when(account.getDevices()).thenReturn(List.of(device));
+
+    assertThrows(MismatchedDevicesException.class, () -> messageSender.sendMessages(account,
+        new AciServiceIdentifier(UUID.randomUUID()),
+        Collections.emptyMap(),
+        Collections.emptyMap(),
+        null));
+  }
 }