From 48c324fe8684433efc2af1727b663be8ebf0a207 Mon Sep 17 00:00:00 2001
From: Ehren Kret <ehren@signal.org>
Date: Wed, 29 Apr 2020 17:31:43 -0700
Subject: [PATCH] Use a static sequence of randomness in tests

The RemoteConfigControllerTest#testMath unit test would occassionally
fail because randomness doesn't necessarily group into expected ranges
over a finite trial count. This changes the test to use a predefined
PRNG sequence instead of one that varies with each test so that the
test will no long randomly fail.
---
 .../controllers/RemoteConfigControllerTest.java    | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/tests/controllers/RemoteConfigControllerTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/tests/controllers/RemoteConfigControllerTest.java
index d1b3856dd..35cbe54db 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/tests/controllers/RemoteConfigControllerTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/tests/controllers/RemoteConfigControllerTest.java
@@ -22,6 +22,7 @@ import javax.ws.rs.core.MediaType;
 import javax.ws.rs.core.Response;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedList;
@@ -263,12 +264,13 @@ public class RemoteConfigControllerTest {
     Map<String, Integer> enabledMap       = new HashMap<>();
     MessageDigest        digest           = MessageDigest.getInstance("SHA1");
     int                  iterations       = 100000;
+    SecureRandom         secureRandom     = new SecureRandom(new byte[]{42});  // the seed value doesn't matter so much as it's constant to make the test not flaky
 
     for (int i=0;i<iterations;i++) {
       for (RemoteConfig config : remoteConfigList) {
         int count  = enabledMap.getOrDefault(config.getName(), 0);
 
-        if (RemoteConfigController.isInBucket(digest, UUID.randomUUID(), config.getName().getBytes(), config.getPercentage(), new HashSet<>())) {
+        if (RemoteConfigController.isInBucket(digest, getRandomUUID(secureRandom), config.getName().getBytes(), config.getPercentage(), new HashSet<>())) {
           count++;
         }
 
@@ -285,5 +287,13 @@ public class RemoteConfigControllerTest {
 
   }
 
-
+  private static UUID getRandomUUID(SecureRandom secureRandom) {
+    long mostSignificantBits  = secureRandom.nextLong();
+    long leastSignificantBits = secureRandom.nextLong();
+    mostSignificantBits  &= 0xffffffffffff0fffL;
+    mostSignificantBits  |= 0x0000000000004000L;
+    leastSignificantBits &= 0x3fffffffffffffffL;
+    leastSignificantBits |= 0x8000000000000000L;
+    return new UUID(mostSignificantBits, leastSignificantBits);
+  }
 }