Bug 1447685 Move nsIRandomGenerator creation out of a lock to avoid a deadlock r=keeler

do_GetService("@mozilla.org/security/random-generator;1" may initialize NSS.
NSS Initialization occurs on Main Thread only.

If we lock on a subthread, then try to initialize NSS, it _might_ be the case
that the main thread is blocked on the same lock (same location or a different
one.) NSS can't initialize on Main Thread - deadlock.

Move do_GetService out of the lock. Now if NSS tries to initialize on a
subthread, the main thead can't be blocked (because the subthread hasn't locked
anything.)

Now, the only statements that occur in locks are pointer asignment, new, memcpy,
and randomGenerator->GenerateRandomBytes.

MozReview-Commit-ID: 9C1Ok910A11

--HG--
extra : rebase_source : 637ca346a343722bd7a4fc68c4fd43a85916d5a6

toolkit/components/resistfingerprinting/nsRFPService.cpp

@@ -376,12 +376,12 @@ nsRFPService::RandomMidpoint(long long aClampedTimeUSec,
 
     // If we don't have a seed, we need to get one.
     if(MOZ_UNLIKELY(!sSecretMidpointSeed)) {
+      nsCOMPtr<nsIRandomGenerator> randomGenerator =
+        do_GetService("@mozilla.org/security/random-generator;1", &rv);
+      if (NS_WARN_IF(NS_FAILED(rv))) { return rv; }
+
       StaticMutexAutoLock lock(sLock);
       if(MOZ_LIKELY(!sSecretMidpointSeed)) {
-        nsCOMPtr<nsIRandomGenerator> randomGenerator =
-            do_GetService("@mozilla.org/security/random-generator;1", &rv);
-        if (NS_WARN_IF(NS_FAILED(rv))) { return rv; }
-
         rv = randomGenerator->GenerateRandomBytes(kSeedSize, &sSecretMidpointSeed);
         if (NS_WARN_IF(NS_FAILED(rv))) { return rv; }
       }