Use private queues in message decrypter and batch processor.

// FREEBIE

SignalServiceKit/src/Messages/OWSBatchMessageProcessor.m

@@ -5,6 +5,7 @@
 #import "OWSBatchMessageProcessor.h"
 #import "NSArray+OWS.h"
 #import "OWSMessageManager.h"
+#import "OWSQueues.h"
 #import "OWSSignalServiceProtos.pb.h"
 #import "TSDatabaseView.h"
 #import "TSStorageManager.h"
@@ -277,6 +278,16 @@ NSString *const OWSMessageContentJobFinderExtensionGroup = @"OWSBatchMessageProc
 
 #pragma mark - instance methods
 
+- (dispatch_queue_t)serialQueue
+{
+    static dispatch_queue_t queue = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        queue = dispatch_queue_create("org.whispersystems.message.process", DISPATCH_QUEUE_SERIAL);
+    });
+    return queue;
+}
+
 - (void)enqueueEnvelopeData:(NSData *)envelopeData plaintextData:(NSData *_Nullable)plaintextData
 {
     OWSAssert(envelopeData);
@@ -287,7 +298,7 @@ NSString *const OWSMessageContentJobFinderExtensionGroup = @"OWSBatchMessageProc
 
 - (void)drainQueue
 {
-    DispatchMainThreadSafe(^{
+    dispatch_async(self.serialQueue, ^{
         if ([TSDatabaseView hasPendingViewRegistrations]) {
             // We don't want to process incoming messages until database
             // view registration is complete.
@@ -305,7 +316,7 @@ NSString *const OWSMessageContentJobFinderExtensionGroup = @"OWSBatchMessageProc
 
 - (void)drainQueueWorkStep
 {
-    AssertIsOnMainThread();
+    AssertOnDispatchQueue(self.serialQueue);
 
     NSArray<OWSMessageContentJob *> *jobs = [self.finder nextJobsForBatchSize:kIncomingMessageBatchSize];
     OWSAssert(jobs);
@@ -315,9 +326,8 @@ NSString *const OWSMessageContentJobFinderExtensionGroup = @"OWSBatchMessageProc
         return;
     }
 
-    [self processJobs:jobs
-           completion:^{
-               dispatch_async(dispatch_get_main_queue(), ^{
+    [self processJobs:jobs];
+
     [self.finder removeJobsWithIds:jobs.uniqueIds];
 
     DDLogVerbose(@"%@ completed %zd jobs. %zd jobs left.",
@@ -329,17 +339,15 @@ NSString *const OWSMessageContentJobFinderExtensionGroup = @"OWSBatchMessageProc
     // This delay won't affect the first message to arrive when this queue is idle,
     // so by definition we're receiving more than one message and can benefit from
     // batching.
-                   dispatch_after(
-                       dispatch_time(DISPATCH_TIME_NOW, (int64_t)0.1f * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
+    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1f * NSEC_PER_SEC)), self.serialQueue, ^{
         [self drainQueueWorkStep];
     });
-               });
-           }];
 }
 
-- (void)processJobs:(NSArray<OWSMessageContentJob *> *)jobs completion:(void (^)())completion
+- (void)processJobs:(NSArray<OWSMessageContentJob *> *)jobs
 {
-    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    AssertOnDispatchQueue(self.serialQueue);
+
     [self.dbReadWriteConnection readWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
         for (OWSMessageContentJob *job in jobs) {
             [self.messagesManager processEnvelope:job.envelopeProto
@@ -347,8 +355,6 @@ NSString *const OWSMessageContentJobFinderExtensionGroup = @"OWSBatchMessageProc
                                       transaction:transaction];
         }
     }];
-        completion();
-    });
 }
 
 #pragma mark Logging

SignalServiceKit/src/Messages/OWSMessageReceiver.m

@@ -6,6 +6,7 @@
 #import "NSArray+OWS.h"
 #import "OWSBatchMessageProcessor.h"
 #import "OWSMessageDecrypter.h"
+#import "OWSQueues.h"
 #import "OWSSignalServiceProtos.pb.h"
 #import "TSDatabaseView.h"
 #import "TSStorageManager.h"
@@ -249,6 +250,16 @@ NSString *const OWSMessageDecryptJobFinderExtensionGroup = @"OWSMessageProcessin
 
 #pragma mark - instance methods
 
+- (dispatch_queue_t)serialQueue
+{
+    static dispatch_queue_t queue = nil;
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        queue = dispatch_queue_create("org.whispersystems.message.decrypt", DISPATCH_QUEUE_SERIAL);
+    });
+    return queue;
+}
+
 - (void)enqueueEnvelopeForProcessing:(OWSSignalServiceProtosEnvelope *)envelope
 {
     [self.finder addJobForEnvelope:envelope];
@@ -256,7 +267,7 @@ NSString *const OWSMessageDecryptJobFinderExtensionGroup = @"OWSMessageProcessin
 
 - (void)drainQueue
 {
-    DispatchMainThreadSafe(^{
+    dispatch_async(self.serialQueue, ^{
         if ([TSDatabaseView hasPendingViewRegistrations]) {
             // We don't want to process incoming messages until database
             // view registration is complete.
@@ -274,7 +285,7 @@ NSString *const OWSMessageDecryptJobFinderExtensionGroup = @"OWSMessageProcessin
 
 - (void)drainQueueWorkStep
 {
-    AssertIsOnMainThread();
+    AssertOnDispatchQueue(self.serialQueue);
 
     OWSMessageDecryptJob *_Nullable job = [self.finder nextJob];
     if (!job) {
@@ -296,10 +307,10 @@ NSString *const OWSMessageDecryptJobFinderExtensionGroup = @"OWSMessageProcessin
 
 - (void)processJob:(OWSMessageDecryptJob *)job completion:(void (^)(BOOL))completion
 {
+    AssertOnDispatchQueue(self.serialQueue);
     OWSAssert(job);
 
     OWSSignalServiceProtosEnvelope *envelope = job.envelopeProto;
-    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     [self.messageDecrypter decryptEnvelope:envelope
         successBlock:^(NSData *_Nullable plaintextData) {
 
@@ -307,16 +318,15 @@ NSString *const OWSMessageDecryptJobFinderExtensionGroup = @"OWSMessageProcessin
             // the decrypted envelope data ASAP to prevent data loss.
             [self.batchMessageProcessor enqueueEnvelopeData:job.envelopeData plaintextData:plaintextData];
 
-                dispatch_async(dispatch_get_main_queue(), ^{
+            dispatch_async(self.serialQueue, ^{
                 completion(YES);
             });
         }
         failureBlock:^{
-                dispatch_async(dispatch_get_main_queue(), ^{
+            dispatch_async(self.serialQueue, ^{
                 completion(NO);
             });
         }];
-    });
 }
 
 #pragma mark Logging