NSRunLoop methods should only be accessed on it's corresponding thread.

SignalServiceKit/src/Util/NSTimer+OWS.h

@@ -14,12 +14,6 @@ NS_ASSUME_NONNULL_BEGIN
                                        userInfo:(nullable id)userInfo
                                         repeats:(BOOL)repeats;
 
-+ (NSTimer *)weakTimerWithTimeInterval:(NSTimeInterval)timeInterval
-                                target:(id)target
-                              selector:(SEL)selector
-                              userInfo:(nullable id)userInfo
-                               repeats:(BOOL)repeats;
-
 @end
 
 NS_ASSUME_NONNULL_END

SignalServiceKit/src/Util/NSTimer+OWS.m

@@ -64,24 +64,6 @@ static void *kNSTimer_OWS_Proxy = &kNSTimer_OWS_Proxy;
     return timer;
 }
 
-+ (NSTimer *)weakTimerWithTimeInterval:(NSTimeInterval)timeInterval
-                                target:(id)target
-                              selector:(SEL)selector
-                              userInfo:(nullable id)userInfo
-                               repeats:(BOOL)repeats
-{
-    NSTimerProxy *proxy = [NSTimerProxy new];
-    proxy.target = target;
-    proxy.selector = selector;
-    NSTimer *timer = [NSTimer timerWithTimeInterval:timeInterval
-                                             target:proxy
-                                           selector:@selector(timerFired:)
-                                           userInfo:userInfo
-                                            repeats:repeats];
-    [timer ows_setProxy:proxy];
-    return timer;
-}
-
 @end
 
 NS_ASSUME_NONNULL_END

SignalServiceKit/src/Util/OWSOperation.m

@@ -192,25 +192,23 @@ NSString *const OWSOperationKeyIsFinished = @"isFinished";
 
     self.remainingRetries--;
 
-    dispatch_sync(self.retryTimerSerialQueue, ^{
-        OWSAssertDebug(self.retryTimer == nil);
-        [self.retryTimer invalidate];
-        NSTimer *retryTimer = [NSTimer weakTimerWithTimeInterval:self.retryInterval
-                                                          target:self
-                                                        selector:@selector(runAnyQueuedRetry)
-                                                        userInfo:nil
-                                                         repeats:NO];
-        
-        self.retryTimer = retryTimer;
-
-        // The `scheduledTimerWith*` methods add the timer to the current thread's RunLoop.
-        // Since Operations typically run on a background thread, that would mean the background
-        // thread's RunLoop. However, the OS can spin down background threads if there's no work
-        // being done, so we run the risk of the timer's RunLoop being deallocated before it's
-        // fired.
-        //
-        // To ensure the timer's thread sticks around, we schedule it on the main RunLoop.
-        [NSRunLoop.mainRunLoop addTimer:retryTimer forMode:NSDefaultRunLoopMode];
+    dispatch_async(dispatch_get_main_queue(), ^{
+        dispatch_sync(self.retryTimerSerialQueue, ^{
+            OWSAssertDebug(self.retryTimer == nil);
+            [self.retryTimer invalidate];
+            // The `scheduledTimerWith*` methods add the timer to the current thread's RunLoop.
+            // Since Operations typically run on a background thread, that would mean the background
+            // thread's RunLoop. However, the OS can spin down background threads if there's no work
+            // being done, so we run the risk of the timer's RunLoop being deallocated before it's
+            // fired.
+            //
+            // To ensure the timer's thread sticks around, we schedule it while on the main RunLoop.
+            self.retryTimer = [NSTimer weakScheduledTimerWithTimeInterval:self.retryInterval
+                                                                   target:self
+                                                                 selector:@selector(runAnyQueuedRetry)
+                                                                 userInfo:nil
+                                                                  repeats:NO];
+        });
     });
 }