Enhance RTCUIApplicationStatusObserver thread safety.

webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "RTCUIApplicationStatusObserver.h"
 
 #if defined(WEBRTC_IOS)
 
 #import <UIKit/UIKit.h>
 
 #include "webrtc/rtc_base/checks.h"
 
 @implementation RTCUIApplicationStatusObserver {
   BOOL _initialized;
   dispatch_block_t _initializeBlock;
+  dispatch_semaphore_t _waitForInitializeSemaphore;
   UIApplicationState _state;
 }
 
 + (instancetype)sharedInstance {
   static id sharedInstance;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
     sharedInstance = [[self alloc] init];
   });
 
@@ skipping 10 matching lines @@
                       _state = [UIApplication sharedApplication].applicationState;
                     }];
 
     [center addObserverForName:UIApplicationDidEnterBackgroundNotification
                         object:nil
                          queue:[NSOperationQueue mainQueue]
                     usingBlock:^(NSNotification *note) {
                       _state = [UIApplication sharedApplication].applicationState;
                     }];
 
+    _waitForInitializeSemaphore = dispatch_semaphore_create(1);
     _initialized = NO;
     _initializeBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
       _state = [UIApplication sharedApplication].applicationState;
       _initialized = YES;
     });
 
     dispatch_async(dispatch_get_main_queue(), _initializeBlock);
   }
 
   return self;
 }
 
 - (BOOL)isApplicationActive {
   if (!_initialized) {
-    long ret = dispatch_block_wait(_initializeBlock,
-                                   dispatch_time(DISPATCH_TIME_NOW, 10.0 * NSEC_PER_SEC));
-    RTC_DCHECK_EQ(ret, 0);
+    dispatch_semaphore_wait(_waitForInitializeSemaphore, DISPATCH_TIME_FOREVER);

[daniela-webrtc, 2017/09/13 09:02:50]

This code is becoming slightly complex now. Would you mind adding comments to
describe why it was done like it is?

[andersc, 2017/09/13 09:19:51]

Acknowledged.

+    if (!_initialized) {

[daniela-webrtc, 2017/09/13 09:02:50]

I guess we want to synchronize only if the observation wasn't initialized?
Wouldn't moving the semaphore in the if statement make more sense?

[andersc, 2017/09/13 09:19:51]

Not sure what you mean. Since the `dispatch_block_wait` function can only
legally be called once, this if statement basically checks if we were stuck in
the semaphore waiting for the other thread. If we were, and the other thread has
now finished waiting for the initialization block to run and exited the critical
section, then we are already initialized and should not call
`dispatch_block_wait` but just proceed and read the _state variable. If
_initialized is still false after we enter the critical section, we are the
first thread to come here and should call `dispatch_block_wait`.

I clarified this in a comment.

[daniela-webrtc, 2017/09/13 09:30:49]

Acknowledged.

[daniela-webrtc, 2017/09/13 09:30:49]

Missed that there were two if statements. The comments make it clearer. Thanks!

+      long ret = dispatch_block_wait(_initializeBlock,
+                                     dispatch_time(DISPATCH_TIME_NOW, 10.0 * NSEC_PER_SEC));
+      RTC_DCHECK_EQ(ret, 0);
+    }
+    dispatch_semaphore_signal(_waitForInitializeSemaphore);
   }
   return _state == UIApplicationStateActive;
 }
 
 @end
 
 #endif  // WEBRTC_IOS