Enhance RTCUIApplicationStatusObserver thread safety.

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

Index: webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
diff --git a/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m b/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
index 168832d9ed08a2653f2595f4f425203bde3d5f7a..84325b27f1616d5ec25dfa6e2d887ea57b141688 100644
--- a/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
+++ b/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
@@ -19,6 +19,7 @@
 @implementation RTCUIApplicationStatusObserver {
   BOOL _initialized;
   dispatch_block_t _initializeBlock;
+  dispatch_semaphore_t _waitForInitializeSemaphore;
   UIApplicationState _state;
 }
 
@@ -49,6 +50,7 @@
                       _state = [UIApplication sharedApplication].applicationState;
                     }];
 
+    _waitForInitializeSemaphore = dispatch_semaphore_create(1);
     _initialized = NO;
     _initializeBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
       _state = [UIApplication sharedApplication].applicationState;
@@ -63,9 +65,13 @@
 
 - (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;
 }