Enhance RTCUIApplicationStatusObserver thread safety.

Add locking around waiting for initialization to finish, since calling
dispatch_block_wait from multiple threads leads to undefined behavior.

Initialize RTCUIApplicationStatusObserver earlier to give the
initialization block more time to run on the main thread before
starting to query the application state.

http://www.dailymotion.com/video/x2mckmh

BUG=b/65558688

Review-Url: https://codereview.webrtc.org/3009383002
Cr-Commit-Position: refs/heads/master@{#19822}

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

@@ -15,6 +15,7 @@
 @interface RTCUIApplicationStatusObserver : NSObject
 
 + (instancetype)sharedInstance;
++ (void)prepareForUse;
 
 - (BOOL)isApplicationActive;
 

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

@@ -26,6 +26,7 @@
 @implementation RTCUIApplicationStatusObserver {
   BOOL _initialized;
   dispatch_block_t _initializeBlock;
+  dispatch_semaphore_t _waitForInitializeSemaphore;
   UIApplicationState _state;
 
   id<NSObject> _activeObserver;
@@ -45,6 +46,12 @@
   return sharedInstance;
 }
 
+// Method to make sure observers are added and the initialization block is
+// scheduled to run on the main queue.
++ (void)prepareForUse {
+  __unused RTCUIApplicationStatusObserver *observer = [self sharedInstance];
+}
+
 - (id)init {
   if (self = [super init]) {
     NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
@@ -65,6 +72,7 @@
                                                 [UIApplication sharedApplication].applicationState;
                                           }];
 
+    _waitForInitializeSemaphore = dispatch_semaphore_create(1);
     _initialized = NO;
     _initializeBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
       weakSelf.state = [UIApplication sharedApplication].applicationState;
@@ -84,10 +92,19 @@
 }
 
 - (BOOL)isApplicationActive {
+  // NOTE: The function `dispatch_block_wait` can only legally be called once.
+  // Because of this, if several threads call the `isApplicationActive` method before
+  // the `_initializeBlock` has been executed, instead of multiple threads calling
+  // `dispatch_block_wait`, the other threads need to wait for the first waiting thread
+  // instead.
   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);
+    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_signal(_waitForInitializeSemaphore);
   }
   return _state == UIApplicationStateActive;
 }

webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm

@@ -67,6 +67,16 @@ void decompressionOutputCallback(void *decoder,
   RTCVideoDecoderCallback _callback;
 }
 
+- (instancetype)init {
+  if (self = [super init]) {
+#if defined(WEBRTC_IOS)
+    [RTCUIApplicationStatusObserver prepareForUse];
+#endif
+  }
+
+  return self;
+}
+
 - (void)dealloc {
   [self destroyDecompressionSession];
   [self setVideoFormat:nullptr];

webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoEncoderH264.mm

@@ -301,6 +301,10 @@ CFStringRef ExtractProfile(const cricket::VideoCodec &codec) {
     _profile = ExtractProfile([codecInfo nativeVideoCodec]);
     LOG(LS_INFO) << "Using profile " << CFStringToString(_profile);
     RTC_CHECK([codecInfo.name isEqualToString:@"H264"]);
+
+#if defined(WEBRTC_IOS)
+    [RTCUIApplicationStatusObserver prepareForUse];
+#endif
   }
   return self;
 }