[iOS] Fix incorrectly oriented frames when rapidly switching between cameras.

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m

Index: webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m
diff --git a/webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m b/webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m
index a2290c2458405a1d39458f64eae820abfdc14bc9..0ac16488b4bfd14116c6934408dd5f8d6e9218d2 100644
--- a/webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m
+++ b/webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCCameraVideoCapturer.m
@@ -19,6 +19,7 @@
 #endif
 
 #import "RTCDispatcher+Private.h"
+#import "RTCImageHelper.h"
 
 const int64_t kNanosecondsPerSecond = 1000000000;
 
@@ -29,21 +30,26 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
 
 @interface RTCCameraVideoCapturer ()<AVCaptureVideoDataOutputSampleBufferDelegate>
 @property(nonatomic, readonly) dispatch_queue_t frameQueue;
+@property(assign) BOOL switchingCameras;

[magjed_webrtc, 2017/07/24 10:12:50]

It looks like we are accessing this variable from multiple threads without
synchronization.

 @end
 
 @implementation RTCCameraVideoCapturer {
   AVCaptureVideoDataOutput *_videoDataOutput;
   AVCaptureSession *_captureSession;
   AVCaptureDevice *_currentDevice;
-  RTCVideoRotation _rotation;
   BOOL _hasRetriedOnFatalError;
   BOOL _isRunning;
   // Will the session be running once all asynchronous operations have been completed?
   BOOL _willBeRunning;
+  BOOL _switchingCameras;
+#if TARGET_OS_IPHONE
+  UIDeviceOrientation _orientation;
+#endif
 }
 
 @synthesize frameQueue = _frameQueue;
 @synthesize captureSession = _captureSession;
+@synthesize switchingCameras = _switchingCameras;
 
 - (instancetype)initWithDelegate:(__weak id<RTCVideoCapturerDelegate>)delegate {
   if (self = [super initWithDelegate:delegate]) {
@@ -55,7 +61,9 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
       return nil;
     }
     NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
+    self.switchingCameras = NO;
 #if TARGET_OS_IPHONE
+    _orientation = UIDeviceOrientationPortrait;
     [center addObserver:self
                selector:@selector(deviceOrientationDidChange:)
                    name:UIDeviceOrientationDidChangeNotification
@@ -117,7 +125,7 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
 - (void)startCaptureWithDevice:(AVCaptureDevice *)device
                         format:(AVCaptureDeviceFormat *)format
                            fps:(NSInteger)fps {
-  _willBeRunning = true;
+  _willBeRunning = YES;
   [RTCDispatcher
       dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                     block:^{
@@ -136,17 +144,19 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
                         return;
                       }
 
+                      self.switchingCameras = YES;
                       [self reconfigureCaptureSessionInput];
                       [self updateOrientation];
                       [_captureSession startRunning];
                       [self updateDeviceCaptureFormat:format fps:fps];
                       [_currentDevice unlockForConfiguration];
-                      _isRunning = true;
+                      self.switchingCameras = NO;
+                      _isRunning = YES;
                     }];
 }
 
 - (void)stopCapture {
-  _willBeRunning = false;
+  _willBeRunning = NO;
   [RTCDispatcher
       dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
                     block:^{
@@ -160,7 +170,7 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
 #if TARGET_OS_IPHONE
                       [[UIDevice currentDevice] endGeneratingDeviceOrientationNotifications];
 #endif
-                      _isRunning = false;
+                      _isRunning = NO;
                     }];
 }
 
@@ -192,11 +202,49 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
     return;
   }
 
+#if TARGET_OS_IPHONE
+  // Default to portrait orientation on iPhone.
+  RTCVideoRotation rotation = RTCVideoRotation_90;
+  // Check here, which camera this frame is from, to avoid any race conditions.
+  AVCaptureDeviceInput *deviceInput =
+      (AVCaptureDeviceInput *)((AVCaptureInputPort *)connection.inputPorts.firstObject).input;
+  BOOL usingFrontCamera = deviceInput.device.position == AVCaptureDevicePositionFront;
+  // We gate checking the image's EXIF only if we're switching cameras as we don't need to parse
+  // the image's attachments and dictionaries for every video image.
+  if (self.switchingCameras) {
+    // Check the image's EXIF for the actual camera the image came as the image could have been
+    // delayed as we set alwaysDiscardsLateVideoFrames to NO.
+    usingFrontCamera = [RTCImageHelper isFrontCameraFromSampleBuffer:sampleBuffer];

[magjed_webrtc, 2017/07/24 10:12:50]

Is this method guaranteed to work on all devices? How expensive is it to call
it? If it's reliable and cheap to call it, can we always use it to simplify the
code?

[jtt_webrtc, 2017/07/24 16:40:36]

I've tested it with an iPhone SE and iPhone 7 and 7+. 

Elapsed time to call is ~0.004ms, so I'll remove the switchingCameras flag.

+  }
+  switch (_orientation) {
+    case UIDeviceOrientationPortrait:
+      rotation = RTCVideoRotation_90;
+      break;
+    case UIDeviceOrientationPortraitUpsideDown:
+      rotation = RTCVideoRotation_270;
+      break;
+    case UIDeviceOrientationLandscapeLeft:
+      rotation = usingFrontCamera ? RTCVideoRotation_180 : RTCVideoRotation_0;
+      break;
+    case UIDeviceOrientationLandscapeRight:
+      rotation = usingFrontCamera ? RTCVideoRotation_0 : RTCVideoRotation_180;
+      break;
+    case UIDeviceOrientationFaceUp:
+    case UIDeviceOrientationFaceDown:
+    case UIDeviceOrientationUnknown:
+      // Ignore.
+      break;
+  }
+#else
+  // No rotation on Mac.
+  RTCVideoRotation rotation = RTCVideoRotation_0;
+#endif
+
   RTCCVPixelBuffer *rtcPixelBuffer = [[RTCCVPixelBuffer alloc] initWithPixelBuffer:pixelBuffer];
   int64_t timeStampNs = CMTimeGetSeconds(CMSampleBufferGetPresentationTimeStamp(sampleBuffer)) *
-                        kNanosecondsPerSecond;
+      kNanosecondsPerSecond;
   RTCVideoFrame *videoFrame = [[RTCVideoFrame alloc] initWithBuffer:rtcPixelBuffer
-                                                           rotation:_rotation
+                                                           rotation:rotation
                                                         timeStampNs:timeStampNs];
   [self.delegate capturer:self didCaptureVideoFrame:videoFrame];
 }
@@ -399,26 +447,7 @@ static inline BOOL IsMediaSubTypeSupported(FourCharCode mediaSubType) {
   NSAssert([RTCDispatcher isOnQueueForType:RTCDispatcherTypeCaptureSession],
            @"updateOrientation must be called on the capture queue.");
 #if TARGET_OS_IPHONE
-  BOOL usingFrontCamera = _currentDevice.position == AVCaptureDevicePositionFront;
-  switch ([UIDevice currentDevice].orientation) {
-    case UIDeviceOrientationPortrait:
-      _rotation = RTCVideoRotation_90;
-      break;
-    case UIDeviceOrientationPortraitUpsideDown:
-      _rotation = RTCVideoRotation_270;
-      break;
-    case UIDeviceOrientationLandscapeLeft:
-      _rotation = usingFrontCamera ? RTCVideoRotation_180 : RTCVideoRotation_0;
-      break;
-    case UIDeviceOrientationLandscapeRight:
-      _rotation = usingFrontCamera ? RTCVideoRotation_0 : RTCVideoRotation_180;
-      break;
-    case UIDeviceOrientationFaceUp:
-    case UIDeviceOrientationFaceDown:
-    case UIDeviceOrientationUnknown:
-      // Ignore.
-      break;
-  }
+  _orientation = [UIDevice currentDevice].orientation;
 #endif
 }