AVFMediaAssetWriter - fix race conditions

Apple recommends starting/setting up a session (AVCaptureSession) on a
special queue and in the past we did this on the writer's queue.
Unfortunately, we also can access the same session from a different thread
and this results in race conditions and different weird crashes, for example,
we're calling start/stopRunning from the writer's queue, while the session
is being configured (in between beginConfiguration/commitConfiguration
on the recorder control's thread). So we have to limit access to the session
by the control's thread.

Apple docs say we have to ensure all appendSampleBuffer calls done
_before_ finishWriting. We ensure this by dispatching_sync an
empty block on the writer's queue after store-release of m_state.

We also do the same with video queue to ensure it does not try to
access viewfinder's data after stop/abort executed.

All these changes also make lock/mutex unneeded.

Task-number: QTBUG-54890
Change-Id: I38e86c879b6b62306bdfbeade65405d6ac3be9f3
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>

1 files changed, 5 insertions, 8 deletions

diff --git a/src/plugins/avfoundation/camera/avfmediaassetwriter.h b/src/plugins/avfoundation/camera/avfmediaassetwriter.h
index 77e2a2f32..c36caee8b 100644
--- a/src/plugins/avfoundation/camera/avfmediaassetwriter.h
+++ b/src/plugins/avfoundation/camera/avfmediaassetwriter.h
@@ -38,7 +38,6 @@
 
 #include <QtCore/qglobal.h>
 #include <QtCore/qatomic.h>
-#include <QtCore/qmutex.h>
 
 #include <AVFoundation/AVFoundation.h>
 
@@ -63,12 +62,12 @@ QT_END_NAMESPACE
     QT_PREPEND_NAMESPACE(AVFScopedPointer)<AVAssetWriterInput> m_audioWriterInput;
     AVCaptureDevice *m_audioCaptureDevice;
 
+    // Queue to write sample buffers:
+    QT_PREPEND_NAMESPACE(AVFScopedPointer)<dispatch_queue_t> m_writerQueue;
     // High priority serial queue for video output:
     QT_PREPEND_NAMESPACE(AVFScopedPointer)<dispatch_queue_t> m_videoQueue;
     // Serial queue for audio output:
     QT_PREPEND_NAMESPACE(AVFScopedPointer)<dispatch_queue_t> m_audioQueue;
-    // Queue to write sample buffers:
-    QT_PREPEND_NAMESPACE(AVFScopedPointer)<dispatch_queue_t> m_writerQueue;
 
     QT_PREPEND_NAMESPACE(AVFScopedPointer)<AVAssetWriter> m_assetWriter;
 
@@ -77,7 +76,6 @@ QT_END_NAMESPACE
     bool m_setStartTime;
 
     QT_PREPEND_NAMESPACE(QAtomicInt) m_state;
-    QT_PREPEND_NAMESPACE(QMutex) m_writerMutex;
 @public
     QT_PREPEND_NAMESPACE(AVFAtomicInt64) m_durationInMs;
 @private
@@ -88,8 +86,7 @@ QT_END_NAMESPACE
     NSDictionary *m_videoSettings;
 }
 
-- (id)initWithQueue:(dispatch_queue_t)writerQueue
-      delegate:(QT_PREPEND_NAMESPACE(AVFMediaRecorderControlIOS) *)delegate;
+- (id)initWithDelegate:(QT_PREPEND_NAMESPACE(AVFMediaRecorderControlIOS) *)delegate;
 
 - (bool)setupWithFileURL:(NSURL *)fileURL
         cameraService:(QT_PREPEND_NAMESPACE(AVFCameraService) *)service
@@ -97,10 +94,10 @@ QT_END_NAMESPACE
         videoSettings:(NSDictionary *)videoSettings
         transform:(CGAffineTransform)transform;
 
+// This to be called from the recorder control's thread:
 - (void)start;
 - (void)stop;
-// This to be called if control's dtor gets called,
-// on the control's thread.
+// This to be called from the recorder control's dtor:
 - (void)abort;
 
 @end