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AVFMediaAssetWriter - fix race conditions

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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>
This commit is contained in:
Timur Pocheptsov
2016-07-22 14:29:45 +02:00
parent ab940d8fe9
commit 7788feea5d
4 changed files with 67 additions and 65 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
src/plugins/avfoundation/camera/avfmediaassetwriter.h
View File

@@ -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

91
src/plugins/avfoundation/camera/avfmediaassetwriter.mm
View File

@@ -85,17 +85,11 @@ enum WriterState
@implementation QT_MANGLE_NAMESPACE(AVFMediaAssetWriter)
- (id)initWithQueue:(dispatch_queue_t)writerQueue
delegate:(AVFMediaRecorderControlIOS *)delegate
- (id)initWithDelegate:(AVFMediaRecorderControlIOS *)delegate
{
Q_ASSERT(writerQueue);
Q_ASSERT(delegate);
if (self = [super init]) {
// "Shared" queue:
dispatch_retain(writerQueue);
m_writerQueue.reset(writerQueue);
m_delegate = delegate;
m_setStartTime = true;
m_state.store(WriterStateIdle);
@@ -126,6 +120,12 @@ enum WriterState
m_audioSettings = audioSettings;
m_videoSettings = videoSettings;
m_writerQueue.reset(dispatch_queue_create("asset-writer-queue", DISPATCH_QUEUE_SERIAL));
if (!m_writerQueue) {
qDebugCamera() << Q_FUNC_INFO << "failed to create an asset writer's queue";
return false;
}
m_videoQueue.reset(dispatch_queue_create("video-output-queue", DISPATCH_QUEUE_SERIAL));
if (!m_videoQueue) {
qDebugCamera() << Q_FUNC_INFO << "failed to create video queue";
@@ -172,20 +172,10 @@ enum WriterState
- (void)start
{
// 'start' is executed on a special writer's queue.
// We lock a mutex for the whole function body
// - render control probably was deleted
// 'immediately' after submitting 'start'
// and before 'start' is actually executed ...
// So if we're starting, render control can not
// abort us 'in the middle'.
const QMutexLocker lock(&m_writerMutex);
if (m_state.load() == WriterStateAborted)
return;
[self setQueues];
m_setStartTime = true;
m_state.storeRelease(WriterStateActive);
[m_assetWriter startWriting];
@@ -196,40 +186,65 @@ enum WriterState
- (void)stop
{
// To be executed on a writer's queue.
{
const QMutexLocker lock(&m_writerMutex);
if (m_state.load() != WriterStateActive)
if (m_state.loadAcquire() != WriterStateActive)
return;
}
if ([m_assetWriter status] != AVAssetWriterStatusWriting)
return;
// Do this here so that -
// 1. '-abort' should not try calling finishWriting again and
// 2. async block (see below) will know if recorder control was deleted
// before the block's execution:
m_state.storeRelease(WriterStateIdle);
// Now, since we have to ensure no sample buffers are
// appended after a call to finishWriting, we must
// ensure writer's queue sees this change in m_state
// _before_ we call finishWriting:
dispatch_sync(m_writerQueue, ^{});
// Done, but now we also want to prevent video queue
// from updating our viewfinder:
dispatch_sync(m_videoQueue, ^{});
// Now we're safe to stop:
[m_assetWriter finishWritingWithCompletionHandler:^{
// This block is async, so by the time it's executed,
// it's possible that render control was deleted already ...
// We lock so that nobody can call 'abort' in the middle ...
const QMutexLocker lock(&m_writerMutex);
if (m_state.load() != WriterStateActive)
if (m_state.loadAcquire() == WriterStateAborted)
return;
AVCaptureSession *session = m_service->session()->captureSession();
[session stopRunning];
if (session.running)
[session stopRunning];
[session removeOutput:m_audioOutput];
[session removeInput:m_audioInput];
m_state.storeRelease(WriterStateIdle);
QMetaObject::invokeMethod(m_delegate, "assetWriterFinished", Qt::QueuedConnection);
}];
}
- (void)abort
{
// To be executed on any thread (presumably, it's the main thread),
// prevents writer from accessing any shared object.
const QMutexLocker lock(&m_writerMutex);
// -abort is to be called from recorder control's dtor.
if (m_state.fetchAndStoreRelease(WriterStateAborted) != WriterStateActive) {
// Not recording, nothing to stop.
return;
}
// From Apple's docs:
// "To guarantee that all sample buffers are successfully written,
// you must ensure that all calls to appendSampleBuffer: and
// appendPixelBuffer:withPresentationTime: have returned before
// invoking this method."
//
// The only way we can ensure this is:
dispatch_sync(m_writerQueue, ^{});
// At this point next block (if any) on the writer's queue
// will see m_state preventing it from any further processing.
dispatch_sync(m_videoQueue, ^{});
// After this point video queue will not try to modify our
// viewfider, so we're safe to delete now.
[m_assetWriter finishWritingWithCompletionHandler:^{
}];
}
@@ -240,13 +255,12 @@ enum WriterState
Q_ASSERT(m_setStartTime);
Q_ASSERT(sampleBuffer);
const QMutexLocker lock(&m_writerMutex);
if (m_state.load() != WriterStateActive)
if (m_state.loadAcquire() != WriterStateActive)
return;
QMetaObject::invokeMethod(m_delegate, "assetWriterStarted", Qt::QueuedConnection);
m_durationInMs.store(0);
m_durationInMs.storeRelease(0);
m_startTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
m_lastTimeStamp = m_startTime;
[m_assetWriter startSessionAtSourceTime:m_startTime];
@@ -255,9 +269,9 @@ enum WriterState
- (void)writeVideoSampleBuffer:(CMSampleBufferRef)sampleBuffer
{
// This code is executed only on a writer's queue.
Q_ASSERT(sampleBuffer);
// This code is executed only on a writer's queue.
if (m_state.loadAcquire() == WriterStateActive) {
if (m_setStartTime)
[self setStartTimeFrom:sampleBuffer];
@@ -268,7 +282,6 @@ enum WriterState
}
}
CFRelease(sampleBuffer);
}
@@ -296,9 +309,6 @@ enum WriterState
{
Q_UNUSED(connection)
// This method can be called on either video or audio queue,
// never on a writer's queue, it needs access to a shared data, so
// lock is required.
if (m_state.loadAcquire() != WriterStateActive)
return;
@@ -310,7 +320,6 @@ enum WriterState
CFRetain(sampleBuffer);
if (captureOutput != m_audioOutput.data()) {
const QMutexLocker lock(&m_writerMutex);
if (m_state.load() != WriterStateActive) {
CFRelease(sampleBuffer);
return;
@@ -450,7 +459,7 @@ enum WriterState
if (!CMTimeCompare(duration, kCMTimeInvalid))
return;
m_durationInMs.store(CMTimeGetSeconds(duration) * 1000);
m_durationInMs.storeRelease(CMTimeGetSeconds(duration) * 1000);
m_lastTimeStamp = newTimeStamp;
}
}

2
src/plugins/avfoundation/camera/avfmediarecordercontrol_ios.h
View File

@@ -91,8 +91,6 @@ private:
void stopWriter();
AVFCameraService *m_service;
AVFScopedPointer<dispatch_queue_t> m_writerQueue;
AVFScopedPointer<QT_MANGLE_NAMESPACE(AVFMediaAssetWriter)> m_writer;
QUrl m_outputLocation;

26
src/plugins/avfoundation/camera/avfmediarecordercontrol_ios.mm
View File

@@ -86,13 +86,7 @@ AVFMediaRecorderControlIOS::AVFMediaRecorderControlIOS(AVFCameraService *service
{
Q_ASSERT(service);
m_writerQueue.reset(dispatch_queue_create("asset-writer-queue", DISPATCH_QUEUE_SERIAL));
if (!m_writerQueue) {
qDebugCamera() << Q_FUNC_INFO << "failed to create an asset writer's queue";
return;
}
m_writer.reset([[QT_MANGLE_NAMESPACE(AVFMediaAssetWriter) alloc] initWithQueue:m_writerQueue delegate:this]);
m_writer.reset([[QT_MANGLE_NAMESPACE(AVFMediaAssetWriter) alloc] initWithDelegate:this]);
if (!m_writer) {
qDebugCamera() << Q_FUNC_INFO << "failed to create an asset writer";
return;
@@ -289,9 +283,15 @@ void AVFMediaRecorderControlIOS::setState(QMediaRecorder::State state)
Q_EMIT stateChanged(m_state);
Q_EMIT statusChanged(m_lastStatus);
dispatch_async(m_writerQueue, ^{
[m_writer start];
});
// Apple recommends to call startRunning and do all
// setup on a special queue, and that's what we had
// initially (dispatch_async to writerQueue). Unfortunately,
// writer's queue is not the only queue/thread that can
// access/modify the session, and as a result we have
// all possible data/race-conditions with Obj-C exceptions
// at best and something worse in general.
// Now we try to only modify session on the same thread.
[m_writer start];
} else {
[session startRunning];
Q_EMIT error(QMediaRecorder::FormatError, tr("Failed to start recording"));
@@ -318,7 +318,7 @@ void AVFMediaRecorderControlIOS::setMuted(bool muted)
void AVFMediaRecorderControlIOS::setVolume(qreal volume)
{
Q_UNUSED(volume);
Q_UNUSED(volume)
qDebugCamera() << Q_FUNC_INFO << "not implemented";
}
@@ -403,9 +403,7 @@ void AVFMediaRecorderControlIOS::stopWriter()
Q_EMIT stateChanged(m_state);
Q_EMIT statusChanged(m_lastStatus);
dispatch_async(m_writerQueue, ^{
[m_writer stop];
});
[m_writer stop];
}
}