Re: [FFmpeg-devel] [PATCH v2 02/03] libavdevice/avfoundation.m: Replace mutex-based concurrency handling in avfoundation.m by a thread-safe fifo queue with maximum length.

Mon, 13 Dec 2021 10:57:03 -0800 



On 13 Dec 2021, at 17:39, Romain Beauxis wrote:


This is the second patch of a series of 3 that cleanup and enhance the
avfoundation implementation for libavdevice.


This patch fixes the concurrency model. Avfoundation runs its own 
producing thread
to send produced frames and ffmpeg runs its own thread to consume 
them.



The existing implementation stores the last transmitted frame and uses 
a mutex
to avoid concurrent access. However, this leads to situations where 
upcoming frames
can be dropped if the ffmpeg thread is acessing the latest frame. This 
happens
even when the thread would otherwise catch up and process frames fast 
enought.



This patches changes this implementation to use a buffer queue with a 
max queue length
and encapsulated thread-safety. This greatly simplifies the logic of 
the calling code
and gives the consuming thread a chance to process all frames 
concurrently to the producing

thread while avoiding memory leaks.



Couldn't this just use CMSimpleQueue 
https://developer.apple.com/documentation/coremedia/cmsimplequeue?language=objc

or CMBufferQueue?


The implementation of the queue in this patch does not seem right, see 
review below.




Signed-off-by: Romain Beauxis <to...@rastageeks.org>
---
libavdevice/avfoundation.m | 220 +++++++++++++++++++++----------------
1 file changed, 127 insertions(+), 93 deletions(-)

diff --git a/libavdevice/avfoundation.m b/libavdevice/avfoundation.m
index 79c9207cfa..95414fd16a 100644
--- a/libavdevice/avfoundation.m
+++ b/libavdevice/avfoundation.m
@@ -26,7 +26,6 @@
 */

#import <AVFoundation/AVFoundation.h>
-#include <pthread.h>

#include "libavutil/channel_layout.h"
#include "libavutil/pixdesc.h"
@@ -80,13 +79,97 @@
    { AV_PIX_FMT_NONE, 0 }
};

+#define MAX_QUEUED_OBJECTS 10
+
+@interface AvdeviceAvfoundationBuffer : NSObject

++ (AvdeviceAvfoundationBuffer *) 
fromCMSampleBufferRef:(CMSampleBufferRef)sampleBuffer;

+- (CMSampleBufferRef) getCMSampleBuffer;
+@end
+
+@implementation AvdeviceAvfoundationBuffer {
+    CMSampleBufferRef sampleBuffer;
+}
+

++ (AvdeviceAvfoundationBuffer *) 
fromCMSampleBufferRef:(CMSampleBufferRef)sampleBuffer {

+    return [[AvdeviceAvfoundationBuffer alloc] init:sampleBuffer];
+}
+
+- (id) init:(CMSampleBufferRef)buffer {
+    sampleBuffer = buffer;
+    return self;
+}
+
+- (CMSampleBufferRef) getCMSampleBuffer {
+    return sampleBuffer;
+}
+@end
+
+@interface AvdeviceAvfoundationBufferQueue : NSObject
+- (CMSampleBufferRef) dequeue;
+- (NSUInteger) count;
+- (void) enqueue:(CMSampleBufferRef)obj;
+@end
+
+@implementation AvdeviceAvfoundationBufferQueue {
+    NSLock *mutex;
+    NSMutableArray *queue;
+}
+
+- (id) init {
+    mutex = [[[NSLock alloc] init] retain];
+    queue = [[[NSMutableArray alloc] init] retain];
+    return self;
+}
+
+- (oneway void) release {
+    NSEnumerator *enumerator = [queue objectEnumerator];
+    AvdeviceAvfoundationBuffer *buffer;
+
+    while (buffer = [enumerator nextObject]) {
+        CFRelease([buffer getCMSampleBuffer]);
+    }
+
+    [mutex release];
+    [queue release];
+}


Shouldn't this be done in dealloc instead of release?
Especially as retain is not subclassed, so this seems
like it could lead to over-releasing resources.


+
+- (NSUInteger) count {
+    [mutex lock];
+    NSUInteger c = [queue count];
+    [mutex unlock];
+    return c;
+}


This does not look right, the count can change after it is returned
and the caller does not hold a lock to prevent this.


+
+- (CMSampleBufferRef) dequeue {
+    [mutex lock];
+
+    if ([queue count] < 1) {
+      [mutex unlock];
+      return nil;
+    }
+
+    AvdeviceAvfoundationBuffer *buffer = [queue objectAtIndex:0];
+    CMSampleBufferRef sampleBuffer = [buffer getCMSampleBuffer];
+    [queue removeObjectAtIndex:0];
+    [mutex unlock];
+
+    return sampleBuffer;
+}
+
+- (void) enqueue:(CMSampleBufferRef)buffer {
+    [mutex lock];
+    while (MAX_QUEUED_OBJECTS < [queue count]) {
+      [queue removeObjectAtIndex:0];
+    }

+    [queue addObject:[AvdeviceAvfoundationBuffer 
fromCMSampleBufferRef:(CMSampleBufferRef)CFRetain(buffer)]];

+    [mutex unlock];
+}
+@end
+
typedef struct
{
    AVClass*        class;

-    int             frames_captured;
-    int             audio_frames_captured;
-    pthread_mutex_t frame_lock;
    id              avf_delegate;
    id              avf_audio_delegate;

@@ -121,8 +204,8 @@
    AVCaptureSession         *capture_session;
    AVCaptureVideoDataOutput *video_output;
    AVCaptureAudioDataOutput *audio_output;
-    CMSampleBufferRef         current_frame;
-    CMSampleBufferRef         current_audio_frame;
+    AvdeviceAvfoundationBufferQueue *audio_frames;
+    AvdeviceAvfoundationBufferQueue *video_frames;

    AVCaptureDevice          *observed_device;
#if !TARGET_OS_IPHONE && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1070
@@ -131,16 +214,6 @@
    int                      observed_quit;
} AVFContext;

-static void lock_frames(AVFContext* ctx)
-{
-    pthread_mutex_lock(&ctx->frame_lock);
-}
-
-static void unlock_frames(AVFContext* ctx)
-{
-    pthread_mutex_unlock(&ctx->frame_lock);
-}
-
/** FrameReciever class - delegate for AVCaptureSession
 */
@interface AVFFrameReceiver : NSObject

@@ -218,17 +291,7 @@ - (void)  captureOutput:(AVCaptureOutput 
*)captureOutput

  didOutputSampleBuffer:(CMSampleBufferRef)videoFrame
         fromConnection:(AVCaptureConnection *)connection
{
-    lock_frames(_context);
-
-    if (_context->current_frame != nil) {
-        CFRelease(_context->current_frame);
-    }
-

-    _context->current_frame = 
(CMSampleBufferRef)CFRetain(videoFrame);

-
-    unlock_frames(_context);
-
-    ++_context->frames_captured;
+    [_context->video_frames enqueue:videoFrame];
}

@end

@@ -262,17 +325,7 @@ - (void)  captureOutput:(AVCaptureOutput 
*)captureOutput

  didOutputSampleBuffer:(CMSampleBufferRef)audioFrame
         fromConnection:(AVCaptureConnection *)connection
{
-    lock_frames(_context);
-
-    if (_context->current_audio_frame != nil) {
-        CFRelease(_context->current_audio_frame);
-    }
-

-    _context->current_audio_frame = 
(CMSampleBufferRef)CFRetain(audioFrame);

-
-    unlock_frames(_context);
-
-    ++_context->audio_frames_captured;
+    [_context->audio_frames enqueue:audioFrame];
}

@end
@@ -284,12 +337,16 @@ static void destroy_context(AVFContext* ctx)
    [ctx->capture_session release];
    [ctx->video_output    release];
    [ctx->audio_output    release];
+    [ctx->video_frames    release];
+    [ctx->audio_frames    release];
    [ctx->avf_delegate    release];
    [ctx->avf_audio_delegate release];

    ctx->capture_session = NULL;
    ctx->video_output    = NULL;
    ctx->audio_output    = NULL;
+    ctx->video_frames    = NULL;
+    ctx->audio_frames    = NULL;
    ctx->avf_delegate    = NULL;
    ctx->avf_audio_delegate = NULL;

@@ -297,12 +354,6 @@ static void destroy_context(AVFContext* ctx)
      AudioConverterDispose(ctx->audio_converter);
      ctx->audio_converter = NULL;
    }
-
-    pthread_mutex_destroy(&ctx->frame_lock);
-
-    if (ctx->current_frame) {
-        CFRelease(ctx->current_frame);
-    }
}

static void parse_device_name(AVFormatContext *s)
@@ -630,18 +681,18 @@ static int get_video_config(AVFormatContext *s)
    }

    // Take stream info from the first frame.
-    while (ctx->frames_captured < 1) {
+    while ([ctx->video_frames count] < 1) {
        CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.1, YES);
    }

-    lock_frames(ctx);
+    CMSampleBufferRef frame = [ctx->video_frames dequeue];

    ctx->video_stream_index = stream->index;

    avpriv_set_pts_info(stream, 64, 1, avf_time_base);

-    image_buffer = CMSampleBufferGetImageBuffer(ctx->current_frame);
-    block_buffer = CMSampleBufferGetDataBuffer(ctx->current_frame);
+    image_buffer = CMSampleBufferGetImageBuffer(frame);
+    block_buffer = CMSampleBufferGetDataBuffer(frame);

    if (image_buffer) {
        image_buffer_size = CVImageBufferGetEncodedSize(image_buffer);
@@ -657,10 +708,7 @@ static int get_video_config(AVFormatContext *s)
        stream->codecpar->format     = ctx->pixel_format;
    }

-    CFRelease(ctx->current_frame);
-    ctx->current_frame = nil;
-
-    unlock_frames(ctx);
+    CFRelease(frame);

    return 0;
}
@@ -680,27 +728,27 @@ static int get_audio_config(AVFormatContext *s)
    }

    // Take stream info from the first frame.
-    while (ctx->audio_frames_captured < 1) {
+    while ([ctx->audio_frames count] < 1) {
        CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0.1, YES);
    }

-    lock_frames(ctx);
+    CMSampleBufferRef frame = [ctx->audio_frames dequeue];

    ctx->audio_stream_index = stream->index;

    avpriv_set_pts_info(stream, 64, 1, avf_time_base);


-    format_desc = 
CMSampleBufferGetFormatDescription(ctx->current_audio_frame);

+    format_desc = CMSampleBufferGetFormatDescription(frame);

    const AudioStreamBasicDescription *input_format = 
CMAudioFormatDescriptionGetStreamBasicDescription(format_desc);


    if (!input_format) {
-        unlock_frames(ctx);
+        CFRelease(frame);
        av_log(s, AV_LOG_ERROR, "audio format not available\n");
        return 1;
    }

    if (input_format->mFormatID != kAudioFormatLinearPCM) {
-        unlock_frames(ctx);
+        CFRelease(frame);

        av_log(s, AV_LOG_ERROR, "only PCM audio format are supported 
at the moment\n");

        return 1;
    }
@@ -778,16 +826,13 @@ static int get_audio_config(AVFormatContext *s)
    if (must_convert) {

        OSStatus ret = AudioConverterNew(input_format, &output_format, 
&ctx->audio_converter);

        if (ret != noErr) {
-            unlock_frames(ctx);
+            CFRelease(frame);

            av_log(s, AV_LOG_ERROR, "Error while allocating audio 
converter\n");

            return 1;
        }
    }

-    CFRelease(ctx->current_audio_frame);
-    ctx->current_audio_frame = nil;
-
-    unlock_frames(ctx);
+    CFRelease(frame);

    return 0;
}
@@ -805,8 +850,6 @@ static int avf_read_header(AVFormatContext *s)

    ctx->num_video_devices = [devices count] + [devices_muxed count];

-    pthread_mutex_init(&ctx->frame_lock, NULL);
-
#if !TARGET_OS_IPHONE && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1070
    CGGetActiveDisplayList(0, NULL, &num_screens);
#endif
@@ -1006,6 +1049,8 @@ static int avf_read_header(AVFormatContext *s)

    // Initialize capture session
    ctx->capture_session = [[AVCaptureSession alloc] init];

+    ctx->video_frames    = [[AvdeviceAvfoundationBufferQueue alloc] 
init];
+    ctx->audio_frames    = [[AvdeviceAvfoundationBufferQueue alloc] 
init];


    if (video_device && add_video_device(s, video_device)) {
        goto fail;

@@ -1088,35 +1133,31 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

    AVFContext* ctx = (AVFContext*)s->priv_data;

    do {
-        CVImageBufferRef image_buffer;
-        CMBlockBufferRef block_buffer;
-        lock_frames(ctx);
-
-        if (ctx->current_frame != nil) {
+        if (1 <= [ctx->video_frames count]) {
            int status;
            int length = 0;
-

-            image_buffer = 
CMSampleBufferGetImageBuffer(ctx->current_frame);
-            block_buffer = 
CMSampleBufferGetDataBuffer(ctx->current_frame);
+            CMSampleBufferRef video_frame = [ctx->video_frames 
dequeue];
+            CVImageBufferRef image_buffer = 
CMSampleBufferGetImageBuffer(video_frame);;
+            CMBlockBufferRef block_buffer = 
CMSampleBufferGetDataBuffer(video_frame);


            if (image_buffer != nil) {
                length = (int)CVPixelBufferGetDataSize(image_buffer);
            } else if (block_buffer != nil) {

                length = 
(int)CMBlockBufferGetDataLength(block_buffer);

            } else  {
-                unlock_frames(ctx);
+                CFRelease(video_frame);
                return AVERROR(EINVAL);
            }

            if (av_new_packet(pkt, length) < 0) {
-                unlock_frames(ctx);
+                CFRelease(video_frame);
                return AVERROR(EIO);
            }

            CMItemCount count;
            CMSampleTimingInfo timing_info;


-            if 
(CMSampleBufferGetOutputSampleTimingInfoArray(ctx->current_frame, 1, 
&timing_info, &count) == noErr) {
+            if 
(CMSampleBufferGetOutputSampleTimingInfoArray(video_frame, 1, 
&timing_info, &count) == noErr) {
                AVRational timebase_q = av_make_q(1, 
timing_info.presentationTimeStamp.timescale);
                pkt->pts = pkt->dts = 
av_rescale_q(timing_info.presentationTimeStamp.value, timebase_q, 
avf_time_base_q);

            }

@@ -1133,15 +1174,14 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

                    status = AVERROR(EIO);
                }
             }
-            CFRelease(ctx->current_frame);
-            ctx->current_frame = nil;
+            CFRelease(video_frame);

            if (status < 0) {
-                unlock_frames(ctx);
                return status;
            }
-        } else if (ctx->current_audio_frame != nil) {

-            CMBlockBufferRef block_buffer = 
CMSampleBufferGetDataBuffer(ctx->current_audio_frame);

+        } else if (1 <= [ctx->audio_frames count]) {

+            CMSampleBufferRef audio_frame = [ctx->audio_frames 
dequeue];
+            CMBlockBufferRef block_buffer = 
CMSampleBufferGetDataBuffer(audio_frame);



            size_t input_size = 
CMBlockBufferGetDataLength(block_buffer);

            int buffer_size = input_size / ctx->audio_buffers;

@@ -1151,12 +1191,12 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

            UInt32 size = sizeof(output_size);

            ret = AudioConverterGetProperty(ctx->audio_converter, 
kAudioConverterPropertyCalculateOutputBufferSize, &size, 
&output_size);

            if (ret != noErr) {
-                unlock_frames(ctx);
+                CFRelease(audio_frame);
                return AVERROR(EIO);
            }

            if (av_new_packet(pkt, output_size) < 0) {
-                unlock_frames(ctx);
+                CFRelease(audio_frame);
                return AVERROR(EIO);
            }


@@ -1173,7 +1213,7 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)


                    if (ret != kCMBlockBufferNoErr) {
                        av_free(input_buffer);
-                        unlock_frames(ctx);
+                        CFRelease(audio_frame);
                        return AVERROR(EIO);
                    }
                }

@@ -1191,7 +1231,7 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

                av_free(input_buffer);

                if (ret != noErr) {
-                    unlock_frames(ctx);
+                    CFRelease(audio_frame);
                    return AVERROR(EIO);
                }


@@ -1199,7 +1239,7 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

            } else {

                 ret = CMBlockBufferCopyDataBytes(block_buffer, 0, 
pkt->size, pkt->data);

                 if (ret != kCMBlockBufferNoErr) {
-                     unlock_frames(ctx);
+                     CFRelease(audio_frame);
                     return AVERROR(EIO);
                 }
            }

@@ -1207,7 +1247,7 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

            CMItemCount count;
            CMSampleTimingInfo timing_info;


-            if 
(CMSampleBufferGetOutputSampleTimingInfoArray(ctx->current_audio_frame, 
1, &timing_info, &count) == noErr) {
+            if 
(CMSampleBufferGetOutputSampleTimingInfoArray(audio_frame, 1, 
&timing_info, &count) == noErr) {
                AVRational timebase_q = av_make_q(1, 
timing_info.presentationTimeStamp.timescale);
                pkt->pts = pkt->dts = 
av_rescale_q(timing_info.presentationTimeStamp.value, timebase_q, 
avf_time_base_q);

            }

@@ -1215,21 +1255,15 @@ static int avf_read_packet(AVFormatContext *s, 
AVPacket *pkt)

            pkt->stream_index  = ctx->audio_stream_index;
            pkt->flags        |= AV_PKT_FLAG_KEY;

-            CFRelease(ctx->current_audio_frame);
-            ctx->current_audio_frame = nil;
-
-            unlock_frames(ctx);
+            CFRelease(audio_frame);
        } else {
            pkt->data = NULL;
-            unlock_frames(ctx);
            if (ctx->observed_quit) {
                return AVERROR_EOF;
            } else {
                return AVERROR(EAGAIN);
            }
        }
-
-        unlock_frames(ctx);
    } while (!pkt->data);

    return 0;
--
2.30.1 (Apple Git-130)

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