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avdevice/avfoundation: wait for frame consumption to avoid dropping A/V frames

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The capture callback dropped the previous frame instead of waiting for it
to be consumed, so frames were lost when avf_read_packet() fell behind;
39fbd06314 (return EAGAIN instead of waiting) made it easy to hit.

Add back a condition variable: the capture callback blocks until
avf_read_packet() takes the current frame, and the reader waits on it when
no frame is ready. An is_stopping flag set in destroy_context() wakes both
sides so teardown cannot deadlock. observed_quit is now set under the lock
and broadcast as well, so a blocked reader wakes and returns EOF when a
transport-control device stops delivering frames.

Based on a patch by Zhongxin Zhuang <zx.zhuang@hotmail.com>; here
unlock_frames() only broadcasts and unlocks, so a read no longer releases
the other stream's still-unconsumed frame.

Verified by capturing camera+mic for 8s and comparing delivered packet
counts before/after this change:

  ffmpeg -f avfoundation -pixel_format nv12 -framerate 30 -i "0:0" -t 8 \
         -vf "scale=2560:1440,hqdn3d" -c:v libx264 -preset medium \
         -c:a aac out.mp4
  ffprobe -count_packets -show_entries stream=nb_read_packets out.mp4

  slow consumer (ideal: video ~240, audio ~375)
                     video   audio
    before (EAGAIN)   174     163   (audio ~57% dropped)
    after  (condvar)  234     376   (no drops)

  fast consumer (-preset ultrafast, no filter)
    before            240     328
    after             238     376

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>
This commit is contained in:
Jun Zhao
2026-06-18 23:14:21 +08:00
committed by Jun Zhao
parent 2f779272e0
commit ddf8f40301
1 changed files with 47 additions and 9 deletions
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56
libavdevice/avfoundation.m
View File

@@ -89,6 +89,8 @@ typedef struct
int frames_captured;
int audio_frames_captured;
pthread_mutex_t frame_lock;
pthread_cond_t frame_wait_cond;
int is_stopping;
id avf_delegate;
id avf_audio_delegate;
@@ -147,6 +149,7 @@ static void lock_frames(AVFContext* ctx)
static void unlock_frames(AVFContext* ctx)
{
pthread_cond_broadcast(&ctx->frame_wait_cond);
pthread_mutex_unlock(&ctx->frame_lock);
}
@@ -210,7 +213,12 @@ static void unlock_frames(AVFContext* ctx)
if (mode != _context->observed_mode) {
if (mode == AVCaptureDeviceTransportControlsNotPlayingMode) {
// Set under the lock and broadcast so a reader blocked in
// avf_read_packet() wakes up and returns EOF instead of
// hanging once the device stops delivering frames.
lock_frames(_context);
_context->observed_quit = 1;
unlock_frames(_context);
}
_context->observed_mode = mode;
}
@@ -229,8 +237,13 @@ static void unlock_frames(AVFContext* ctx)
{
lock_frames(_context);
if (_context->current_frame != nil) {
CFRelease(_context->current_frame);
while ((_context->current_frame != nil) && !_context->is_stopping) {
pthread_cond_wait(&_context->frame_wait_cond, &_context->frame_lock);
}
if (_context->is_stopping) {
unlock_frames(_context);
return;
}
_context->current_frame = (CMSampleBufferRef)CFRetain(videoFrame);
@@ -273,8 +286,13 @@ static void unlock_frames(AVFContext* ctx)
{
lock_frames(_context);
if (_context->current_audio_frame != nil) {
CFRelease(_context->current_audio_frame);
while ((_context->current_audio_frame != nil) && !_context->is_stopping) {
pthread_cond_wait(&_context->frame_wait_cond, &_context->frame_lock);
}
if (_context->is_stopping) {
unlock_frames(_context);
return;
}
_context->current_audio_frame = (CMSampleBufferRef)CFRetain(audioFrame);
@@ -288,6 +306,12 @@ static void unlock_frames(AVFContext* ctx)
static void destroy_context(AVFContext* ctx)
{
// Wake any capture callback blocked waiting for the consumer and make it
// bail out, so stopRunning() can drain the session without a deadlock.
lock_frames(ctx);
ctx->is_stopping = 1;
unlock_frames(ctx);
[ctx->capture_session stopRunning];
[ctx->capture_session release];
@@ -305,10 +329,17 @@ static void destroy_context(AVFContext* ctx)
av_freep(&ctx->url);
av_freep(&ctx->audio_buffer);
pthread_cond_destroy(&ctx->frame_wait_cond);
pthread_mutex_destroy(&ctx->frame_lock);
if (ctx->current_frame) {
CFRelease(ctx->current_frame);
ctx->current_frame = nil;
}
if (ctx->current_audio_frame) {
CFRelease(ctx->current_audio_frame);
ctx->current_audio_frame = nil;
}
}
@@ -836,6 +867,8 @@ static int avf_read_header(AVFormatContext *s)
ctx->num_video_devices = [devices count] + [devices_muxed count];
pthread_mutex_init(&ctx->frame_lock, NULL);
pthread_cond_init(&ctx->frame_wait_cond, NULL);
ctx->is_stopping = 0;
#if !TARGET_OS_IPHONE && __MAC_OS_X_VERSION_MIN_REQUIRED >= 1070
CGGetActiveDisplayList(0, NULL, &num_screens);
@@ -1120,10 +1153,10 @@ static int avf_read_packet(AVFormatContext *s, AVPacket *pkt)
{
AVFContext* ctx = (AVFContext*)s->priv_data;
lock_frames(ctx);
do {
CVImageBufferRef image_buffer;
CMBlockBufferRef block_buffer;
lock_frames(ctx);
if (ctx->current_frame != nil) {
int status;
@@ -1253,16 +1286,21 @@ static int avf_read_packet(AVFormatContext *s, AVPacket *pkt)
ctx->current_audio_frame = nil;
} else {
pkt->data = NULL;
unlock_frames(ctx);
if (ctx->observed_quit) {
unlock_frames(ctx);
return AVERROR_EOF;
} else {
return AVERROR(EAGAIN);
}
// No frame available yet: wait until a capture callback delivers
// one (or until the device is being torn down).
pthread_cond_wait(&ctx->frame_wait_cond, &ctx->frame_lock);
}
} while (!pkt->data && !ctx->is_stopping);
if (ctx->is_stopping) {
unlock_frames(ctx);
} while (!pkt->data);
return AVERROR_EOF;
}
unlock_frames(ctx);
return 0;
}