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The following commit(s) were added to refs/heads/master by this push:
new ddf8f40301 avdevice/avfoundation: wait for frame consumption to avoid
dropping A/V frames
ddf8f40301 is described below
commit ddf8f40301af20ad985cf369d1eb6d114be0c8f0
Author: Jun Zhao <[email protected]>
AuthorDate: Thu Jun 18 23:14:21 2026 +0800
Commit: Jun Zhao <[email protected]>
CommitDate: Fri Jul 3 23:53:25 2026 +0000
avdevice/avfoundation: wait for frame consumption to avoid dropping A/V
frames
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 <[email protected]>; 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 <[email protected]>
---
libavdevice/avfoundation.m | 56 ++++++++++++++++++++++++++++++++++++++--------
1 file changed, 47 insertions(+), 9 deletions(-)
diff --git a/libavdevice/avfoundation.m b/libavdevice/avfoundation.m
index ebec1ac4f2..6f52989fc9 100644
--- a/libavdevice/avfoundation.m
+++ b/libavdevice/avfoundation.m
@@ -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;
}
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