tor-browser

commit e4d5b2294c324f34022e2727b932e551997c3fd0
parent b7d5495e2cfe76ed6307941419eee56ac3cf5b47
Author: Alastor Wu <alwu@mozilla.com>
Date:   Fri, 17 Oct 2025 18:28:12 +0000

Bug 1967475 - part2 : check the first sync sample in MP4 if a key frame can't be found from the closest sync sample. r=media-playback-reviewers,jolin

Due to D268192, now only IDR frame will be marked as keyframe on MacOS.

That means other type of I frames will be ignored, in this situation,
seeking from the closet sync sample might not be able to find a key frame,
because the sync sample might be other types of I frames.

Therefore, we should try to search from the first sync sample in a GOP, which
is usually a IDR frame, in order to find a valid key frame.

Differential Revision: https://phabricator.services.mozilla.com/D268213

Diffstat:
M
dom/media/mp4/MP4Demuxer.cpp
 | 
28
+++++++++++++++++++++++++++-
M
dom/media/mp4/SampleIterator.cpp
 | 
5
++++-
M
dom/media/mp4/SampleIterator.h
 | 
9
++++++++-

3 files changed, 39 insertions(+), 3 deletions(-)
diff --git a/dom/media/mp4/MP4Demuxer.cpp b/dom/media/mp4/MP4Demuxer.cpp
@@ -379,11 +379,34 @@ RefPtr<MP4TrackDemuxer::SeekPromise> MP4TrackDemuxer::Seek(
 
   mIterator->Seek(seekTime);
 
+#ifdef MOZ_APPLEMEDIA
+  bool hasSeenValidSamples = false, seekingFromFirstSyncSample = false;
+#endif
   // Check what time we actually seeked to.
   do {
     auto next = GetNextSample();
     if (next.isErr()) {
-      return SeekPromise::CreateAndReject(next.unwrapErr(), __func__);
+      auto error = next.unwrapErr();
+#ifdef MOZ_APPLEMEDIA
+      // On macOS, only IDR frames are marked as key frames. This means some
+      // sync samples (e.g., Recovery SEI or I-slices) are not treated as key
+      // frames, as they may not be supported by Apple’s VideoToolbox H.264
+      // decoder. If we have seen samples but found no key frame, it indicates
+      // we need to look further back for an earlier sample where an IDR frame
+      // should exist per spec. In that case, we retry from the first sync
+      // sample in the GOP, which is typically an IDR frame. This situation
+      // suggests a poorly muxed file that wastes seek time, but unfortunately
+      // such cases do occur in real-world content.
+      if (mType == kH264 && error == NS_ERROR_DOM_MEDIA_END_OF_STREAM &&
+          hasSeenValidSamples && !seekingFromFirstSyncSample) {
+        LOG("Can not find a key frame from the closet sync sample, try again "
+            "from the first sync sample");
+        seekingFromFirstSyncSample = true;
+        mIterator->Seek(seekTime, SampleIterator::SyncSampleMode::First);
+        continue;
+      }
+#endif
+      return SeekPromise::CreateAndReject(error, __func__);
     }
     RefPtr<MediaRawData> sample = next.unwrap();
     if (!sample->Size()) {
@@ -395,6 +418,9 @@ RefPtr<MP4TrackDemuxer::SeekPromise> MP4TrackDemuxer::Seek(
       mQueuedSample = sample;
       seekTime = mQueuedSample->mTime;
     }
+#ifdef MOZ_APPLEMEDIA
+    hasSeenValidSamples = true;
+#endif
   } while (!mQueuedSample);
 
   SetNextKeyFrameTime();
diff --git a/dom/media/mp4/SampleIterator.cpp b/dom/media/mp4/SampleIterator.cpp
@@ -431,7 +431,7 @@ Result<Sample*, nsresult> SampleIterator::Get() {
 
 void SampleIterator::Next() { ++mCurrentSample; }
 
-void SampleIterator::Seek(const TimeUnit& aTime) {
+void SampleIterator::Seek(const TimeUnit& aTime, SyncSampleMode aMode) {
   size_t syncMoof = 0;
   size_t syncSample = 0;
   mCurrentMoof = 0;
@@ -443,6 +443,9 @@ void SampleIterator::Seek(const TimeUnit& aTime) {
     if (sample->mSync) {
       syncMoof = mCurrentMoof;
       syncSample = mCurrentSample;
+      if (aMode == SyncSampleMode::First) {
+        break;
+      }
     }
     if (sample->mCompositionRange.start == aTime) {
       break;
diff --git a/dom/media/mp4/SampleIterator.h b/dom/media/mp4/SampleIterator.h
@@ -28,7 +28,14 @@ class SampleIterator {
   bool HasNext();
   already_AddRefed<mozilla::MediaRawData> GetNextHeader();
   Result<already_AddRefed<mozilla::MediaRawData>, MediaResult> GetNext();
-  void Seek(const media::TimeUnit& aTime);
+
+  // The default seek mode finds the closest sync sample at or before the target
+  // time. Setting the mode to `First` allows seeking to the earliest sync
+  // sample instead, which is only used in a specific case.
+  enum class SyncSampleMode { Closest, First };
+  void Seek(const media::TimeUnit& aTime,
+            SyncSampleMode aMode = SyncSampleMode::Closest);
+
   media::TimeUnit GetNextKeyframeTime();
 
  private: