2 files changed, 27 insertions, 10 deletions

diff --git a/dom/media/platforms/wrappers/MediaChangeMonitor.cpp b/dom/media/platforms/wrappers/MediaChangeMonitor.cpp
index 46989840bf..bb7b015fab 100644
--- a/dom/media/platforms/wrappers/MediaChangeMonitor.cpp
+++ b/dom/media/platforms/wrappers/MediaChangeMonitor.cpp
@@ -800,6 +800,7 @@ RefPtr<ShutdownPromise> MediaChangeMonitor::ShutdownDecoder() {
   AssertOnThread();
   mConversionRequired.reset();
   if (mDecoder) {
+    MutexAutoLock lock(mMutex);
     RefPtr<MediaDataDecoder> decoder = std::move(mDecoder);
     return decoder->Shutdown();
   }
@@ -847,6 +848,7 @@ MediaChangeMonitor::CreateDecoder() {
           ->Then(
               GetCurrentSerialEventTarget(), __func__,
               [self = RefPtr{this}, this](RefPtr<MediaDataDecoder>&& aDecoder) {
+                MutexAutoLock lock(mMutex);
                 mDecoder = std::move(aDecoder);
                 DDLINKCHILD("decoder", mDecoder.get());
                 return CreateDecoderPromise::CreateAndResolve(true, __func__);
@@ -1095,6 +1097,11 @@ void MediaChangeMonitor::FlushThenShutdownDecoder(
       ->Track(mFlushRequest);
 }
 
+MediaDataDecoder* MediaChangeMonitor::GetDecoderOnNonOwnerThread() const {
+  MutexAutoLock lock(mMutex);
+  return mDecoder;
+}
+
 #undef LOG
 
 }  // namespace mozilla
diff --git a/dom/media/platforms/wrappers/MediaChangeMonitor.h b/dom/media/platforms/wrappers/MediaChangeMonitor.h
index a3ee5b5aa0..ff4f6921f6 100644
--- a/dom/media/platforms/wrappers/MediaChangeMonitor.h
+++ b/dom/media/platforms/wrappers/MediaChangeMonitor.h
@@ -41,34 +41,34 @@ class MediaChangeMonitor final
   RefPtr<ShutdownPromise> Shutdown() override;
   bool IsHardwareAccelerated(nsACString& aFailureReason) const override;
   nsCString GetDescriptionName() const override {
-    if (mDecoder) {
-      return mDecoder->GetDescriptionName();
+    if (RefPtr<MediaDataDecoder> decoder = GetDecoderOnNonOwnerThread()) {
+      return decoder->GetDescriptionName();
     }
     return "MediaChangeMonitor decoder (pending)"_ns;
   }
   nsCString GetProcessName() const override {
-    if (mDecoder) {
-      return mDecoder->GetProcessName();
+    if (RefPtr<MediaDataDecoder> decoder = GetDecoderOnNonOwnerThread()) {
+      return decoder->GetProcessName();
     }
     return "MediaChangeMonitor"_ns;
   }
   nsCString GetCodecName() const override {
-    if (mDecoder) {
-      return mDecoder->GetCodecName();
+    if (RefPtr<MediaDataDecoder> decoder = GetDecoderOnNonOwnerThread()) {
+      return decoder->GetCodecName();
     }
     return "MediaChangeMonitor"_ns;
   }
   void SetSeekThreshold(const media::TimeUnit& aTime) override;
   bool SupportDecoderRecycling() const override {
-    if (mDecoder) {
-      return mDecoder->SupportDecoderRecycling();
+    if (RefPtr<MediaDataDecoder> decoder = GetDecoderOnNonOwnerThread()) {
+      return decoder->SupportDecoderRecycling();
     }
     return false;
   }
 
   ConversionRequired NeedsConversion() const override {
-    if (mDecoder) {
-      return mDecoder->NeedsConversion();
+    if (RefPtr<MediaDataDecoder> decoder = GetDecoderOnNonOwnerThread()) {
+      return decoder->NeedsConversion();
     }
     // Default so no conversion is performed.
     return ConversionRequired::kNeedNone;
@@ -100,6 +100,9 @@ class MediaChangeMonitor final
     MOZ_ASSERT(!mThread || mThread->IsOnCurrentThread());
   }
 
+  // This is used for getting decoder debug info on other threads. Thread-safe.
+  MediaDataDecoder* GetDecoderOnNonOwnerThread() const;
+
   bool CanRecycleDecoder() const;
 
   typedef MozPromise<bool, MediaResult, true /* exclusive */>
@@ -140,6 +143,13 @@ class MediaChangeMonitor final
   const CreateDecoderParamsForAsync mParams;
   // Keep any seek threshold set for after decoder creation and initialization.
   Maybe<media::TimeUnit> mPendingSeekThreshold;
+
+  // This lock is used for mDecoder specifically, but it doens't need to be used
+  // for every places accessing mDecoder which is mostly on the owner thread.
+  // However, when requesting decoder debug info, it can happen on other
+  // threads, so we need this mutex to avoid the data race of
+  // creating/destroying decoder and accessing decoder's debug info.
+  mutable Mutex MOZ_ANNOTATED mMutex{"MediaChangeMonitor"};
 };
 
 }  // namespace mozilla