[M90-LTS] Protect HRTF database loader thread from access by differen…

…t threads

This patch add a new mutex locker around the HRTF database loader
thread to ensure the safe exclusive access of the loader thread
and the HRTF database.

(cherry picked from commit 6811e85)

Bug: 1233564
Change-Id: Ie12b99ffe520d3747e34af387a37637a10aab38a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3068260
Auto-Submit: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#908269}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3097764
Reviewed-by: Artem Sumaneev <asumaneev@google.com>
Owners-Override: Artem Sumaneev <asumaneev@google.com>
Commit-Queue: Roger Felipe Zanoni da Silva <rzanoni@google.com>
Cr-Commit-Position: refs/branch-heads/4430@{#1568}
Cr-Branched-From: e5ce7dc-refs/heads/master@{#857950}

third_party/blink/renderer/platform/audio/hrtf_database_loader.cc

@@ -86,6 +86,8 @@ void HRTFDatabaseLoader::LoadTask() {
 void HRTFDatabaseLoader::LoadAsynchronously() {
   DCHECK(IsMainThread());
 
+  MutexLocker locker(lock_);
+
   // m_hrtfDatabase and m_thread should both be unset because this should be a
   // new HRTFDatabaseLoader object that was just created by
   // createAndLoadAsynchronouslyIfNecessary and because we haven't started
@@ -122,6 +124,10 @@ void HRTFDatabaseLoader::CleanupTask(base::WaitableEvent* sync) {
 }
 
 void HRTFDatabaseLoader::WaitForLoaderThreadCompletion() {
+  // We can lock this because this is called from either the main thread or
+  // the offline audio rendering thread.
+  MutexLocker locker(lock_);
+
   if (!thread_)
     return;
 

third_party/blink/renderer/platform/audio/hrtf_database_loader.h

@@ -64,8 +64,8 @@ class PLATFORM_EXPORT HRTFDatabaseLoader final
   // must be called from the audio thread.
   bool IsLoaded() { return Database(); }
 
-  // waitForLoaderThreadCompletion() may be called more than once and is
-  // thread-safe.
+  // May be called from both main and audio thread, and also can be called more
+  // than once.
   void WaitForLoaderThreadCompletion();
 
   // Returns the database or nullptr if the database doesn't yet exist.  Must
@@ -87,11 +87,10 @@ class PLATFORM_EXPORT HRTFDatabaseLoader final
   void LoadTask();
   void CleanupTask(base::WaitableEvent*);
 
-  // Holding a m_lock is required when accessing m_hrtfDatabase since we access
-  // it from multiple threads.
+  // |lock_| MUST be held when accessing |hrtf_database_| or |thread_| because
+  // it can be accessed by multiple threads (e.g multiple AudioContexts).
   Mutex lock_;
   std::unique_ptr<HRTFDatabase> hrtf_database_;
-
   std::unique_ptr<Thread> thread_;
 
   float database_sample_rate_;