Bug 1646348 - Introduce the AudioThreadRegistry. r=achronop

This keeps track of how many users of an audio thread there is, and takes care
of registring or unregistring those threads to the Gecko Profiler.

Differential Revision: https://phabricator.services.mozilla.com/D80464

dom/media/AudioThreadRegistry.h

@@ -0,0 +1,83 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef AUDIOTHREADREGISTRY_H
+#define AUDIOTHREADREGISTRY_H
+
+#include <cstdint>
+#include <mozilla/DataMutex.h>
+#include <nsTArray.h>
+#include <thread>
+#include <GeckoProfiler.h>
+
+namespace mozilla {
+
+// This class is a singleton that tracks all audio callback threads and makes
+// sure they are registered or unregistered to the profiler safely and
+// consistently.
+//
+// Register and Unregister are fairly expensive and shouldn't be used in a hot
+// path.
+class AudioThreadRegistry final {
+ public:
+  AudioThreadRegistry() : mThreadIds("AudioThreadId") {}
+  ~AudioThreadRegistry() {
+    // It would be nice to be able to assert that all threads have be
+    // unregistered, but we can't: it's legal to suspend an audio stream, so
+    // that the callback isn't called, and then immediately destroy it.
+  }
+
+  // This is intended to be called when an object starts an audio callback
+  // thread.
+  void Register(int aThreadId) {
+    auto threadIds = mThreadIds.Lock();
+    for (uint32_t i = 0; i < threadIds->Length(); i++) {
+      if ((*threadIds)[i].mId == aThreadId) {
+        (*threadIds)[i].mUserCount++;
+        return;
+      }
+    }
+    ThreadUserCount tuc;
+    tuc.mId = aThreadId;
+    tuc.mUserCount = 1;
+    threadIds->AppendElement(tuc);
+    PROFILER_REGISTER_THREAD("NativeAudioCallback");
+  }
+
+  // This is intended to be called when an object stops an audio callback thread
+  void Unregister(int aThreadId) {
+    auto threadIds = mThreadIds.Lock();
+    for (uint32_t i = 0; i < threadIds->Length(); i++) {
+      if ((*threadIds)[i].mId == aThreadId) {
+        MOZ_ASSERT((*threadIds)[i].mUserCount > 0);
+        (*threadIds)[i].mUserCount--;
+
+        if ((*threadIds)[i].mUserCount == 0) {
+          PROFILER_UNREGISTER_THREAD();
+          threadIds->RemoveElementAt(i);
+        }
+        return;
+      }
+    }
+    MOZ_ASSERT(false);
+  }
+
+ private:
+  AudioThreadRegistry(const AudioThreadRegistry&) = delete;
+  AudioThreadRegistry& operator=(const AudioThreadRegistry&) = delete;
+  AudioThreadRegistry(AudioThreadRegistry&&) = delete;
+  AudioThreadRegistry& operator=(AudioThreadRegistry&&) = delete;
+
+  struct ThreadUserCount {
+    int mId;  // from profiler_current_thread_id
+    int mUserCount;
+  };
+  DataMutex<nsTArray<ThreadUserCount>> mThreadIds;
+};
+
+}  // namespace mozilla
+
+#endif  // AUDIOTHREADREGISTRY_H

dom/media/moz.build

@@ -117,6 +117,7 @@ EXPORTS += [
     'AudioSampleFormat.h',
     'AudioSegment.h',
     'AudioStream.h',
+    'AudioThreadRegistry.h',
     'AutoplayPolicy.h',
     'BackgroundVideoDecodingPermissionObserver.h',
     'Benchmark.h',