ppapi: VideoEncoder: prevent scheduling encoding multiple times

The ScheduleNextEncode() can be called at multiple stages in the
example depending on whether it needs to reconfigure the
MediaVideoStreamTrack. This can lead the example to encode at twice
30fps. At this speed we can exhaust the 33ms lifespan of video
buffers, leading to errors in the example plugins which can't retrieve
new frames to respect the 30fps framerate.

BUG=503153
TEST=run video_encoder example from NaCl SDK with VP8 codec and verify there are no error messages after 1 minute

Review URL: https://codereview.chromium.org/1218513003

Cr-Commit-Position: refs/heads/master@{#336850}

native_client_sdk/src/examples/api/video_encode/video_encode.cc

@@ -46,6 +46,10 @@
 
 namespace {
 
+double clamp(double min, double max, double value) {
+  return std::max(std::min(value, max), min);
+}
+
 // IVF container writer. It is possible to parse H264 bitstream using
 // NAL units but for VP8 we need a container to at least find encoded
 // pictures as well as the picture sizes.
@@ -161,6 +165,7 @@ class VideoEncoderInstance : public pp::Instance {
   VideoProfileToStringMap profile_to_string_;
 
   bool is_encoding_;
+  bool is_encode_ticking_;
   bool is_receiving_track_frames_;
 
   pp::VideoEncoder video_encoder_;
@@ -186,6 +191,7 @@ VideoEncoderInstance::VideoEncoderInstance(PP_Instance instance,
                                            pp::Module* module)
     : pp::Instance(instance),
       is_encoding_(false),
+      is_encode_ticking_(false),
       callback_factory_(this),
 #if defined(USE_VP8_INSTEAD_OF_H264)
       video_profile_(PP_VIDEOPROFILE_VP8_ANY),
@@ -340,15 +346,28 @@ void VideoEncoderInstance::OnInitializedEncoder(int32_t result) {
 }
 
 void VideoEncoderInstance::ScheduleNextEncode() {
+  // Avoid scheduling more than once at a time.
+  if (is_encode_ticking_)
+    return;
+
   PP_Time now = pp::Module::Get()->core()->GetTime();
+  PP_Time tick = 1.0 / 30;
+  // If the callback was triggered late, we need to account for that
+  // delay for the next tick.
+  PP_Time delta = tick - clamp(0, tick, now - last_encode_tick_ - tick);
+
   pp::Module::Get()->core()->CallOnMainThread(
-      std::min(std::max(now - last_encode_tick_, 0.0), 1000.0 / 30),
+      delta * 1000,
       callback_factory_.NewCallback(&VideoEncoderInstance::GetEncoderFrameTick),
       0);
+
   last_encode_tick_ = now;
+  is_encode_ticking_ = true;
 }
 
 void VideoEncoderInstance::GetEncoderFrameTick(int32_t result) {
+  is_encode_ticking_ = false;
+
   if (is_encoding_) {
     if (!current_track_frame_.is_null()) {
       pp::VideoFrame frame = current_track_frame_;

ppapi/examples/video_encode/video_encode.cc

@@ -44,6 +44,10 @@
 
 namespace {
 
+double clamp(double min, double max, double value) {
+  return std::max(std::min(value, max), min);
+}
+
 // IVF container writer. It is possible to parse H264 bitstream using
 // NAL units but for VP8 we need a container to at least find encoded
 // pictures as well as the picture sizes.
@@ -160,6 +164,7 @@ class VideoEncoderInstance : public pp::Instance {
   VideoProfileToStringMap profile_to_string_;
 
   bool is_encoding_;
+  bool is_encode_ticking_;
   bool is_receiving_track_frames_;
 
   pp::VideoEncoder video_encoder_;
@@ -185,6 +190,7 @@ VideoEncoderInstance::VideoEncoderInstance(PP_Instance instance,
                                            pp::Module* module)
     : pp::Instance(instance),
       is_encoding_(false),
+      is_encode_ticking_(false),
       callback_factory_(this),
 #if defined(USE_VP8_INSTEAD_OF_H264)
       video_profile_(PP_VIDEOPROFILE_VP8_ANY),
@@ -343,15 +349,28 @@ void VideoEncoderInstance::OnInitializedEncoder(int32_t result) {
 }
 
 void VideoEncoderInstance::ScheduleNextEncode() {
+  // Avoid scheduling more than once at a time.
+  if (is_encode_ticking_)
+    return;
+
   PP_Time now = pp::Module::Get()->core()->GetTime();
+  PP_Time tick = 1.0 / 30;
+  // If the callback was triggered late, we need to account for that
+  // delay for the next tick.
+  PP_Time delta = tick - clamp(0, tick, now - last_encode_tick_ - tick);
+
   pp::Module::Get()->core()->CallOnMainThread(
-      std::min(std::max(now - last_encode_tick_, 0.0), 1000.0 / 30),
+      delta * 1000,
       callback_factory_.NewCallback(&VideoEncoderInstance::GetEncoderFrameTick),
       0);
+
   last_encode_tick_ = now;
+  is_encode_ticking_ = true;
 }
 
 void VideoEncoderInstance::GetEncoderFrameTick(int32_t result) {
+  is_encode_ticking_ = false;
+
   if (is_encoding_) {
     if (!current_track_frame_.is_null()) {
       pp::VideoFrame frame = current_track_frame_;