ppapi: VideoEncoder: prevent scheduling encoding multiple times

ppapi/examples/video_encode/video_encode.cc

Index: ppapi/examples/video_encode/video_encode.cc
diff --git a/ppapi/examples/video_encode/video_encode.cc b/ppapi/examples/video_encode/video_encode.cc
index 7a55df294cfa2e89ce2ea21f64d974761d554871..c5c536b204489d74a7d2b4f9118d535b829881db 100644
--- a/ppapi/examples/video_encode/video_encode.cc
+++ b/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 +349,27 @@ 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;
+  PP_Time delta =
+      tick - clamp(0, 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_;