Destroy the fallback decoder after enough sw frames are decoded.

We only need to hold onto the GPU decoder long enough that previous
frames have been rendered; once we exceed kMaxFrames we know we've
exhausted all hardware frames and can destruct the decoder.

BUG=none
TEST=media_unittests

Review URL: https://codereview.chromium.org/1417183002

Cr-Commit-Position: refs/heads/master@{#355930}

media/filters/decoder_stream.cc

@@ -12,6 +12,7 @@
 #include "base/trace_event/trace_event.h"
 #include "media/base/bind_to_current_loop.h"
 #include "media/base/decoder_buffer.h"
+#include "media/base/limits.h"
 #include "media/base/media_log.h"
 #include "media/base/timestamp_constants.h"
 #include "media/base/video_decoder.h"
@@ -50,11 +51,11 @@ DecoderStream<StreamType>::DecoderStream(
       decoder_selector_(new DecoderSelector<StreamType>(task_runner,
                                                         decoders.Pass(),
                                                         media_log)),
+      decoded_frames_since_fallback_(0),
       active_splice_(false),
       decoding_eos_(false),
       pending_decode_requests_(0),
-      weak_factory_(this) {
-}
+      weak_factory_(this) {}
 
 template <DemuxerStream::Type StreamType>
 DecoderStream<StreamType>::~DecoderStream() {
@@ -243,6 +244,7 @@ void DecoderStream<StreamType>::OnDecoderSelected(
   }
 
   previous_decoder_ = decoder_.Pass();
+  decoded_frames_since_fallback_ = 0;
   decoder_ = selected_decoder.Pass();
   if (decrypting_demuxer_stream) {
     decrypting_demuxer_stream_ = decrypting_demuxer_stream.Pass();
@@ -413,6 +415,13 @@ void DecoderStream<StreamType>::OnDecodeOutputReady(
 
   // Store decoded output.
   ready_outputs_.push_back(output);
+
+  // Destruct any previous decoder once we've decoded enough frames to ensure
+  // that it's no longer in use.
+  if (previous_decoder_ &&
+      ++decoded_frames_since_fallback_ > limits::kMaxVideoFrames) {
+    previous_decoder_.reset();
+  }
 }
 
 template <DemuxerStream::Type StreamType>

media/filters/decoder_stream.h

@@ -113,6 +113,10 @@ class MEDIA_EXPORT DecoderStream {
     config_change_observer_cb_ = config_change_observer;
   }
 
+  const Decoder* get_previous_decoder_for_testing() const {
+    return previous_decoder_.get();
+  }
+
  private:
   enum State {
     STATE_UNINITIALIZED,
@@ -186,9 +190,11 @@ class MEDIA_EXPORT DecoderStream {
   scoped_ptr<DecoderSelector<StreamType> > decoder_selector_;
 
   scoped_ptr<Decoder> decoder_;
-  // TODO(watk): When falling back from H/W decoding to S/W decoding,
-  // destructing the GpuVideoDecoder too early results in black frames being
-  // displayed. |previous_decoder_| is used to keep it alive.
+  // When falling back from H/W decoding to S/W decoding, destructing the
+  // GpuVideoDecoder too early results in black frames being displayed.
+  // |previous_decoder_| is used to keep it alive.  It is destroyed once we've
+  // decoded at least media::limits::kMaxVideoFrames frames after fallback.
+  int decoded_frames_since_fallback_;
   scoped_ptr<Decoder> previous_decoder_;
   scoped_ptr<DecryptingDemuxerStream> decrypting_demuxer_stream_;
 

media/filters/video_frame_stream_unittest.cc

@@ -825,7 +825,9 @@ TEST_P(VideoFrameStreamTest, FallbackDecoderSelectedOnFailureToReinitialize) {
   Initialize();
   decoder1_->SimulateFailureToInit();
   ReadUntilDecoderReinitialized(decoder1_);
+  ASSERT_TRUE(video_frame_stream_->get_previous_decoder_for_testing());
   ReadAllFrames();
+  ASSERT_FALSE(video_frame_stream_->get_previous_decoder_for_testing());
 }
 
 TEST_P(VideoFrameStreamTest,