memory: implemented basic memory limiting functionality

Keeps track of total memory usage and uses it as a guide towards
allocating/reusing more memory.

app/codec/ffmpeg/ffmpegdecoder.cpp

@@ -889,13 +889,15 @@ QWaitCondition *FFmpegDecoderInstance::cache_wait_cond()
   return &cache_wait_cond_;
 }
 
-bool FFmpegDecoderInstance::IsWorking() const
+bool FFmpegDecoderInstance::IsWorking()
 {
+  QMutexLocker locker(&is_working_mutex_);
   return is_working_;
 }
 
 void FFmpegDecoderInstance::SetWorking(bool working)
 {
+  QMutexLocker locker(&is_working_mutex_);
   is_working_ = working;
 }
 
@@ -1113,7 +1115,12 @@ FFmpegFramePool::ElementPtr FFmpegDecoderInstance::RetrieveFrame(const int64_t&
       }
 
       // Clear early frames
-      TruncateCacheRangeTo(second_ts_);
+      int removed = TruncateCacheRangeTo(second_ts_);
+
+      if (removed == 0 && MemoryPoolLimitReached()) {
+        // Relinquish a frame if we have to conserve memory
+        RemoveFirstFrame();
+      }
 
       // Append this frame and signal to other threads that a new frame has arrived
       cached_frames_.append(cached);
@@ -1295,19 +1302,30 @@ FFmpegFramePool::ElementPtr FFmpegDecoderInstance::GetFrameFromCache(const int64
 void FFmpegDecoderInstance::RemoveFramesBefore(const qint64 &t)
 {
   // We keep one frame in memory as an identifier for what pts the decoder is up to
-  while (cached_frames_.size() > 1 && cached_frames_.first()->last_accessed() < t) {
-    cached_frames_.removeFirst();
-    cache_at_zero_ = false;
+  int min_frames = (MemoryPoolLimitReached() && !IsWorking()) ? 0 : 1;
+
+  while (cached_frames_.size() > min_frames && cached_frames_.first()->last_accessed() < t) {
+    RemoveFirstFrame();
   }
 }
 
-void FFmpegDecoderInstance::TruncateCacheRangeTo(const qint64 &t)
+int FFmpegDecoderInstance::TruncateCacheRangeTo(const qint64 &t)
 {
+  int counter = 0;
+
   // We keep one frame in memory as an identifier for what pts the decoder is up to
   while (cached_frames_.size() > 1 && (RangeEnd() - RangeStart()) > t) {
-    cached_frames_.removeFirst();
-    cache_at_zero_ = false;
+    RemoveFirstFrame();
+    counter++;
   }
+
+  return counter;
+}
+
+void FFmpegDecoderInstance::RemoveFirstFrame()
+{
+  cached_frames_.removeFirst();
+  cache_at_zero_ = false;
 }
 
 FFmpegDecoderInstance::FFmpegDecoderInstance(const char *filename, int stream_index) :

app/codec/ffmpeg/ffmpegdecoder.h

@@ -62,7 +62,8 @@ public:
   FFmpegFramePool::ElementPtr GetFrameFromCache(const int64_t& t) const;
 
   void RemoveFramesBefore(const qint64& t);
-  void TruncateCacheRangeTo(const qint64& t);
+  int TruncateCacheRangeTo(const qint64& t);
+  void RemoveFirstFrame();
 
   AVFormatContext* fmt_ctx() const
   {
@@ -90,7 +91,7 @@ public:
   QMutex* cache_lock();
   QWaitCondition* cache_wait_cond();
 
-  bool IsWorking() const;
+  bool IsWorking();
   void SetWorking(bool working);
 
 private:
@@ -119,6 +120,7 @@ private:
   int64_t cache_target_time_;
 
   bool is_working_;
+  QMutex is_working_mutex_;
 
   bool cache_at_zero_;
   bool cache_at_eof_;