[cscore] SourceImpl waits for any new frame

cscore/src/main/native/cpp/SourceImpl.cpp

@@ -78,20 +78,34 @@ Frame SourceImpl::GetCurFrame() {
 
 Frame SourceImpl::GetNextFrame() {
   std::unique_lock lock{m_frameMutex};
-  auto oldTime = m_frame.GetTime();
-  m_frameCv.wait(
-      lock, [=, this] { return oldTime == 0 || m_frame.GetTime() != oldTime; });
+
+  // Wait unitl m_frame has a timestamp other than the last time we called
+  // GetNextFrame
+  m_frameCv.wait(lock, [=, this] {
+    return m_lastGetNextFrameTime == 0 ||
+           m_frame.GetTime() != m_lastGetNextFrameTime;
+  });
+
+  // Write down this frame's time (treating time as a frame uuid)
+  m_lastGetNextFrameTime = m_frame.GetTime();
+
   return m_frame;
 }
 
 Frame SourceImpl::GetNextFrame(double timeout) {
   std::unique_lock lock{m_frameMutex};
-  auto oldTime = m_frame.GetTime();
+
+  // Wait unitl m_frame has a timestamp other than the last time we called
+  // GetNextFrame
   if (!m_frameCv.wait_for(
           lock, std::chrono::milliseconds(static_cast<int>(timeout * 1000)),
-          [=, this] { return m_frame.GetTime() != oldTime; })) {
+          [=, this] { return m_frame.GetTime() != m_lastGetNextFrameTime; })) {
     m_frame = Frame{*this, "timed out getting frame", wpi::Now()};
   }
+
+  // Write down this frame's time (treating time as a frame uuid)
+  m_lastGetNextFrameTime = m_frame.GetTime();
+
   return m_frame;
 }
 

cscore/src/main/native/cpp/SourceImpl.h

@@ -188,6 +188,9 @@ class SourceImpl : public PropertyContainer {
   // MUST be located below m_poolMutex as the Frame destructor calls back
   // into SourceImpl::ReleaseImage, which locks m_poolMutex.
   Frame m_frame;
+
+  // The timestamp of the last image we handed on to a sink in GetNextFrame
+  Frame::Time m_lastGetNextFrameTime{0};
 };
 
 }  // namespace cs