fix block

web_viewer/app.py

@@ -60,15 +60,18 @@ async def init_video_fifo():
         return
 
     try:
-        print(f"Opening FIFO {VIDEO_FIFO_PATH} for reading (will block until C++ opens for writing)...")
+        print(f"Opening FIFO {VIDEO_FIFO_PATH} for reading (non-blocking mode)...")
         loop = asyncio.get_event_loop()
 
-        # Open file in BLOCKING mode - this will unblock C++ when it opens for writing
+        # Open file in NON-BLOCKING mode to prevent deadlock
+        # GStreamer filesink opens FIFO lazily (only when first frame arrives)
+        # Using O_NONBLOCK prevents Python from blocking while waiting for C++ writer
         def open_fifo():
-            return open(VIDEO_FIFO_PATH, 'rb', buffering=0)
+            fd = os.open(VIDEO_FIFO_PATH, os.O_RDONLY | os.O_NONBLOCK)
+            return os.fdopen(fd, 'rb', buffering=0)
 
         video_fifo_file = await loop.run_in_executor(None, open_fifo)
-        print(f"FIFO opened successfully - C++ should now be writing")
+        print(f"FIFO opened successfully in non-blocking mode")
     except Exception as e:
         print(f"Failed to open video FIFO: {e}")
 
@@ -333,16 +336,39 @@ async def stream_video_from_fifo():
         async def read_from_fifo():
             """Read from FIFO and pipe to ffmpeg stdin"""
             nonlocal bytes_written
+            writer_connected = False
+
             try:
                 while streaming_active:
                     def read_chunk():
-                        return fifo.read(chunk_size)
+                        try:
+                            return fifo.read(chunk_size)
+                        except BlockingIOError:
+                            # No data available yet (writer not connected or no data)
+                            return None
 
                     data = await loop.run_in_executor(None, read_chunk)
+
+                    if data is None:
+                        # BlockingIOError: no data available, wait a bit
+                        if not writer_connected:
+                            print("Waiting for C++ to start writing to FIFO...")
+                        await asyncio.sleep(0.1)
+                        continue
+
                     if not data:
-                        print("FIFO EOF reached")
+                        # Empty data: EOF (writer closed the FIFO)
+                        if writer_connected:
+                            print("FIFO EOF reached - C++ stopped writing")
+                        else:
+                            print("FIFO closed before C++ started writing")
                         break
 
+                    # Got data! Mark writer as connected
+                    if not writer_connected:
+                        writer_connected = True
+                        print("C++ started writing to FIFO successfully")
+
                     if ffmpeg_process and ffmpeg_process.stdin:
                         try:
                             ffmpeg_process.stdin.write(data)