Reuse a AVPacket read buffer when demuxing

Author: lgeiger

av/container/input.py

@@ -153,6 +153,7 @@ def demux(self, *args, **kwargs):
 
         i: cython.uint
         packet: Packet
+        read_packet: Packet
         ret: cython.int
 
         self.set_timeout(self.read_timeout)
@@ -165,22 +166,31 @@ def demux(self, *args, **kwargs):
                     raise ValueError(f"stream index {i} out of range")
                 include_stream[i] = True
 
+            # Pre-allocate a AVPacket that is reused as the read buffer.
+            read_packet = Packet()
             while True:
-                packet = Packet()
+                # Reset the read buffer
+                with cython.nogil:
+                    lib.av_packet_unref(read_packet.ptr)
                 try:
                     self.start_timeout()
                     with cython.nogil:
-                        ret = lib.av_read_frame(self.ptr, packet.ptr)
+                        ret = lib.av_read_frame(self.ptr, read_packet.ptr)
                     self.err_check(ret)
                 except EOFError:
                     break
 
-                if include_stream[packet.ptr.stream_index]:
+                if include_stream[read_packet.ptr.stream_index]:
                     # If AVFMTCTX_NOHEADER is set in ctx_flags, then new streams
                     # may also appear in av_read_frame().
                     # http://ffmpeg.org/doxygen/trunk/structAVFormatContext.html
                     # TODO: find better way to handle this
-                    if packet.ptr.stream_index < len(self.streams):
+                    if read_packet.ptr.stream_index < len(self.streams):
+                        # Move the encoded data out of the read buffer into a
+                        # fresh Packet for the caller.
+                        packet = Packet()
+                        with cython.nogil:
+                            lib.av_packet_move_ref(packet.ptr, read_packet.ptr)
                         packet._stream = self.streams[packet.ptr.stream_index]
                         # Keep track of this so that remuxing is easier.
                         packet.ptr.time_base = packet._stream.ptr.time_base

include/avcodec.pxd

@@ -407,6 +407,8 @@ cdef extern from "libavcodec/avcodec.h" nogil:
     cdef void av_packet_free(AVPacket **)
     cdef int av_new_packet(AVPacket*, int)
     cdef int av_packet_ref(AVPacket *dst, const AVPacket *src)
+    cdef void av_packet_unref(AVPacket *pkt)
+    cdef void av_packet_move_ref(AVPacket *dst, AVPacket *src)
     cdef void av_packet_rescale_ts(AVPacket *pkt, AVRational src_tb, AVRational dst_tb)
 
     cdef enum AVSubtitleType: