Copy template flags if creating a new Codec object

This fixes #1903

Author: WyattBlue

av/container/output.py

@@ -130,7 +130,9 @@ def add_stream(self, codec_name, rate=None, options: dict | None = None, **kwarg
 
         return py_stream
 
-    def add_stream_from_template(self, template: Stream, opaque=None, **kwargs):
+    def add_stream_from_template(
+        self, template: Stream, opaque: bool | None = None, **kwargs
+    ):
         """
         Creates a new stream from a template. Supports video, audio, and subtitle streams.
 
@@ -170,6 +172,11 @@ def add_stream_from_template(self, template: Stream, opaque=None, **kwargs):
         if self.ptr.oformat.flags & lib.AVFMT_GLOBALHEADER:
             ctx.flags |= lib.AV_CODEC_FLAG_GLOBAL_HEADER
 
+        # Copy flags If we're creating a new codec object. This fixes some muxing issues.
+        # Overwriting `ctx.flags |= lib.AV_CODEC_FLAG_GLOBAL_HEADER` is intentional.
+        if not opaque:
+            ctx.flags = template.codec_context.flags
+
         # Initialize stream codec parameters to populate the codec type. Subsequent changes to
         # the codec context will be applied just before encoding starts in `start_encoding()`.
         err_check(lib.avcodec_parameters_from_context(stream.codecpar, ctx))

tests/test_remux.py

@@ -0,0 +1,33 @@
+import av
+import av.datasets
+
+
+def test_video_remux() -> None:
+    input_path = av.datasets.curated("pexels/time-lapse-video-of-night-sky-857195.mp4")
+    input_ = av.open(input_path)
+    output = av.open("remuxed.mkv", "w")
+
+    in_stream = input_.streams.video[0]
+    out_stream = output.add_stream_from_template(in_stream)
+
+    for packet in input_.demux(in_stream):
+        if packet.dts is None:
+            continue
+
+        packet.stream = out_stream
+        output.mux(packet)
+
+    input_.close()
+    output.close()
+
+    with av.open("remuxed.mkv") as container:
+        # Assert output is a valid media file
+        assert len(container.streams.video) == 1
+        assert len(container.streams.audio) == 0
+        assert container.streams.video[0].codec.name == "h264"
+
+        packet_count = 0
+        for packet in container.demux(video=0):
+            packet_count += 1
+
+        assert packet_count > 50