On Thu, 9 Nov 2017, James Cowgill wrote:


On 09/11/17 14:02, Hendrik Leppkes wrote:
On Thu, Nov 9, 2017 at 1:21 PM, James Cowgill <jcowg...@debian.org> wrote:
In commit 061a0c14bb57 ("decode: restructure the core decoding code"), the
deprecated avcodec_decode_* APIs were reworked so that they called into the
new avcodec_send_packet / avcodec_receive_frame API. This had the side effect
of prohibiting sending new packets containing data after a drain
packet, but in previous versions of FFmpeg this "worked" and some
applications relied on it.

To restore some compatibility, reset the codec if we receive a new non-drain
packet using the old API after draining has completed. While this does
not give the same behaviour as the old API did, in the majority of cases
it works and it does not require changes to any other part of the decoding

Fixes ticket #6775
Signed-off-by: James Cowgill <jcowg...@debian.org>
 libavcodec/decode.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libavcodec/decode.c b/libavcodec/decode.c
index 86fe5aef52..2f1932fa85 100644
--- a/libavcodec/decode.c
+++ b/libavcodec/decode.c
@@ -726,6 +726,11 @@ static int compat_decode(AVCodecContext *avctx, AVFrame 

     av_assert0(avci->compat_decode_consumed == 0);

+    if (avci->draining_done && pkt && pkt->size != 0) {
+        av_log(avctx, AV_LOG_WARNING, "Got unexpected packet after EOF\n");
+        avcodec_flush_buffers(avctx);
+    }

I don't think this is a good idea. Draining and not flushing
afterwards is a bug in the calling code, and even before recent
changes it would result in inconsistent behavior and even crashes
(with select decoders).

I am fully aware that this will only trigger if the calling code is
buggy. I am trying to avoid silent breakage of those applications doing
this when upgrading to ffmpeg 3.4.

I was looking at the documentation of avcodec_decode_* recently because
of this and I had some trouble deciding if using the API this way was
incorrect. I expect the downstreams affected thought that what they were
doing was fine and then got angry when ffmpeg suddenly "broke" their
code. This patch at least allows some sort of "transitional period"
until downstreams update.

I think the intent was to flush the codec by passing the NULL packets to it, so it makes a lot of sense to actually do that. Especially since by implicitly doing a flush, we can avoid the undefined behaviour/crashes on the codec side.

Also this is only compatibility code, which probably will be removed at the next bump, I see no harm in making it as compatible as realistically possible.

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