From 57c0b0ffa1508f6400ea034d6c0403e686794fdf Mon Sep 17 00:00:00 2001
From: Steve Lhomme <robux4@ycbcr.xyz>
Date: Fri, 12 Feb 2021 11:10:03 +0100
Subject: [PATCH 2/2] avcodec/mpeg12dec: don't end a slice without first_slice

If first_slice is set that means the first slice/field is not started yet. We
should not end the slice. In particular calling hwaccel->end_frame may crash as
we're ending a frame that was not started.

We also need to reset first_slice once the slice_end is finished handling
for this check to work.
---
 libavcodec/mpeg12dec.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
index 335c3713c2..42c580b8a5 100644
--- a/libavcodec/mpeg12dec.c
+++ b/libavcodec/mpeg12dec.c
@@ -2484,13 +2484,19 @@ static int decode_chunks(AVCodecContext *avctx, AVFrame *picture,
                         s2->er.error_count += s2->thread_context[i]->er.error_count;
                 }
 
-                ret = slice_end(avctx, picture);
-                if (ret < 0)
-                    return ret;
-                else if (ret) {
-                    // FIXME: merge with the stuff in mpeg_decode_slice
-                    if (s2->last_picture_ptr || s2->low_delay || s2->pict_type == AV_PICTURE_TYPE_B)
-                        *got_output = 1;
+                if (s->first_slice) // not started yet. don't end it
+                    ret = 0;
+                else {
+                    ret = slice_end(avctx, picture);
+                    if (ret < 0)
+                        return ret;
+                    else if (ret) {
+                        // FIXME: merge with the stuff in mpeg_decode_slice
+                        if (s2->last_picture_ptr || s2->low_delay || s2->pict_type == AV_PICTURE_TYPE_B)
+                            *got_output = 1;
+                    }
+                    // slice ended, don't end it again later
+                    s->first_slice = 1;
                 }
             }
             s2->pict_type = 0;
-- 
2.27.0.windows.1