h264: enable low delay only if no delayed frames were seen

Dropping frames is undesirable but that is the only way by which the
decoder could return to low delay mode. Instead emit a warning and
continue with delayed frames.
Fixes a crash in fuzzed sample nasa-8s2.ts_s20033 caused by a larger
than expected has_b_frames value. Low delay keeps getting re-enabled
from a presumely broken SPS.

CC: libav-stable@libav.org
(cherry picked from commit 706acb558a38eba633056773280155d66c2f4b24)

Conflicts:

	libavcodec/h264.c

1 files changed, 10 insertions, 3 deletions

diff --git a/libavcodec/h264.c b/libavcodec/h264.c
index b866917e5f..1c5b841889 100644
--- a/libavcodec/h264.c
+++ b/libavcodec/h264.c
@@ -4030,9 +4030,16 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
                 ff_h264_decode_seq_parameter_set(h);
             }
 
-            if (s->flags& CODEC_FLAG_LOW_DELAY ||
-                (h->sps.bitstream_restriction_flag && !h->sps.num_reorder_frames))
-                s->low_delay=1;
+            if (s->flags & CODEC_FLAG_LOW_DELAY ||
+                (h->sps.bitstream_restriction_flag &&
+                 !h->sps.num_reorder_frames)) {
+                if (s->avctx->has_b_frames > 1 || h->delayed_pic[0])
+                    av_log(avctx, AV_LOG_WARNING, "Delayed frames seen "
+                           "reenabling low delay requires a codec "
+                           "flush.\n");
+                else
+                    s->low_delay = 1;
+            }
 
             if(avctx->has_b_frames < 2)
                 avctx->has_b_frames= !s->low_delay;