avcodec/dvdec: Retry decoding seemingly damaged MBs while skiping likely damaged parts

Improves decoding of Ticket1589
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dvdec.c

@@ -291,10 +291,15 @@ static int dv_decode_video_segment(AVCodecContext *avctx, void *arg)
     LOCAL_ALIGNED_16(uint8_t, vs_bit_buffer, [80 * 5 + FF_INPUT_BUFFER_PADDING_SIZE]); /* allow some slack */
     const int log2_blocksize = 3-s->avctx->lowres;
     int is_field_mode[5];
+    int vs_bit_buffer_damaged = 0;
+    int mb_bit_buffer_damaged[5] = {0};
+    int retried = 0;
 
     av_assert1((((int) mb_bit_buffer) & 7) == 0);
     av_assert1((((int) vs_bit_buffer) & 7) == 0);
 
+retry:
+
     memset(sblock, 0, 5 * DV_MAX_BPM * sizeof(*sblock));
 
     /* pass 1: read DC and AC coefficients in blocks */
@@ -349,11 +354,16 @@ static int dv_decode_video_segment(AVCodecContext *avctx, void *arg)
              * block is finished */
             if (mb->pos >= 64)
                 bit_copy(&pb, &gb);
+            if (mb->pos >= 64 && mb->pos < 127)
+                vs_bit_buffer_damaged = mb_bit_buffer_damaged[mb_index] = 1;
 
             block += 64;
             mb++;
         }
 
+        if (mb_bit_buffer_damaged[mb_index] > 0)
+            continue;
+
         /* pass 2: we can do it just after */
         ff_dlog(avctx, "***pass 2 size=%d MB#=%d\n", put_bits_count(&pb), mb_index);
         block = block1;
@@ -367,6 +377,8 @@ static int dv_decode_video_segment(AVCodecContext *avctx, void *arg)
                 /* if still not finished, no need to parse other blocks */
                 if (mb->pos < 64)
                     break;
+                if (mb->pos < 127)
+                    vs_bit_buffer_damaged = mb_bit_buffer_damaged[mb_index] = 1;
             }
         }
         /* all blocks are finished, so the extra bytes can be used at
@@ -384,17 +396,25 @@ static int dv_decode_video_segment(AVCodecContext *avctx, void *arg)
     flush_put_bits(&vs_pb);
     for (mb_index = 0; mb_index < 5; mb_index++) {
         for (j = 0; j < s->sys->bpm; j++) {
-            if (mb->pos < 64 && get_bits_left(&gb) > 0) {
+            if (mb->pos < 64 && get_bits_left(&gb) > 0 && !vs_bit_buffer_damaged) {
                 ff_dlog(avctx, "start %d:%d\n", mb_index, j);
                 dv_decode_ac(&gb, mb, block);
             }
-            if (mb->pos >= 64 && mb->pos < 127)
+
+            if (mb->pos >= 64 && mb->pos < 127) {
                 av_log(avctx, AV_LOG_ERROR,
                        "AC EOB marker is absent pos=%d\n", mb->pos);
+                vs_bit_buffer_damaged = 1;
+            }
             block += 64;
             mb++;
         }
     }
+    if (vs_bit_buffer_damaged && !retried) {
+        av_log(avctx, AV_LOG_ERROR, "Concealing bitstream errors\n");
+        retried = 1;
+        goto retry;
+    }
 
     /* compute idct and place blocks */
     block = &sblock[0][0];