avcodec/h264_slice: Check picture structure before setting the related fields

This might fix a hypothetical race condition

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit f111831ed61103f9fa8fdda41473a23da016bdaa)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Conflicts:

	libavcodec/h264_slice.c

Conflicts:

	libavcodec/h264.c

1 files changed, 16 insertions, 14 deletions

diff --git a/libavcodec/h264.c b/libavcodec/h264.c
index dfbab6687a..3e621041ff 100644
--- a/libavcodec/h264.c
+++ b/libavcodec/h264.c
@@ -3148,7 +3148,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
     int must_reinit;
     int needs_reinit = 0;
     int first_slice = h == h0 && !h0->current_slice;
-    int frame_num;
+    int frame_num, picture_structure, droppable;
     PPS *pps;
 
     h->me.qpel_put = h->h264qpel.put_h264_qpel_pixels_tab;
@@ -3342,37 +3342,32 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             return AVERROR_INVALIDDATA;
         }
     }
-    h->frame_num = frame_num;
 
     h->mb_mbaff        = 0;
     h->mb_aff_frame    = 0;
     last_pic_structure = h0->picture_structure;
     last_pic_droppable = h0->droppable;
-    h->droppable       = h->nal_ref_idc == 0;
+    droppable          = h->nal_ref_idc == 0;
     if (h->sps.frame_mbs_only_flag) {
-        h->picture_structure = PICT_FRAME;
+        picture_structure = PICT_FRAME;
     } else {
         if (!h->sps.direct_8x8_inference_flag && slice_type == AV_PICTURE_TYPE_B) {
             av_log(h->avctx, AV_LOG_ERROR, "This stream was generated by a broken encoder, invalid 8x8 inference\n");
             return -1;
         }
         if (get_bits1(&h->gb)) { // field_pic_flag
-            h->picture_structure = PICT_TOP_FIELD + get_bits1(&h->gb); // bottom_field_flag
+            picture_structure = PICT_TOP_FIELD + get_bits1(&h->gb); // bottom_field_flag
         } else {
-            h->picture_structure = PICT_FRAME;
+            picture_structure = PICT_FRAME;
             h->mb_aff_frame      = h->sps.mb_aff;
         }
     }
-    h->mb_field_decoding_flag = h->picture_structure != PICT_FRAME;
-
-    if (h0->current_slice != 0) {
-        if (last_pic_structure != h->picture_structure ||
-            last_pic_droppable != h->droppable) {
+    if (h0->current_slice) {
+        if (last_pic_structure != picture_structure ||
+            last_pic_droppable != droppable) {
             av_log(h->avctx, AV_LOG_ERROR,
                    "Changing field mode (%d -> %d) between slices is not allowed\n",
                    last_pic_structure, h->picture_structure);
-            h->picture_structure = last_pic_structure;
-            h->droppable         = last_pic_droppable;
             return AVERROR_INVALIDDATA;
         } else if (!h0->cur_pic_ptr) {
             av_log(h->avctx, AV_LOG_ERROR,
@@ -3380,7 +3375,14 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
                    h0->current_slice + 1);
             return AVERROR_INVALIDDATA;
         }
-    } else {
+    }
+
+    h->picture_structure = picture_structure;
+    h->droppable         = droppable;
+    h->frame_num         = frame_num;
+    h->mb_field_decoding_flag = picture_structure != PICT_FRAME;
+
+    if (h0->current_slice == 0) {
         /* Shorten frame num gaps so we don't have to allocate reference
          * frames just to throw them away */
         if (h->frame_num != h->prev_frame_num) {