calculate frame_num for recovery point "keyframe"

recovery points that have non-zero recovery_frame_cnt resulted in
flagging incomplete frames as keyframes. This delays setting the
keyframe flag till the frame has been fully constructed.

libavcodec/h264.c

@@ -3714,9 +3714,19 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             if((err = decode_slice_header(hx, h)))
                break;
 
+            if (h->sei_recovery_frame_cnt >= 0) {
+                h->recovery_frame = (h->frame_num + h->sei_recovery_frame_cnt) %
+                                    (1 << h->sps.log2_max_frame_num);
+            }
+
             s->current_picture_ptr->f.key_frame |=
-                    (hx->nal_unit_type == NAL_IDR_SLICE) ||
-                    (h->sei_recovery_frame_cnt >= 0);
+                    (hx->nal_unit_type == NAL_IDR_SLICE);
+
+            if (h->recovery_frame == h->frame_num) {
+                s->current_picture_ptr->f.key_frame |= 1;
+                h->recovery_frame = -1;
+            }
+
             h->sync |= !!s->current_picture_ptr->f.key_frame;
             h->sync |= 3*!!(s->flags2 & CODEC_FLAG2_SHOW_ALL);
             s->current_picture_ptr->sync = h->sync;

libavcodec/h264.h

@@ -571,6 +571,13 @@ typedef struct H264Context{
      * frames.
      */
     int sei_recovery_frame_cnt;
+    /**
+     * recovery_frame is the frame_num at which the next frame should
+     * be fully constructed.
+     *
+     * Set to -1 when not expecting a recovery point.
+     */
+    int recovery_frame;
 
     int luma_weight_flag[2];   ///< 7.4.3.2 luma_weight_lX_flag
     int chroma_weight_flag[2]; ///< 7.4.3.2 chroma_weight_lX_flag