lavd/v4l2: produce a 0 byte packet when a dequeued buffer is flagged with V4L2_BUF_FLAG_ERROR

Fixes ticket #4030.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/v4l2.c

@@ -499,13 +499,14 @@ static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
     };
     int res;
 
+    pkt->size = 0;
+
     /* FIXME: Some special treatment might be needed in case of loss of signal... */
     while ((res = v4l2_ioctl(s->fd, VIDIOC_DQBUF, &buf)) < 0 && (errno == EINTR));
     if (res < 0) {
-        if (errno == EAGAIN) {
-            pkt->size = 0;
+        if (errno == EAGAIN)
             return AVERROR(EAGAIN);
-        }
+
         res = AVERROR(errno);
         av_log(ctx, AV_LOG_ERROR, "ioctl(VIDIOC_DQBUF): %s\n",
                av_err2str(res));
@@ -520,19 +521,25 @@ static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
     // always keep at least one buffer queued
     av_assert0(avpriv_atomic_int_get(&s->buffers_queued) >= 1);
 
+    if (buf.flags & V4L2_BUF_FLAG_ERROR) {
+        av_log(ctx, AV_LOG_WARNING,
+               "Dequeued v4l2 buffer contains corrupted data (%d bytes).\n",
+               buf.bytesused);
+        buf.bytesused = 0;
+    } else {
         /* CPIA is a compressed format and we don't know the exact number of bytes
-     * used by a frame, so set it here as the driver announces it.
-     */
+         * used by a frame, so set it here as the driver announces it. */
         if (ctx->video_codec_id == AV_CODEC_ID_CPIA)
             s->frame_size = buf.bytesused;
 
         if (s->frame_size > 0 && buf.bytesused != s->frame_size) {
             av_log(ctx, AV_LOG_ERROR,
-               "The v4l2 frame is %d bytes, but %d bytes are expected. Flags: 0x%08X\n",
+                   "Dequeued v4l2 buffer contains %d bytes, but %d were expected. Flags: 0x%08X.\n",
                    buf.bytesused, s->frame_size, buf.flags);
             enqueue_buffer(s, &buf);
             return AVERROR_INVALIDDATA;
         }
+    }
 
     /* Image is at s->buff_start[buf.index] */
     if (avpriv_atomic_int_get(&s->buffers_queued) == FFMAX(s->buffers / 8, 1)) {
@@ -586,7 +593,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
     pkt->pts = buf.timestamp.tv_sec * INT64_C(1000000) + buf.timestamp.tv_usec;
     convert_timestamp(ctx, &pkt->pts);
 
-    return s->buf_len[buf.index];
+    return pkt->size;
 }
 
 static int mmap_start(AVFormatContext *ctx)