avcodec/pthread_frame, decode: allow errors to happen on draining

So, all frames and errors are correctly reported in order.
Also limit the numbers of error during draining to prevent infinite loop.

This fix fate failure with THREADS>=4:
  make fate-h264-attachment-631 THREADS=4
This also reverts a755b725.

Suggested-by: wm4, Ronald S. Bultje, Marton Balint
Reviewed-by: w4 <nfxjfg@googlemail.com>
Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>

libavcodec/decode.c

@@ -568,8 +568,24 @@ FF_ENABLE_DEPRECATION_WARNINGS
         avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
 #endif
 
-    if (avctx->internal->draining && !got_frame)
-        avci->draining_done = 1;
+    /* do not stop draining when got_frame != 0 or ret < 0 */
+    if (avctx->internal->draining && !got_frame) {
+        if (ret < 0) {
+            /* prevent infinite loop if a decoder wrongly always return error on draining */
+            /* reasonable nb_errors_max = maximum b frames + thread count */
+            int nb_errors_max = 20 + (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME ?
+                                avctx->thread_count : 1);
+
+            if (avci->nb_draining_errors++ >= nb_errors_max) {
+                av_log(avctx, AV_LOG_ERROR, "Too many errors when draining, this is a bug. "
+                       "Stop draining and force EOF.\n");
+                avci->draining_done = 1;
+                ret = AVERROR_BUG;
+            }
+        } else {
+            avci->draining_done = 1;
+        }
+    }
 
     avci->compat_decode_consumed += ret;
 
@@ -1659,6 +1675,7 @@ void avcodec_flush_buffers(AVCodecContext *avctx)
 {
     avctx->internal->draining      = 0;
     avctx->internal->draining_done = 0;
+    avctx->internal->nb_draining_errors = 0;
     av_frame_unref(avctx->internal->buffer_frame);
     av_frame_unref(avctx->internal->compat_decode_frame);
     av_packet_unref(avctx->internal->buffer_pkt);

libavcodec/internal.h

@@ -200,6 +200,9 @@ typedef struct AVCodecInternal {
     int showed_multi_packet_warning;
 
     int skip_samples_multiplier;
+
+    /* to prevent infinite loop on errors when draining */
+    int nb_draining_errors;
 } AVCodecInternal;
 
 struct AVCodecDefault {

libavcodec/pthread_frame.c

@@ -509,8 +509,8 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
     /*
      * Return the next available frame from the oldest thread.
      * If we're at the end of the stream, then we have to skip threads that
-     * didn't output a frame, because we don't want to accidentally signal
-     * EOF (avpkt->size == 0 && *got_picture_ptr == 0).
+     * didn't output a frame/error, because we don't want to accidentally signal
+     * EOF (avpkt->size == 0 && *got_picture_ptr == 0 && err >= 0).
      */
 
     do {
@@ -526,20 +526,19 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
         av_frame_move_ref(picture, p->frame);
         *got_picture_ptr = p->got_frame;
         picture->pkt_dts = p->avpkt.dts;
-
-        if (p->result < 0)
-            err = p->result;
+        err = p->result;
 
         /*
          * A later call with avkpt->size == 0 may loop over all threads,
-         * including this one, searching for a frame to return before being
+         * including this one, searching for a frame/error to return before being
          * stopped by the "finished != fctx->next_finished" condition.
-         * Make sure we don't mistakenly return the same frame again.
+         * Make sure we don't mistakenly return the same frame/error again.
          */
         p->got_frame = 0;
+        p->result = 0;
 
         if (finished >= avctx->thread_count) finished = 0;
-    } while (!avpkt->size && !*got_picture_ptr && finished != fctx->next_finished);
+    } while (!avpkt->size && !*got_picture_ptr && err >= 0 && finished != fctx->next_finished);
 
     update_context_from_thread(avctx, p->avctx, 1);