avfilter/internal: Add flag to free filtered AVFrames generically

Up until now, an AVFilterPad's filter_frame callback was always
responsible for passing the frame to the next filter or freeing it.
Yet there are several filter_frame callbacks that need to free this
frame on every exit, so it makes sense to do this generically.
This commit adds a flag which if set shifts the responsibility to free
the frame to the caller of filter_frame.

In the future this might be extended to avoid allocating+freeing the
frame for filters with this flag set.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/avfilter.c

@@ -958,7 +958,7 @@ static int ff_filter_frame_framed(AVFilterLink *link, AVFrame *frame)
     int (*filter_frame)(AVFilterLink *, AVFrame *);
     AVFilterContext *dstctx = link->dst;
     AVFilterPad *dst = link->dstpad;
-    int ret;
+    int ret, needs_free = dst->flags & AVFILTERPAD_FLAG_GENERIC_FREE;
 
     if (!(filter_frame = dst->filter_frame))
         filter_frame = default_filter_frame;
@@ -973,9 +973,13 @@ static int ff_filter_frame_framed(AVFilterLink *link, AVFrame *frame)
     dstctx->is_disabled = !ff_inlink_evaluate_timeline_at_frame(link, frame);
 
     if (dstctx->is_disabled &&
-        (dstctx->filter->flags & AVFILTER_FLAG_SUPPORT_TIMELINE_GENERIC))
+        (dstctx->filter->flags & AVFILTER_FLAG_SUPPORT_TIMELINE_GENERIC)) {
         filter_frame = default_filter_frame;
+        needs_free   = 0;
+    }
     ret = filter_frame(link, frame);
+    if (needs_free)
+        av_frame_free(&frame);
     link->frame_count_out++;
     return ret;
 

libavfilter/internal.h

@@ -68,6 +68,14 @@ struct AVFilterPad {
      */
 #define AVFILTERPAD_FLAG_NEEDS_WRITABLE                  (1 << 0)
 
+    /**
+     * AVFilterPad.filter_frame does not free its input frame at all
+     * and expects its caller to do so all the time.
+     *
+     * input pads only.
+     */
+#define AVFILTERPAD_FLAG_GENERIC_FREE                    (1 << 1)
+
     /**
      * A combination of AVFILTERPAD_FLAG_* flags.
      */
@@ -97,7 +105,8 @@ struct AVFilterPad {
      *
      * @return >= 0 on success, a negative AVERROR on error. This function
      * must ensure that frame is properly unreferenced on error if it
-     * hasn't been passed on to another filter.
+     * hasn't been passed on to another filter unless the
+     * AVFILTERPAD_FLAG_GENERIC_FREE flag is set.
      */
     int (*filter_frame)(AVFilterLink *link, AVFrame *frame);