fftools/ffmpeg_filter: close all no-longer needed inputs

Currently, the thread loop of ffmpeg_filter essentially works like this:

while (1) {
    frame, idx = get_from_decoder();
    err = send_to_filter_graph(frame);
    if (err) { // i.e. EOF
        close_input(idx);
        continue;
    }

    while (filtered_frame = get_filtered_frame())
        send_to_encoder(filtered_frame);
}

The exact details are not 100% correct since the actual control flow is a bit
more complicated as a result of the scheduler, but this is the general flow.

Notably, this leaves the possibility of leaving a no-longer-needed input
permanently open if the filter graph starts producing infinite frames (during
the second loop) *after* it finishes reading from an input, e.g. in a filter
graph like -af atrim,apad.

This patch avoids this issue by always querying the status of all filter graph
inputs and explicitly closing any that were closed downstream; after each round
of reading output frames. As a result, information about the filtergraph being
closed can now propagate back upstream, even if the filter is no longer
requesting any input frames (i.e. input_idx == fg->nb_inputs).

Fixes: https://trac.ffmpeg.org/ticket/11061
See-Also: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20457#issuecomment-6208

Backported-from: 38a5fcc02c

During backporting, I had to change the signature of `close_input()` to
`void close_input(FilterGraph *fg, int input_idx)` since this version of
FFmpeg does not contain a reference to the input index in the InputFilterPriv.

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2024-03-07
 
 API changes, most recent first:
 
+2025-09-xx - xxxxxxxxxx - lavfi 10.5.100 - buffersrc.h
+  Add av_buffersrc_get_status().
+
 2024-09-23 - 6940a6de2f0 - lavu 59.38.100 - frame.h
   Add AV_FRAME_DATA_VIEW_ID.
 

fftools/ffmpeg_filter.c

@@ -2365,6 +2365,18 @@ finish:
     fps->dropped_keyframe |= fps->last_dropped && (frame->flags & AV_FRAME_FLAG_KEY);
 }
 
+static void close_input(FilterGraph *fg, int input_idx)
+{
+
+    InputFilterPriv *ifp = ifp_from_ifilter(fg->inputs[input_idx]);
+    FilterGraphPriv *fgp = fgp_from_fg(ifp->ifilter.graph);
+
+    if (!ifp->eof) {
+        sch_filter_receive_finish(fgp->sch, fgp->sch_idx, input_idx);
+        ifp->eof = 1;
+    }
+}
+
 static int close_output(OutputFilterPriv *ofp, FilterGraphThread *fgt)
 {
     FilterGraphPriv *fgp = fgp_from_fg(ofp->ofilter.graph);
@@ -3035,7 +3047,7 @@ static int filter_thread(void *arg)
         if (ret == AVERROR_EOF) {
             av_log(fg, AV_LOG_VERBOSE, "Input %u no longer accepts new data\n",
                    input_idx);
-            sch_filter_receive_finish(fgp->sch, fgp->sch_idx, input_idx);
+            close_input(fg, input_idx);
             continue;
         }
         if (ret < 0)
@@ -3052,6 +3064,13 @@ read_frames:
                    av_err2str(ret));
             goto finish;
         }
+
+        // ensure all inputs no longer accepting data are closed
+        for (int i = 0; fgt.graph && i < fg->nb_inputs; i++) {
+            InputFilterPriv *ifp = ifp_from_ifilter(fg->inputs[i]);
+            if (av_buffersrc_get_status(ifp->filter))
+                close_input(fg, i);
+        }
     }
 
     for (unsigned i = 0; i < fg->nb_outputs; i++) {

libavfilter/buffersrc.c

@@ -283,6 +283,16 @@ int av_buffersrc_close(AVFilterContext *ctx, int64_t pts, unsigned flags)
     return (flags & AV_BUFFERSRC_FLAG_PUSH) ? push_frame(ctx->graph) : 0;
 }
 
+int av_buffersrc_get_status(AVFilterContext *ctx)
+{
+    BufferSourceContext *s = ctx->priv;
+
+    if (!s->eof && ff_outlink_get_status(ctx->outputs[0]))
+        s->eof = 1;
+
+    return s->eof ? AVERROR(EOF) : 0;
+}
+
 static av_cold int init_video(AVFilterContext *ctx)
 {
     BufferSourceContext *c = ctx->priv;

libavfilter/buffersrc.h

@@ -208,6 +208,14 @@ int av_buffersrc_add_frame_flags(AVFilterContext *buffer_src,
  */
 int av_buffersrc_close(AVFilterContext *ctx, int64_t pts, unsigned flags);
 
+/**
+ * Returns 0 or a negative AVERROR code. Currently, this will only ever
+ * return AVERROR(EOF), to indicate that the buffer source has been closed,
+ * either as a result of av_buffersrc_close(), or because the downstream
+ * filter is no longer accepting new data.
+ */
+int av_buffersrc_get_status(AVFilterContext *ctx);
+
 /**
  * @}
  */

libavfilter/version.h

@@ -31,7 +31,7 @@
 
 #include "version_major.h"
 
-#define LIBAVFILTER_VERSION_MINOR   4
+#define LIBAVFILTER_VERSION_MINOR   5
 #define LIBAVFILTER_VERSION_MICRO 100