avfilter/buffersrc: add av_buffersrc_get_status()

There is currently no way for API users to know that a buffersrc is no longer
accepting input, except by trying to feed it a frame and seeing what happens.

Of course, this is not possible if the user does not *have* a frame to feed,
but may still wish to know if the filter is still accepting input or not.

Since passing `frame == NULL` to `av_buffersrc_add_frame()` is already treated
as closing the input, we are left with no choice but to introduce a new
function for this.

We don't explicitly return the result of `ff_outlink_get_status()` to avoid
leaking internal status codes, and instead translate them all to AVERROR(EOF).

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
 
 API changes, most recent first:
 
+2025-09-xx - xxxxxxxxxx - lavfi 11.10.100 - buffersrc.h
+  Add av_buffersrc_get_status().
+
 2025-11-18 - xxxxxxxxxx - lavu 60.19.100 - hwcontext_amf.h
   avutil/hwcontext_amf: add lock and unlock for AVAMFDeviceContext.
 

libavfilter/buffersrc.c

@@ -286,6 +286,16 @@ 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

@@ -216,6 +216,14 @@ 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   9
+#define LIBAVFILTER_VERSION_MINOR  10
 #define LIBAVFILTER_VERSION_MICRO 100