lavfi: add ff_inlink_acknowledge_status().

Also introduce libavfilter/filters.h for all functions needed
to implement filters.

libavfilter/filters.h

@@ -0,0 +1,53 @@
+/*
+ * Filters implementation helper functions
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with FFmpeg; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVFILTER_FILTERS_H
+#define AVFILTER_FILTERS_H
+
+/**
+ * Filters implementation helper functions
+ */
+
+#include "avfilter.h"
+
+/**
+ * Test and acknowledge the change of status on the link.
+ *
+ * Status means EOF or an error condition; a change from the normal (0)
+ * status to a non-zero status can be queued in a filter's input link, it
+ * becomes relevant after the frames queued in the link's FIFO are
+ * processed. This function tests if frames are still queued and if a queued
+ * status change has not yet been processed. In that case it performs basic
+ * treatment (updating the link's timestamp) and returns a positive value to
+ * let the filter do its own treatments (flushing...).
+ *
+ * Filters implementing the activate callback should call this function when
+ * they think it might succeed (usually after checking unsuccessfully for a
+ * queued frame).
+ * Filters implementing the filter_frame and request_frame callbacks do not
+ * need to call that since the same treatment happens in ff_filter_frame().
+ *
+ * @param[out] rstatus  new or current status
+ * @param[out] rpts     current timestamp of the link in link time base
+ * @return  >0 if status changed, <0 if status already acked, 0 otherwise
+ */
+int ff_inlink_acknowledge_status(AVFilterLink *link, int *rstatus, int64_t *rpts);
+
+#endif /* AVFILTER_FILTERS_H */