avformat: Do not search through the AVOption table for a option not in it repeatedly on each packet

This search takes alot of time especially when compared with small packets

46631 decicycles -> 15719 decicycles in read_frame_internal() for amr-nb in 3gp

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/demux.c

@@ -1407,12 +1407,15 @@ FF_ENABLE_DEPRECATION_WARNINGS
         }
     }
 
-    av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);
-    if (metadata) {
-        s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
-        av_dict_copy(&s->metadata, metadata, 0);
-        av_dict_free(&metadata);
-        av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);
+    if (!si->metafree) {
+        int metaret = av_opt_get_dict_val(s, "metadata", AV_OPT_SEARCH_CHILDREN, &metadata);
+        if (metadata) {
+            s->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
+            av_dict_copy(&s->metadata, metadata, 0);
+            av_dict_free(&metadata);
+            av_opt_set_dict_val(s, "metadata", NULL, AV_OPT_SEARCH_CHILDREN);
+        }
+        si->metafree = metaret == AVERROR_OPTION_NOT_FOUND;
     }
 
     if (s->debug & FF_FDEBUG_TS)

libavformat/internal.h

@@ -183,6 +183,11 @@ typedef struct FFFormatContext {
      * Set if chapter ids are strictly monotonic.
      */
     int chapter_ids_monotonic;
+
+    /**
+     * Contexts and child contexts do not contain a metadata option
+     */
+    int metafree;
 } FFFormatContext;
 
 static av_always_inline FFFormatContext *ffformatcontext(AVFormatContext *s)