lavfi/metadata: allow deleting all metadata

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

doc/filters.texi

@@ -16059,7 +16059,8 @@ Modify value of already present key.
 
 @item delete
 If @code{value} is set, delete only keys that have such value.
-Otherwise, delete key.
+Otherwise, delete key. If @code{key} is not set, delete all metadata values in
+the frame.
 
 @item print
 Print key and its value if metadata was found. If @code{key} is not set print all
@@ -16067,7 +16068,7 @@ metadata values available in frame.
 @end table
 
 @item key
-Set key used with all modes. Must be set for all modes except @code{print}.
+Set key used with all modes. Must be set for all modes except @code{print} and @code{delete}.
 
 @item value
 Set metadata value which will be used. This option is mandatory for

libavfilter/f_metadata.c

@@ -194,7 +194,7 @@ static av_cold int init(AVFilterContext *ctx)
     MetadataContext *s = ctx->priv;
     int ret;
 
-    if (!s->key && s->mode != METADATA_PRINT) {
+    if (!s->key && s->mode != METADATA_PRINT && s->mode != METADATA_DELETE) {
         av_log(ctx, AV_LOG_WARNING, "Metadata key must be set\n");
         return AVERROR(EINVAL);
     }
@@ -328,7 +328,9 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
         return ff_filter_frame(outlink, frame);
         break;
     case METADATA_DELETE:
-        if (e && e->value && s->value && s->compare(s, e->value, s->value)) {
+        if (!s->key) {
+            av_dict_free(metadata);
+        } else if (e && e->value && s->value && s->compare(s, e->value, s->value)) {
             av_dict_set(metadata, s->key, NULL, 0);
         } else if (e && e->value) {
             av_dict_set(metadata, s->key, NULL, 0);