avformat/utils: Stream specifier enhancement 2.

In some cases, mainly working with multiprogram mpeg-ts containers as
input, it would be handy to select sub stream of a specific program by
their metadata.
This patch makes it possible to narrow the stream selection among
streams of the specified program by stream metadata.

Examples:
p:601:m:language:hun  will select all sub streams of program with id 601
where sub streams have metadata key named 'language' with value 'hun'.
p:602:m:guide  will select all sub streams of program with id 602 where
sub streams have metadata key named 'guide'.
Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/fftools-common-opts.texi

@@ -42,14 +42,20 @@ streams, 'V' only matches video streams which are not attached pictures, video
 thumbnails or cover arts.  If @var{stream_index} is given, then it matches
 stream number @var{stream_index} of this type. Otherwise, it matches all
 streams of this type.
-@item p:@var{program_id}[:@var{stream_index}] or p:@var{program_id}[:@var{stream_type}[:@var{stream_index}]]
+@item p:@var{program_id}[:@var{stream_index}] or p:@var{program_id}[:@var{stream_type}[:@var{stream_index}]] or
+p:@var{program_id}:m:@var{key}[:@var{value}]
 In first version, if @var{stream_index} is given, then it matches the stream with number @var{stream_index}
 in the program with the id @var{program_id}. Otherwise, it matches all streams in the
-program. In the latter version, @var{stream_type} is one of following: 'v' for video, 'a' for audio, 's'
+program. In the second version, @var{stream_type} is one of following: 'v' for video, 'a' for audio, 's'
 for subtitle, 'd' for data. If @var{stream_index} is also given, then it matches
 stream number @var{stream_index} of this type in the program with the id @var{program_id}.
 Otherwise, if only @var{stream_type} is given, it matches all
 streams of this type in the program with the id @var{program_id}.
+In the third version matches streams in the program with the id @var{program_id} with the metadata
+tag @var{key} having the specified value. If
+@var{value} is not given, matches streams that contain the given tag with any
+value.
+
 @item #@var{stream_id} or i:@var{stream_id}
 Match the stream by stream id (e.g. PID in MPEG-TS container).
 @item m:@var{key}[:@var{value}]

libavformat/utils.c

@@ -5124,6 +5124,34 @@ FF_ENABLE_DEPRECATION_WARNINGS
                             }
                         return 0;
                     }
+
+                } else if ( *endptr == 'm') { // p:<id>:m:<metadata_spec>
+                    AVDictionaryEntry *tag;
+                    char *key, *val;
+                    int ret = 0;
+
+                    if (*(++endptr) != ':') {
+                        av_log(s, AV_LOG_ERROR, "Invalid stream specifier syntax, missing ':' sign after :m.\n");
+                        return AVERROR(EINVAL);
+                    }
+
+                    val = strchr(++endptr, ':');
+                    key = val ? av_strndup(endptr, val - endptr) : av_strdup(endptr);
+                    if (!key)
+                        return AVERROR(ENOMEM);
+
+                    for (j = 0; j < s->programs[i]->nb_stream_indexes; j++)
+                        if (st->index == s->programs[i]->stream_index[j]) {
+                            tag = av_dict_get(st->metadata, key, NULL, 0);
+                            if (tag && (!val || !strcmp(tag->value, val + 1)))
+                                ret = 1;
+
+                            break;
+                        }
+
+                    av_freep(&key);
+                    return ret;
+
                 } else {  // p:<id>:<index>
                     int stream_idx = strtol(endptr, NULL, 0);
                     return stream_idx >= 0 &&