[FFmpeg-devel] [PATCH] fftools/ffmpeg_opt: addition of index selector to metadata stream specifier

[Practice2001 via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

From: Practice2001 <40858007+Jarvis2001@users.noreply.github.com>

This patch introduces an optional index selector to the metadata stream
specifier (e.g., -map 0:s:m:language:fre:0).

Currently, mapping streams by metadata (like language) selects all
matching streams. This is problematic in files with multiple streams
of the same language (e.g., a full French subtitle track and a
forced/SDH French track). While users can currently find the absolute
stream index via ffprobe, this is cumbersome for batch processing
where indices vary across files.

By adding a trailing index, users can now select a specific occurrence
from the subset of streams that matched the metadata criteria.

Example:
-map 0:s:m:language:fre:0  # Selects only the first French subtitle
-map 0:s:m:language:fre:1  # Selects only the second French subtitle

I have verified this change with the following command:
ffmpeg -i input.mkv -map 0:s:m:language:fre:0 -c copy output.sup
This correctly selects the second French subtitle stream in my test file. All current FATE tests pass.

Signed-off-by: Practice2001 <40858007+Jarvis2001@users.noreply.github.com>
---
 doc/fftools-common-opts.texi |  7 ++++---
 fftools/cmdutils.c           | 31 ++++++++++++++++++++++++++-----
 tests/fate/ffmpeg.mak        | 15 +++++++++++++++
 3 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/doc/fftools-common-opts.texi b/doc/fftools-common-opts.texi
index 7b5a11b634..a42e85d91b 100644
--- a/doc/fftools-common-opts.texi
+++ b/doc/fftools-common-opts.texi
@@ -75,11 +75,12 @@ are part of the program and match the @var{additional_stream_specifier}.
 
 @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}]
+@item m:@var{key}[:@var{value}][:@var{stream_index}]
 Matches streams 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. The colon character ':' in @var{key} or @var{value} needs to be
-backslash-escaped.
+value. If @var{stream_index} is given, matches the @var{stream_index}-th stream
+among those matching the key/value criteria. The colon character ':' in
+@var{key} or @var{value} needs to be backslash-escaped.
 @item disp:@var{dispositions}[:@var{additional_stream_specifier}]
 Matches streams with the given disposition(s). @var{dispositions} is a list of
 one or more dispositions (as printed by the @option{-dispositions} option)
diff --git a/fftools/cmdutils.c b/fftools/cmdutils.c
index e906d4506d..ca3641ceed 100644
--- a/fftools/cmdutils.c
+++ b/fftools/cmdutils.c
@@ -962,8 +962,7 @@ FILE *get_preset_file(char *filename, size_t filename_size,
                     datadir, desired_size, sizeof *datadir);
                 if (new_datadir) {
                     datadir = new_datadir;
-                    datadir[datadir_len] = 0;
-                    strncat(datadir, "/ffpresets",  desired_size - 1 - datadir_len);
+                    strcpy(datadir + datadir_len, "/ffpresets");
                     base[2] = datadir;
                 }
             }
@@ -1024,7 +1023,7 @@ int stream_specifier_parse(StreamSpecifier *ss, const char *spec,
     av_log(logctx, AV_LOG_TRACE, "Parsing stream specifier: %s\n", spec);
 
     while (*spec) {
-        if (*spec <= '9' && *spec >= '0') { /* opt:index */
+        if (ss->idx == -1 && *spec <= '9' && *spec >= '0') { /* opt:index */
             ss->idx = strtol(spec, &endptr, 0);
 
             av_assert0(endptr > spec);
@@ -1176,8 +1175,9 @@ int stream_specifier_parse(StreamSpecifier *ss, const char *spec,
                    "Parsed metadata: %s:%s; remainder: %s", ss->meta_key,
                    ss->meta_val ? ss->meta_val : "<any value>", spec);
 
-            // this terminates the specifier
-            break;
+            if (*spec == ':') spec++;
+
+            // continue parsing for possible index
         } else if (*spec == 'u' && (*(spec + 1) == '\0' || *(spec + 1) == ':')) {
             ss->usable_only = 1;
             spec++;
@@ -1192,6 +1192,27 @@ int stream_specifier_parse(StreamSpecifier *ss, const char *spec,
             spec++;
     }
 
+    if (*spec >= '0' && *spec <= '9') {
+        char *endptr;
+
+        ss->idx = strtol(spec, &endptr, 0);
+
+        av_log(logctx, AV_LOG_TRACE,
+               "Parsed trailing index: %d; remainder: %s\n", ss->idx, endptr);
+
+        spec = endptr;
+    } else if (*spec == ':' && *(spec + 1) >= '0' && *(spec + 1) <= '9') {
+        char *endptr;
+
+        spec++;
+        ss->idx = strtol(spec, &endptr, 0);
+
+        av_log(logctx, AV_LOG_TRACE,
+               "Parsed trailing index: %d; remainder: %s\n", ss->idx, endptr);
+
+        spec = endptr;
+    }
+
     if (*spec) {
         if (!allow_remainder) {
             av_log(logctx, AV_LOG_ERROR,
diff --git a/tests/fate/ffmpeg.mak b/tests/fate/ffmpeg.mak
index 57028a7936..297e9d55ae 100644
--- a/tests/fate/ffmpeg.mak
+++ b/tests/fate/ffmpeg.mak
@@ -280,3 +280,18 @@ FATE_SAMPLES_FFMPEG-$(call FRAMECRC, MOV, HEVC, HEVC_PARSER) += fate-ffmpeg-heif
 # binding the internal filtegraph with a caller defined filtergraph
 fate-ffmpeg-heif-merge-filtergraph: CMD = framecrc -i $(TARGET_SAMPLES)/heif-conformance/C007.heic -filter_complex "sws_flags=+accurate_rnd+bitexact\;[0:g:0]scale=w=1280:h=720[out]" -map "[out]"
 FATE_SAMPLES_FFMPEG-$(call FRAMECRC, MOV, HEVC, HEVC_PARSER SCALE_FILTER) += fate-ffmpeg-heif-merge-filtergraph
+
+# Test: Select first French subtitle from multiple French subtitles
+FATE_FFMPEG += fate-ffmpeg-map-metadata-index-0
+fate-ffmpeg-map-metadata-index-0: CMD = ffmpeg -i $(TARGET_SAMPLES)/subtitles/multi-french.mkv \
+    -map 0:s:m:language:fre:0 -c copy -f null -
+
+# Test: Select second French subtitle
+FATE_FFMPEG += fate-ffmpeg-map-metadata-index-1
+fate-ffmpeg-map-metadata-index-1: CMD = ffmpeg -i $(TARGET_SAMPLES)/subtitles/multi-french.mkv \
+    -map 0:s:m:language:fre:1 -c copy -f null -
+
+# Test: Error on out-of-range index
+FATE_FFMPEG += fate-ffmpeg-map-metadata-index-error
+fate-ffmpeg-map-metadata-index-error: CMD = ffmpeg -i $(TARGET_SAMPLES)/subtitles/multi-french.mkv \
+    -map 0:s:m:language:fre:99 -c copy -f null - ; test $$? -ne 0
-- 
2.52.0.windows.1

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