command: add a way to access properties of a current track

Requested. Should be good for simple use cases. "sub2" is technically
inconsistent (since the option is called --secondary-sid), but fuck the
consistent name.

DOCS/man/input.rst

@@ -2780,6 +2780,26 @@ Property list
                 "replaygain-album-peak" MPV_FORMAT_DOUBLE
                 "replaygain-album-gain" MPV_FORMAT_DOUBLE
 
+``current-tracks/...``
+    This gives access to currently selected tracks. It redirects to the correct
+    entry in ``track-list``.
+
+    The following sub-entries are defined: ``video``, ``audio``, ``sub``,
+    ``sub2``
+
+    For example, ``current-tracks/audio/lang`` returns the current audio track's
+    language field (the same value as ``track-list/N/lang``).
+
+    A sub-entry is accessible only if a track of that type is actually selected.
+    Tracks selected via ``--lavfi-complex`` never appear under this property.
+    ``current-tracks`` and ``current-tracks/`` are currently not accessible, and
+    will not return anything.
+
+    Scripts etc. should not use this. They should use ``track-list``, loop over
+    all tracks, and inspect the ``selected`` field to test whether a track is
+    selected (or compare the ``id`` field to the ``video`` / ``audio`` etc.
+    options).
+
 ``chapter-list``
     List of chapters, current entry marked. Currently, the raw property value
     is useless.

player/command.c

@@ -2025,6 +2025,53 @@ static int property_list_tracks(void *ctx, struct m_property *prop,
                                 get_track_entry, mpctx);
 }
 
+static int property_current_tracks(void *ctx, struct m_property *prop,
+                                   int action, void *arg)
+{
+    MPContext *mpctx = ctx;
+
+    if (action != M_PROPERTY_KEY_ACTION)
+        return M_PROPERTY_UNAVAILABLE;
+
+    int type = -1;
+    int order = 0;
+
+    struct m_property_action_arg *ka = arg;
+    bstr key;
+    char *rem;
+    m_property_split_path(ka->key, &key, &rem);
+
+    if (bstr_equals0(key, "video")) {
+        type = STREAM_VIDEO;
+    } else if (bstr_equals0(key, "audio")) {
+        type = STREAM_AUDIO;
+    } else if (bstr_equals0(key, "sub")) {
+        type = STREAM_SUB;
+    } else if (bstr_equals0(key, "sub2")) {
+        type = STREAM_SUB;
+        order = 1;
+    }
+
+    if (type < 0)
+        return M_PROPERTY_UNKNOWN;
+
+    struct track *t = mpctx->current_track[order][type];
+    if (!t)
+        return M_PROPERTY_UNAVAILABLE;
+
+    int index = -1;
+    for (int n = 0; n < mpctx->num_tracks; n++) {
+        if (mpctx->tracks[n] == t) {
+            index = n;
+            break;
+        }
+    }
+    assert(index >= 0);
+
+    char *name = mp_tprintf(80, "track-list/%d/%s", index, rem);
+    return mp_property_do(name, ka->action, ka->arg, ctx);
+}
+
 static int mp_property_hwdec_current(void *ctx, struct m_property *prop,
                                      int action, void *arg)
 {
@@ -3445,6 +3492,7 @@ static const struct m_property mp_properties_base[] = {
 
     {"chapter-list", mp_property_list_chapters},
     {"track-list", property_list_tracks},
+    {"current-tracks", property_current_tracks},
     {"edition-list", property_list_editions},
 
     {"playlist", mp_property_playlist},
@@ -3584,7 +3632,7 @@ static const char *const *const mp_event_property_change[] = {
     E(MPV_EVENT_END_FILE, "*"),
     E(MPV_EVENT_FILE_LOADED, "*"),
     E(MP_EVENT_CHANGE_ALL, "*"),
-    E(MPV_EVENT_TRACKS_CHANGED, "track-list"),
+    E(MPV_EVENT_TRACKS_CHANGED, "track-list", "current-tracks"),
     E(MPV_EVENT_IDLE, "*"),
     E(MPV_EVENT_TICK, "time-pos", "audio-pts", "stream-pos", "avsync",
       "percent-pos", "time-remaining", "playtime-remaining", "playback-time",