command: always allow setting volume/mute properties

This seems generally easier when using libmpv (and was already requested
and implemented before: see commit 327a779a; it was reverted some time
later).

With the weird internal logic we have to deal with, in particular the
--softvol=no case (using system volume), and using the audio API's mixer
(--softvol=auto on some systems), we still can't avoid all glitches and
corner cases that complicate this issue so much. The API user is either
recommended to use --softvol=yes or auto, or to watch the new
mixer-active property, and assume the volume/mute properties have
significant values if the mixer is active.

Remaining glitches:
- changing the volume/mute properties has no effect if no internal mixer
  is used (--softvol=no) and the mixer is not active; the actual mixer
  controls do not change, only the property values
- --volume/--mute do not have an effect on the volume/mute properties
  before mixer initialization (the options strictly are only applied
  during mixer init)
- volume-max is 100 while the mixer is not active

DOCS/interface-changes.rst

@@ -29,6 +29,9 @@ Interface changes
       instead of "sample"
     - change VO opengl scaler-resizes-only default to enabled
     - add VO opengl "tscale-interpolates-only" suboption
+    - make "volume" and "mute" properties changeable even if no audio output is
+      active (this gives not-ideal behavior if --softvol=no is used)
+    - add "volume-max" and "mixer-active" properties
  --- mpv 0.15.0 ---
     - change "yadif" video filter defaults
  --- mpv 0.14.0 ---

DOCS/man/input.rst

@@ -1224,11 +1224,30 @@ Property list
 ``hr-seek`` (RW)
     See ``--hr-seek``.
 
+``mixer-active``
+    Return ``yes`` if the audio mixer is active, ``no`` otherwise. This has
+    implications for ``--softvol=no`` mode: if the mixer is active, changing
+    ``volume`` doesn't actually change anything on the system mixer. If the
+    ``--volume`` or ``--mute`` option are used, these might not be applied
+    property until the mixer becomes active either. (The options, if set, will
+    just overwrite the mixer state at audio initialization.)
+
+    While the behavior with ``mixer-active==yes`` is relatively well-defined,
+    the ``no`` case will provide possibly wrong or insignificant values.
+
+    Note that an active mixer does not necessarily imply active audio output,
+    although this is implied in the current implementation.
+
 ``volume`` (RW)
-    Current volume (see ``--volume`` for details).
+    Current volume (see ``--volume`` for details). Also see ``mixer-active``
+    property.
+
+``volume-max``
+    Current maximum value the volume property can be set to. (This may depend
+    on the ``--softvol-max`` option.)
 
 ``mute`` (RW)
-    Current mute status (``yes``/``no``).
+    Current mute status (``yes``/``no``). Also see ``mixer-active`` property.
 
 ``audio-delay`` (RW)
     See ``--audio-delay``.

audio/mixer.c

@@ -399,4 +399,5 @@ void mixer_uninit_audio(struct mixer *mixer)
     }
     mixer->ao = NULL;
     mixer->af = NULL;
+    mixer->softvol = false;
 }

player/command.c

@@ -1538,13 +1538,18 @@ static int mp_property_partially_seekable(void *ctx, struct m_property *prop,
     return m_property_flag_ro(action, arg, mpctx->demuxer->partially_seekable);
 }
 
+static int mp_property_mixer_active(void *ctx, struct m_property *prop,
+                                    int action, void *arg)
+{
+    MPContext *mpctx = ctx;
+    return m_property_flag_ro(action, arg, mixer_audio_initialized(mpctx->mixer));
+}
+
 /// Volume (RW)
 static int mp_property_volume(void *ctx, struct m_property *prop,
                               int action, void *arg)
 {
     MPContext *mpctx = ctx;
-    if (!mixer_audio_initialized(mpctx->mixer))
-        return M_PROPERTY_UNAVAILABLE;
     switch (action) {
     case M_PROPERTY_GET:
         mixer_getbothvolume(mpctx->mixer, arg);
@@ -1578,13 +1583,18 @@ static int mp_property_volume(void *ctx, struct m_property *prop,
     return M_PROPERTY_NOT_IMPLEMENTED;
 }
 
+static int mp_property_volume_max(void *ctx, struct m_property *prop,
+                                  int action, void *arg)
+{
+    MPContext *mpctx = ctx;
+    return m_property_float_ro(action, arg, mixer_getmaxvolume(mpctx->mixer));
+}
+
 /// Mute (RW)
 static int mp_property_mute(void *ctx, struct m_property *prop,
                             int action, void *arg)
 {
     MPContext *mpctx = ctx;
-    if (!mixer_audio_initialized(mpctx->mixer))
-        return M_PROPERTY_ERROR;
     switch (action) {
     case M_PROPERTY_SET:
         mixer_setmute(mpctx->mixer, *(int *) arg);
@@ -3553,7 +3563,9 @@ static const struct m_property mp_properties[] = {
     M_PROPERTY_ALIAS("playlist-count", "playlist/count"),
 
     // Audio
+    {"mixer-active", mp_property_mixer_active},
     {"volume", mp_property_volume},
+    {"volume-max", mp_property_volume_max},
     {"mute", mp_property_mute},
     {"audio-delay", mp_property_audio_delay},
     {"audio-codec-name", mp_property_audio_codec_name},
@@ -3734,7 +3746,7 @@ static const char *const *const mp_event_property_change[] = {
     E(MPV_EVENT_AUDIO_RECONFIG, "audio-format", "audio-codec", "audio-bitrate",
       "samplerate", "channels", "audio", "volume", "mute", "balance",
       "volume-restore-data", "current-ao", "audio-codec-name", "audio-params",
-      "audio-out-params"),
+      "audio-out-params", "volume-max", "mixer-active"),
     E(MPV_EVENT_SEEK, "seeking", "core-idle", "eof-reached"),
     E(MPV_EVENT_PLAYBACK_RESTART, "seeking", "core-idle", "eof-reached"),
     E(MPV_EVENT_METADATA_UPDATE, "metadata", "filtered-metadata", "media-title"),