audio: add option for falling back to ao_null

The manpage entry explains this.

(Maybe this option could be always enabled and removed. I don't quite
remember what valid use-cases there are for just disabling audio
entirely, other than that this is also needed for audio decoder init
failure.)

DOCS/interface-changes.rst

@@ -20,6 +20,7 @@ Interface changes
 ::
 
  --- mpv 0.12.0 ---
+    - add --audio-fallback-to-null option
     - replace vf_format outputlevels suboption with "video-output-levels" global
       property/option; also remove "colormatrix-output-range" property
     - vo_opengl: remove sharpen3/sharpen5 scale filters, add sharpen sub-option

DOCS/man/options.rst

@@ -917,6 +917,14 @@ Audio
 
     Currently not implemented for most AOs.
 
+``--audio-fallback-to-null=<yes|no>``
+    If no audio device can be opened, behave as if ``--ao=null`` was given. This
+    is useful in combination with ``--audio-device``: instead of causing an
+    error if the selected device does not exist, the client API user (or a
+    Lua script) could let playback continue normally, and check the
+    ``current-ao`` and ``audio-device-list`` properties to make high-level
+    decisions about how to continue.
+
 ``--ao=<driver1[:suboption1[=value]:...],driver2,...[,]>``
     Specify a priority list of audio output drivers to be used. For
     interactive use one would normally specify a single one to use, but in

audio/out/ao.c

@@ -240,6 +240,7 @@ static void split_ao_device(void *tmp, char *opt, char **out_ao, char **out_dev)
 }
 
 struct ao *ao_init_best(struct mpv_global *global,
+                        bool ao_null_fallback,
                         struct input_ctx *input_ctx,
                         struct encode_lavc_context *encode_lavc_ctx,
                         int samplerate, int format, struct mp_chmap channels)
@@ -282,6 +283,11 @@ struct ao *ao_init_best(struct mpv_global *global,
         }
     }
 
+    if (ao_null_fallback) {
+        MP_TARRAY_APPEND(tmp, ao_list, ao_num,
+            (struct m_obj_settings){.name = "null"});
+    }
+
     for (int n = 0; n < ao_num; n++) {
         struct m_obj_settings *entry = &ao_list[n];
         bool probing = n + 1 != ao_num;

audio/out/ao.h

@@ -72,6 +72,7 @@ struct encode_lavc_context;
 struct mp_audio;
 
 struct ao *ao_init_best(struct mpv_global *global,
+                        bool ao_null_fallback,
                         struct input_ctx *input_ctx,
                         struct encode_lavc_context *encode_lavc_ctx,
                         int samplerate, int format, struct mp_chmap channels);

options/options.c

@@ -383,6 +383,7 @@ const m_option_t mp_opts[] = {
     OPT_SETTINGSLIST("ao-defaults", ao_defs, 0, &ao_obj_list),
     OPT_STRING("audio-device", audio_device, 0),
     OPT_STRING("audio-client-name", audio_client_name, 0),
+    OPT_FLAG("audio-fallback-to-null", ao_null_fallback, 0),
     OPT_CHOICE("force-window", force_vo, 0,
                ({"no", 0}, {"yes", 1}, {"immediate", 2})),
     OPT_FLAG("ontop", vo.ontop, M_OPT_FIXED),

options/options.h

@@ -81,6 +81,7 @@ typedef struct MPOpts {
     struct m_obj_settings *audio_driver_list, *ao_defs;
     char *audio_device;
     char *audio_client_name;
+    int ao_null_fallback;
     int force_vo;
     int softvol;
     float mixer_init_volume;

player/audio.c

@@ -260,11 +260,15 @@ void reinit_audio_chain(struct MPContext *mpctx)
     }
 
     if (!mpctx->ao) {
+        bool spdif_fallback = af_fmt_is_spdif(afs->output.format) &&
+                              mpctx->d_audio->spdif_passthrough;
+        bool ao_null_fallback = opts->ao_null_fallback && !spdif_fallback;
+
         mp_chmap_remove_useless_channels(&afs->output.channels,
                                          &opts->audio_output_channels);
         mp_audio_set_channels(&afs->output, &afs->output.channels);
 
-        mpctx->ao = ao_init_best(mpctx->global, mpctx->input,
+        mpctx->ao = ao_init_best(mpctx->global, ao_null_fallback, mpctx->input,
                                  mpctx->encode_lavc_ctx, afs->output.rate,
                                  afs->output.format, afs->output.channels);
 
@@ -283,9 +287,7 @@ void reinit_audio_chain(struct MPContext *mpctx)
 
         if (!mpctx->ao) {
             // If spdif was used, try to fallback to PCM.
-            if (af_fmt_is_spdif(afs->output.format) &&
-                mpctx->d_audio->spdif_passthrough)
-            {
+            if (spdif_fallback) {
                 mpctx->d_audio->spdif_passthrough = false;
                 if (!audio_init_best_codec(mpctx->d_audio))
                     goto init_error;