From 68b08c5963427b9a9ce1e7c8d9a7c2d12f638c7d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20M=C3=BCller?= <deso@posteo.net>
Date: Sat, 22 Oct 2022 21:00:28 -0700
Subject: [PATCH] Add support for optional slaves to multi device

The "multi" ALSA device requires all defined slaves to be present.
That's typically fine for hardware devices that do not vanish when
turned off, such as sound cards. However, for Bluetooth devices, for
example, that is not the case. Those will fail to open and, hence, cause
the entire multi device to fail. That in turn stops any sound from being
emitted.
With this change we enhance said multi devices with support for optional
slaves. This optionality has to be opted in explicitly by clients, by
setting the "comment" field of the slave to "optional":

  > type multi
  > ...
  > slaves.c.pcm "bluetooth"
  > slaves.c.channels 2;
  > slaves.c.comment "optional";
  > bindings.6.slave c;
  > bindings.6.channel 0;
  > ...

If that is done, a failure to open such a slave that is caused by the
device not being found will no longer lead to overall failure of the
open operation. Instead, it will simply be ignored and the remaining
slaves be used.
---
 src/pcm/pcm_multi.c | 63 +++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 61 insertions(+), 2 deletions(-)

diff --git a/src/pcm/pcm_multi.c b/src/pcm/pcm_multi.c
index bec6d0..1817ce 100644
--- a/src/pcm/pcm_multi.c
+++ b/src/pcm/pcm_multi.c
@@ -1226,6 +1226,8 @@ pcm.quad2 {
 
 */
 
+int snd_pcm_null_open(snd_pcm_t **pcmp, const char *name, snd_pcm_stream_t stream, int mode);
+
 /**
  * \brief Creates a new Multi PCM
  * \param pcmp Returns created PCM handle
@@ -1421,14 +1423,71 @@ int _snd_pcm_multi_open(snd_pcm_t **pcmp, const char *name,
 		channels_schannel[cchannel] = schannel;
 	}
 	
-	for (idx = 0; idx < slaves_count; ++idx) {
+	idx = 0;
+	snd_config_for_each(i, inext, slaves) {
+		snd_config_t *n = snd_config_iterator_entry(i);
+		const char *id;
+		const char *str;
+		int optional = 0;
+
+		if (snd_config_get_id(n, &id) < 0)
+			continue;
+
+		if (snd_config_get_string(n, &str) >= 0) {
+			err = snd_config_search_definition(root, "pcm_slave", str, &n);
+			if (err < 0) {
+				SNDERR("Invalid slave definition");
+				err = -EINVAL;
+				goto _free;
+			}
+		}
+
+		if (snd_config_get_type(n) != SND_CONFIG_TYPE_COMPOUND) {
+			SNDERR("Invalid slave definition");
+			err = -EINVAL;
+			goto _free;
+		}
+
+		snd_config_for_each(j, jnext, n) {
+			snd_config_t *m = snd_config_iterator_entry(j);
+			const char *idm;
+			const char *str;
+
+			if (snd_config_get_id(m, &idm) < 0)
+				continue;
+
+			if (strcmp(idm, "comment") == 0) {
+				err = snd_config_get_string(m, &str);
+				if (err < 0) {
+					SNDERR("Comment field is not a string");
+					goto _free;
+				}
+				if (strcmp(str, "optional") != 0) {
+					SNDERR("Comment field has unrecognized value: %s", str);
+					goto _free;
+				}
+
+				optional = 1;
+			}
+		}
+
 		err = snd_pcm_open_slave(&slaves_pcm[idx], root,
 					 slaves_conf[idx], stream, mode,
 					 conf);
-		if (err < 0)
+		if (err == -ENODEV && optional != 0) {
+			err = snd_pcm_null_open(&slaves_pcm[idx], NULL, stream, mode);
+			if (err < 0) {
+				SNDERR("Failed to open null PCM for non-present optional slave");
+				goto _free;
+			}
+			SNDERR("Optional slave '%s' not found; continuing...", id);
+			idx += 1;
+			continue;
+		} else if (err < 0)
 			goto _free;
 		snd_config_delete(slaves_conf[idx]);
 		slaves_conf[idx] = NULL;
+		idx += 1;
 	}
 	err = snd_pcm_multi_open(pcmp, name, slaves_count, master_slave,
 				 slaves_pcm, slaves_channels,
-- 
2.37.4