pulse-server: Add a message to enable/disable mono mixdown

WirePlumber recently added a mechanism to force mono mixdown on audio
outputs, which is a useful feature for accessibility. Let's also expose
that setting via libpulse for existing audio settings UIs to be able to
use.

src/modules/module-protocol-pulse/client.h

@@ -60,6 +60,11 @@ struct client {
 	struct pw_manager_object *metadata_routes;
 	struct pw_properties *routes;
 
+	struct pw_manager_object *metadata_schema_sm_settings;
+	bool have_force_mono_audio;
+	struct pw_manager_object *metadata_sm_settings;
+	bool force_mono_audio;
+
 	uint32_t connect_tag;
 
 	uint32_t in_index;

src/modules/module-protocol-pulse/defs.h

@@ -323,5 +323,6 @@ static inline uint32_t port_type_value(const char *port_type)
 #define METADATA_CONFIG_DEFAULT_SOURCE  "default.configured.audio.source"
 #define METADATA_TARGET_NODE            "target.node"
 #define METADATA_TARGET_OBJECT          "target.object"
+#define METADATA_FEATURES_AUDIO_MONO    "node.features.audio.mono"
 
 #endif /* PULSE_SERVER_DEFS_H */

src/modules/module-protocol-pulse/message-handler.c

@@ -19,6 +19,7 @@
 
 #include "client.h"
 #include "collect.h"
+#include "defs.h"
 #include "log.h"
 #include "manager.h"
 #include "module.h"
@@ -89,6 +90,46 @@ static int bluez_card_object_message_handler(struct client *client, struct pw_ma
 	return 0;
 }
 
+
+static int core_object_force_mono_output(struct client *client, const char *params, FILE *response)
+{
+	if (!client->have_force_mono_audio) {
+		/* Not supported, return a null value to indicate that */
+		fprintf(response, "null");
+		return 0;
+	}
+
+	if (!params || params[0] == '\0') {
+		/* No parameter => query the current value */
+		fprintf(response, "%s", client->force_mono_audio ? "true" : "false");
+		return 0;
+	} else {
+		/* The caller is trying to set a value or clear with a null */
+		int ret;
+
+		if (spa_streq(params, "true")) {
+			ret = pw_manager_set_metadata(client->manager, client->metadata_sm_settings, PW_ID_CORE,
+					METADATA_FEATURES_AUDIO_MONO, "Spa:String:JSON", "true");
+		} else if (spa_streq(params, "false")) {
+			ret = pw_manager_set_metadata(client->manager, client->metadata_sm_settings, PW_ID_CORE,
+					METADATA_FEATURES_AUDIO_MONO, "Spa:String:JSON", "false");
+		} else if (spa_streq(params, "null")) {
+			ret = pw_manager_set_metadata(client->manager, client->metadata_sm_settings, PW_ID_CORE,
+					METADATA_FEATURES_AUDIO_MONO, NULL, NULL);
+		} else {
+			fprintf(response, "Value must be true, false, or clear");
+			return -EINVAL;
+		}
+
+		if (ret < 0)
+			fprintf(response, "Could not set metadata: %s", spa_strerror(ret));
+		else
+			fprintf(response, "%s", params);
+
+		return ret;
+	}
+}
+
 static int core_object_message_handler(struct client *client, struct pw_manager_object *o, const char *message, const char *params, FILE *response)
 {
 	pw_log_debug(": core %p object message:'%s' params:'%s'", o, message, params);
@@ -103,7 +144,8 @@ static int core_object_message_handler(struct client *client, struct pw_manager_
 				"  pipewire-pulse:malloc-trim     	run malloc_trim\n"
 				"  pipewire-pulse:log-level       	update log level with <params>\n"
 				"  pipewire-pulse:list-modules    	list all module names\n"
-				"  pipewire-pulse:describe-module describe module info for <params>"
+				"  pipewire-pulse:describe-module 	describe module info for <params>\n"
+				"  pipewire-pulse:force-mono-output	force mono mixdown on all hardware outputs"
 				);
 	} else if (spa_streq(message, "list-handlers")) {
 		bool first = true;
@@ -164,6 +206,8 @@ static int core_object_message_handler(struct client *client, struct pw_manager_
 		} else {
 			fprintf(response, "Failed to open module.\n");
 		}
+	} else if (spa_streq(message, "pipewire-pulse:force-mono-output")) {
+		return core_object_force_mono_output(client, params, response);
 	} else {
 		return -ENOSYS;
 	}

src/modules/module-protocol-pulse/pulse-server.c

@@ -404,6 +404,14 @@ static void handle_metadata(struct client *client, struct pw_manager_object *old
 		if (client->metadata_routes == old)
 			client->metadata_routes = new;
 	}
+	else if (spa_streq(name, "sm-settings")) {
+		if (client->metadata_sm_settings == old)
+			client->metadata_sm_settings = new;
+	}
+	else if (spa_streq(name, "schema-sm-settings")) {
+		if (client->metadata_schema_sm_settings == old)
+			client->metadata_schema_sm_settings = new;
+	}
 }
 
 static uint32_t frac_to_bytes_round_up(struct spa_fraction val, const struct sample_spec *ss)
@@ -964,6 +972,14 @@ static void manager_metadata(void *data, struct pw_manager_object *o,
 	}
 	if (subject == PW_ID_CORE && o == client->metadata_routes)
 		client_update_routes(client, key, value);
+	if (subject == PW_ID_CORE && o == client->metadata_schema_sm_settings) {
+		if (spa_streq(key, METADATA_FEATURES_AUDIO_MONO))
+			client->have_force_mono_audio = true;
+	}
+	if (subject == PW_ID_CORE && o == client->metadata_sm_settings) {
+		if (spa_streq(key, METADATA_FEATURES_AUDIO_MONO))
+			client->force_mono_audio = spa_streq(value, "true");
+	}
 }