protocol-pulse: implement readiness notification

Write a newline to the fd in the PIPEWIRE_PULSE_NOTIFICATION_FD env
variable when set.

This is to implement readiness notification as suggested in
https://skarnet.org/software/s6/notifywhenup.html

Fixes #4347

src/modules/module-protocol-pulse.c

@@ -346,6 +346,13 @@
  * Takes an object with the properties to update on the client. Common actions are to
  * tweak the quantum values.
  *
+ * ### startup notification
+ *
+ * A newline will be written into the notification file descriptor when the server has
+ * started if the following environment variable is set:
+ *
+ * - PIPEWIRE_PULSE_NOTIFICATION_FD
+ *
  * ## Example configuration
  *
  *\code{.unparsed}

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

@@ -5559,6 +5559,8 @@ struct pw_protocol_pulse *pw_protocol_pulse_new(struct pw_context *context,
 
 	cmd_run(impl);
 
+	notify_startup();
+
 	return (struct pw_protocol_pulse *) impl;
 
 error_free:

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

@@ -159,7 +159,8 @@ const char *get_server_name(struct pw_context *context)
 	return name;
 }
 
-int create_pid_file(void) {
+int create_pid_file(void)
+{
 	char pid_file[PATH_MAX];
 	FILE *f;
 	int res;
@@ -185,3 +186,41 @@ int create_pid_file(void) {
 
 	return 0;
 }
+
+int notify_startup(void)
+{
+	long v;
+	int fd, res = 0;
+	char *endptr;
+	const char *env = getenv("PIPEWIRE_PULSE_NOTIFICATION_FD");
+
+	if (env == NULL || env[0] == '\0')
+		return 0;
+
+	errno = 0;
+	v = strtol(env, &endptr, 10);
+	if (endptr[0] != '\0')
+		errno = EINVAL;
+	if (errno != 0) {
+		res = -errno;
+		pw_log_error("can't parse PIPEWIRE_PULSE_NOTIFICATION_FD env: %m");
+		goto error;
+	}
+	fd = (int)v;
+	if (v != fd) {
+		res = -ERANGE;
+		pw_log_error("invalid PIPEWIRE_PULSE_NOTIFICATION_FD %ld: %s", v, spa_strerror(res));
+		goto error;
+	}
+	if (dprintf(fd, "\n") < 0) {
+		res = -errno;
+		pw_log_error("can't signal PIPEWIRE_PULSE_NOTIFICATION_FD: %m");
+		goto error;
+	}
+	close(fd);
+	unsetenv("PIPEWIRE_PULSE_NOTIFICATION_FD");
+	return 0;
+
+error:
+	return res;
+}

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

@@ -16,5 +16,6 @@ int check_flatpak(struct client *client, pid_t pid);
 pid_t get_client_pid(struct client *client, int client_fd);
 const char *get_server_name(struct pw_context *context);
 int create_pid_file(void);
+int notify_startup(void);
 
 #endif /* PULSE_SERVER_UTILS_H */