librc: Add initial support for dynamically multiplexed services.

Generate a new directory on the service dir, "multiplexed", which will
hold all the symlinks we generate at runtime, and a new librc function,
"rc_service_multiplex", which takes a basename or service path, and a
variant name, then generated a symlink to the base service from the
multiplex dir.

This allows multiplexing services at runtime, possibly using data
acquired at runtime, such as the name of an user or interface.

src/librc/librc-depend.c

@@ -697,6 +697,7 @@ static const char *const depdirs[] =
 	RC_SVCDIR "/wasinactive",
 	RC_SVCDIR "/failed",
 	RC_SVCDIR "/hotplugged",
+	RC_SVCDIR "/multiplexed",
 	RC_SVCDIR "/daemons",
 	RC_SVCDIR "/options",
 	RC_SVCDIR "/exclusive",

src/librc/librc.c

@@ -636,6 +636,12 @@ rc_service_resolve(const char *service)
 		return file;
 #endif
 
+	/* Check dynamically multiplexed services last */
+	free(file);
+	xasprintf(&file, "%s/multiplexed/%s", RC_SVCDIR, service);
+	if (stat(file, &buf) == 0)
+		return file;
+
 	free(file);
 	return NULL;
 }
@@ -933,6 +939,32 @@ rc_service_state(const char *service)
 	return state;
 }
 
+char *
+rc_service_multiplex(const char *base, const char *variant)
+{
+	char *service = rc_service_resolve(base);
+	char *target;
+	char *name;
+
+	if (!service)
+		return NULL;
+
+	if (base[0] == '/')
+		base = basename_c(base);
+
+	xasprintf(&name, "%s.%s", base, variant);
+
+	xasprintf(&target, "%s/multiplexed/%s", RC_SVCDIR, name);
+	if (!exists(target) && symlink(service, target) == -1) {
+		free(target);
+		target = NULL;
+	}
+
+	free(name);
+	free(service);
+	return target;
+}
+
 char *
 rc_service_value_get(const char *service, const char *option)
 {
@@ -1119,6 +1151,8 @@ rc_service_add(const char *runlevel, const char *service)
 	char *path;
 	char *binit = NULL;
 	char *i;
+	char *multiplex_dir;
+	int len;
 
 	if (!rc_runlevel_exists(runlevel)) {
 		errno = ENOENT;
@@ -1151,10 +1185,20 @@ rc_service_add(const char *runlevel, const char *service)
 		i = binit;
 	}
 
-	xasprintf(&file, "%s/%s/%s", RC_RUNLEVELDIR, runlevel,
-		basename_c(service));
+	/* We shouldn't add any dynamically multiplexed service to a runlevel */
+	len = xasprintf(&multiplex_dir, "%s/multiplexed", RC_SVCDIR);
+	if (strncmp(i, multiplex_dir, len) == 0) {
+		errno = ENOENT;
+		retval = false;
+		goto out;
+	}
+
+	xasprintf(&file, "%s/%s/%s", RC_RUNLEVELDIR, runlevel, basename_c(service));
 	retval = (symlink(i, file) == 0);
 	free(file);
+
+out:
+	free(multiplex_dir);
 	free(binit);
 	free(init);
 	return retval;

src/librc/rc.h.in

@@ -267,6 +267,11 @@ bool rc_service_schedule_clear(const char *);
  * @return state of the service */
 RC_SERVICE rc_service_state(const char *);
 
+/*! Dynamically multiplexes a base service into a variant as basename.variant
+ * @param basename or full path of service being multiplexed
+ * @param variant to dynamically multiplex */
+char *rc_service_multiplex(const char *, const char *);
+
 /*! Check if the service started the daemon
  * @param service to check
  * @param exec to check