loadavg: make cleanup of start_loadavg

Cleanup start_loadavg code:
- add a new external symbol load_daemon_v2 with the pthread_create-like signature
- make hacky casts of pthread_t to int (and reverse) unnecessary for new API users

Related to: #610

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

src/lxcfs.c

@@ -89,13 +89,37 @@ static int start_loadavg(void)
 {
 	char *error;
 	pthread_t (*__load_daemon)(int);
+	int (*__load_daemon_v2)(pthread_t *, int);
 
+	/* try a new load_daemon_v2() API */
+	dlerror();
+	__load_daemon_v2 = (int (*)(pthread_t *, int))dlsym(dlopen_handle, "load_daemon_v2");
+	error = dlerror();
+	if (error)
+		/* try with an old symbol name */
+		goto old_api;
+
+	lxcfs_debug("start_loadavg: using load_daemon_v2");
+
+	if (__load_daemon_v2(&loadavg_pid, 1)) {
+		/* we have to NULLify loadavg_pid as in case of error it's contents are undefined */
+		loadavg_pid = 0;
+		return log_error(-1, "Failed to start loadavg daemon");
+	}
+
+	/* we are done */
+	return 0;
+
+old_api:
+	/* go with an old load_daemon() API */
 	dlerror();
 	__load_daemon = (pthread_t(*)(int))dlsym(dlopen_handle, "load_daemon");
 	error = dlerror();
 	if (error)
 		return log_error(-1, "%s - Failed to start loadavg daemon", error);
 
+	lxcfs_debug("start_loadavg: using load_daemon");
+
 	loadavg_pid = __load_daemon(1);
 	if (!loadavg_pid)
 		return -1;

src/proc_loadavg.c

@@ -628,7 +628,7 @@ static void load_free(void)
 	}
 }
 
-/* Return a positive number on success, return 0 on failure.*/
+/* Return a positive number on success, return 0 on failure. */
 pthread_t load_daemon(int load_use)
 {
 	int ret;
@@ -644,11 +644,31 @@ pthread_t load_daemon(int load_use)
 		return (pthread_t)log_error(0, "Create pthread fails in load_daemon!");
 	}
 
-	/* use loadavg, here loadavg = 1*/
+	/* use loadavg, here loadavg = 1 */
 	loadavg = load_use;
 	return pid;
 }
 
+/* Return 0 on success, return -1 on failure. */
+int load_daemon_v2(pthread_t *thread, int load_use)
+{
+	int ret;
+
+	ret = init_load();
+	if (ret == -1)
+		return log_error(-1, "Initialize hash_table fails in load_daemon!");
+
+	ret = pthread_create(thread, NULL, load_begin, NULL);
+	if (ret != 0) {
+		load_free();
+		return log_error(-1, "%s - Create pthread fails in load_daemon!", strerror(ret));
+	}
+
+	/* use loadavg, here loadavg = 1 */
+	loadavg = load_use;
+	return 0;
+}
+
 /* Returns 0 on success. */
 int stop_load_daemon(pthread_t pid)
 {

src/proc_loadavg.h

@@ -16,6 +16,7 @@
 #include "macro.h"
 
 __visible extern pthread_t load_daemon(int load_use);
+__visible extern int load_daemon_v2(pthread_t *thread, int load_use);
 __visible extern int stop_load_daemon(pthread_t pid);
 
 extern int proc_loadavg_read(char *buf, size_t size, off_t offset, struct fuse_file_info *fi);