sync: add LXC_SYNC_ERROR to report errors from another process.

src/lxc/sync.c

@@ -27,6 +27,7 @@
 #include <errno.h>
 #include <fcntl.h>
 
+#include "sync.h"
 #include "log.h"
 #include "start.h"
 
@@ -46,6 +47,12 @@ static int __sync_wait(int fd, int sequence)
 	if (!ret)
 		return 0;
 
+	if (sync == LXC_SYNC_ERROR) {
+		ERROR("An error occurred in another process "
+		      "(expected sequence number %d)", sequence);
+		return -1;
+	}
+
 	if (sync != sequence) {
 		ERROR("invalid sequence number %d. expected %d",
 		      sync, sequence);

src/lxc/sync.h

@@ -32,6 +32,7 @@ enum {
 	LXC_SYNC_POST_CGROUP,
 	LXC_SYNC_RESTART,
 	LXC_SYNC_POST_RESTART,
+	LXC_SYNC_ERROR = -1 /* Used to report errors from another process */
 };
 
 int lxc_sync_init(struct lxc_handler *handler);