crtools: write a pid of a root task in a specified file

When we restore a pid namespace the root task will get some unknown pid
in the original (i.e. -- the ns crtools a launched from) one. To find
this pid out one can use this option -- it will make the pid obtained by
the new init to be written into a pid file.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -547,6 +547,20 @@ static inline int fork_with_pid(struct pstree_item *item, unsigned long ns_clone
 
 	if (ca.clone_flags & CLONE_NEWPID)
 		item->pid.real = ret;
+
+	if (opts.pidfile && root_item == item) {
+		int fd;
+
+		fd = open(opts.pidfile, O_WRONLY | O_TRUNC | O_CREAT, 0600);
+		if (fd == -1) {
+			pr_perror("Can't open %s", opts.pidfile);
+			kill(ret, SIGKILL);
+		} else {
+			dprintf(fd, "%d", ret);
+			close(fd);
+		}
+	}
+
 err_unlock:
 	if (flock(ca.fd, LOCK_UN))
 		pr_perror("%d: Can't unlock %s", pid, LAST_PID_PATH);

crtools.c

@@ -89,6 +89,7 @@ int main(int argc, char *argv[])
 			{ "log-pid", no_argument, 0, 44},
 			{ "version", no_argument, 0, 'V'},
 			{ "evasive-devices", no_argument, 0, 45},
+			{ "pidfile", required_argument, 0, 46},
 			{ },
 		};
 
@@ -169,6 +170,9 @@ int main(int argc, char *argv[])
 		case 45:
 			opts.evasive_devices = true;
 			break;
+		case 46:
+			opts.pidfile = optarg;
+			break;
 		case 'V':
 			pr_msg("Version: %d.%d\n", CRIU_VERSION_MAJOR, CRIU_VERSION_MINOR);
 			return 0;
@@ -249,6 +253,7 @@ usage:
 	pr_msg("  -d|--restore-detached detach after restore\n");
 	pr_msg("  -s|--leave-stopped    leave tasks in stopped state after checkpoint instead of killing them\n");
 	pr_msg("  -D|--images-dir       directory where to put images to\n");
+	pr_msg("     --pidfile [FILE]	write a pid of a root task in this file\n");
 
 	pr_msg("\n* Special resources support:\n");
 	pr_msg("  -n|--namespaces       checkpoint/restore namespaces - values must be separated by comma\n");

include/crtools.h

@@ -92,6 +92,7 @@ struct cr_options {
 	bool			log_file_per_pid;
 	char			*output;
 	char			*root;
+	char			*pidfile;
 };
 
 extern struct cr_options opts;