restore: Introduce the --restore-sibling option

We have a slight mess with how criu restores root task.
Right now we have the following options.

1) CLI
	a) Usually
	task calling criu
	 `- criu
	     `- root restored task

	b) when --restore-detached AND root has pdeath_sig

	task calling criu
	 `- criu
	 `- root restored task

2) Library/SWRK
	task using lib/swrk
	 `- criu
	 `- root restored task

3) Standalone service
	a) Usually
	service
	 `- service sub task
	     `- root restored task

	b) when root has pdeath_sig
	criu service
	 `- criu sub task
	 `- root restored task

It would be better is CRIU always restored the root task as sibling,
but we have 3 constraints:

First, the case 1.a is kept for zdtm to run tests in pid namespaces
on 3.11, which in turn doesn't allow CLONE_PARENT | CLONE_NEWPID.

Second, CLI w/o --restore-detach waits for the restored task to die and
this behavior can be "expected" already.

Third, in case of standalone service tasks shouldn't become service's
children.

And I have one "plan". The p.haul project while live migrating tasks
on destination node starts a service, which uses library/swrk mode. In
this case the restored processes become p.haul service's kids which is
also not great.

That said, here's the option called --restore-child that pairs the
--restore-detach like this:

* detached AND child:

task
 `- criu restore (exits at the end)
 `- root task

The root task will become task's child.
This will be default to library/swrk.
This is what LXC needs.

* detach AND !child

task
 `- criu restore (exits at the end)
     `- root task

The root task will get re-parented to init.
This will be compatible with 1.3.
This will be default to standalone service and
to my wish with the p.haul case.

* !detach AND child

task
 `- criu restore (waits for root task to die)
 `- root task

This should be deprecated, so that criu restore doesn't mess
 with task <-> root task signalling.

* !detach AND !child

task
 `- criu restore (waits for root task to die)
     `- root task

This is how plain criu restore works now.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@openvz.org>

crtools.c

@@ -133,13 +133,14 @@ int main(int argc, char *argv[], char *envp[])
 	int log_level = LOG_UNSET;
 	char *imgs_dir = ".";
 	char *work_dir = NULL;
-	static const char short_opts[] = "dsRf:F:t:p:hcD:o:n:v::xVr:jlW:L:M:";
+	static const char short_opts[] = "dSsRf:F:t:p:hcD:o:n:v::xVr:jlW:L:M:";
 	static struct option long_opts[] = {
 		{ "tree", required_argument, 0, 't' },
 		{ "pid", required_argument, 0, 'p' },
 		{ "leave-stopped", no_argument, 0, 's' },
 		{ "leave-running", no_argument, 0, 'R' },
 		{ "restore-detached", no_argument, 0, 'd' },
+		{ "restore-sibling", no_argument, 0, 'S' },
 		{ "daemon", no_argument, 0, 'd' },
 		{ "contents", no_argument, 0, 'c' },
 		{ "file", required_argument, 0, 'f' },
@@ -247,6 +248,9 @@ int main(int argc, char *argv[], char *envp[])
 		case 'd':
 			opts.restore_detach = true;
 			break;
+		case 'S':
+			opts.restore_sibling = true;
+			break;
 		case 'D':
 			imgs_dir = optarg;
 			break;
@@ -404,6 +408,11 @@ int main(int argc, char *argv[], char *envp[])
 		}
 	}
 
+	if (!opts.restore_detach && opts.restore_sibling) {
+		pr_msg("--restore-sibling only makes sense with --restore-detach\n");
+		return 1;
+	}
+
 	if (work_dir == NULL)
 		work_dir = imgs_dir;
 
@@ -540,6 +549,7 @@ usage:
 "* Generic:\n"
 "  -t|--tree PID         checkpoint a process tree identified by PID\n"
 "  -d|--restore-detached detach after restore\n"
+"  -S|--restore-sibling  restore root task as sibling\n"
 "  -s|--leave-stopped    leave tasks in stopped state after checkpoint\n"
 "  -R|--leave-running    leave tasks in running state after checkpoint\n"
 "  -D|--images-dir DIR   directory for image files\n"