restore: add interface for creating helper tasks (v3)

They will be used for restoring sid. For example, if a session
group leader is absent, a helper process is created with this id
and it will die after restoring all other tasks.

Before this patch restore failed if anyone exited.
Now we should skip helpers, which exited successfully. It's a bit tricky.
All children are collected in sigchld_handler, but we have a point,
where we want to wait all helpers. For that waitpit is used and ECHLD
is ignored, because it signs that a helper exited and has been waited in
sigchld_handler.

v2: check that me isn't NULL in the sig handler
v3: move code about waiting helpers in a separate function
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -339,10 +339,42 @@ err:
 	return ret;
 }
 
+static int pstree_wait_helpers()
+{
+	struct pstree_item *pi;
+
+	list_for_each_entry(pi, &me->children, list) {
+		int status, ret;
+
+		if (pi->state != TASK_HELPER)
+			continue;
+
+		/* Check, that a helper completed. */
+		ret = waitpid(pi->pid.virt, &status, 0);
+		if (ret == -1) {
+			if (errno == ECHILD)
+				continue; /* It has been waited in sigchld_handler */
+			pr_err("waitpid(%d) failed\n", pi->pid.virt);
+			return -1;
+		}
+		if (!WIFEXITED(status) || WEXITSTATUS(status)) {
+			pr_err("%d exited with non-zero code (%d,%d)", pi->pid.virt,
+				WEXITSTATUS(status), WTERMSIG(status));
+			return -1;
+		}
+
+	}
+
+	return 0;
+}
+
 static int restore_one_alive_task(int pid)
 {
 	pr_info("Restoring resources\n");
 
+	if (pstree_wait_helpers())
+		return -1;
+
 	if (prepare_fds(me))
 		return -1;
 
@@ -405,6 +437,13 @@ static inline int sig_fatal(int sig)
 	return (sig > 0) && (sig < SIGMAX) && (SIG_FATAL_MASK & (1 << sig));
 }
 
+static int restore_one_fake(int pid)
+{
+	/* We should wait here, otherwise last_pid will be changed. */
+	futex_wait_while(&task_entries->start, CR_STATE_FORKING);
+	return 0;
+}
+
 static int restore_one_zombie(int pid, int exit_code)
 {
 	pr_info("Restoring zombie with %d code\n", exit_code);
@@ -473,6 +512,9 @@ static int restore_one_task(int pid)
 {
 	struct task_core_entry tc;
 
+	if (me->state == TASK_HELPER)
+		return restore_one_fake(pid);
+
 	if (check_core_header(pid, &tc))
 		return -1;
 
@@ -557,12 +599,43 @@ err:
 
 static void sigchld_handler(int signal, siginfo_t *siginfo, void *data)
 {
-	if (siginfo->si_code & CLD_EXITED)
-		pr_err("%d exited, status=%d\n",
-			siginfo->si_pid, siginfo->si_status);
-	else if (siginfo->si_code & CLD_KILLED)
-		pr_err("%d killed by signal %d\n",
-			siginfo->si_pid, siginfo->si_status);
+	struct pstree_item *pi;
+	pid_t pid = siginfo->si_pid;
+	int status;
+	int exit;
+
+	exit = siginfo->si_code & CLD_EXITED;
+	status = siginfo->si_status;
+	if (!me || status)
+		goto err;
+
+	/* Skip a helper if it was completed successfully */
+	while (pid) {
+		pid = waitpid(-1, &status, WNOHANG);
+		if (pid <= 0)
+			return;
+
+		exit = WIFEXITED(status);
+		status = exit ? WEXITSTATUS(status) : WTERMSIG(status);
+		if (status)
+			break;
+
+		list_for_each_entry(pi, &me->children, list) {
+			if (pi->state != TASK_HELPER)
+				continue;
+			if (pi->pid.virt == siginfo->si_pid)
+				break;
+		}
+
+		if (&pi->list == &me->children)
+			break; /* The process is not a helper */
+	}
+
+err:
+	if (exit)
+		pr_err("%d exited, status=%d\n", pid, status);
+	else
+		pr_err("%d killed by signal %d\n", pid, status);
 
 	futex_abort_and_wake(&task_entries->nr_in_progress);
 }
@@ -704,8 +777,11 @@ static int restore_root_task(struct pstree_item *init, struct cr_options *opts)
 		return -1;
 	}
 
-	act.sa_flags |= SA_NOCLDWAIT | SA_NOCLDSTOP | SA_SIGINFO | SA_RESTART;
+	act.sa_flags |= SA_NOCLDSTOP | SA_SIGINFO | SA_RESTART;
 	act.sa_sigaction = sigchld_handler;
+	sigemptyset(&act.sa_mask);
+	sigaddset(&act.sa_mask, SIGCHLD);
+
 	ret = sigaction(SIGCHLD, &act, &old_act);
 	if (ret < 0) {
 		perror("sigaction() failed\n");

include/image.h

@@ -499,6 +499,7 @@ struct core_entry {
 #define TASK_ALIVE		0x1
 #define TASK_DEAD		0x2
 #define TASK_STOPPED		0x3 /* FIXME - implement */
+#define TASK_HELPER		0x4
 
 #endif /* CONFIG_X86_64 */