lxccontainer: detect if we should send SIGRTMIN+3

This is required by systemd to cleanly shutdown. Other init systems should not
have SIGRTMIN+3 in the blocked signals set.

Signed-off-by: Christian Brauner <cbrauner@suse.de>

src/lxc/lxccontainer.c

@@ -1604,8 +1604,16 @@ static bool do_lxcapi_shutdown(struct lxc_container *c, int timeout)
 	pid = do_lxcapi_init_pid(c);
 	if (pid <= 0)
 		return true;
+
+	/* Detect whether we should send SIGRTMIN + 3 (e.g. systemd). */
+	if (task_blocking_signal(pid, (SIGRTMIN + 3)))
+		haltsignal = (SIGRTMIN + 3);
+
 	if (c->lxc_conf && c->lxc_conf->haltsignal)
 		haltsignal = c->lxc_conf->haltsignal;
+
+	INFO("Using signal number '%d' as halt signal.", haltsignal);
+
 	kill(pid, haltsignal);
 	retv = do_lxcapi_wait(c, "STOPPED", timeout);
 	return retv;

src/lxc/utils.c

@@ -1838,3 +1838,41 @@ int lxc_strmunmap(void *addr, size_t length)
 {
 	return munmap(addr, length + 1);
 }
+
+/* Check whether a signal is blocked by a process. */
+bool task_blocking_signal(pid_t pid, int signal)
+{
+	bool bret = false;
+	char *line = NULL;
+	long unsigned int sigblk = 0;
+	size_t n = 0;
+	int ret;
+	FILE *f;
+
+	/* The largest integer that can fit into long int is 2^64. This is a
+	 * 20-digit number. */
+	size_t len = /* /proc */ 5 + /* /pid-to-str */ 21 + /* /status */ 7 + /* \0 */ 1;
+	char status[len];
+
+	ret = snprintf(status, len, "/proc/%d/status", pid);
+	if (ret < 0 || ret >= len)
+		return bret;
+
+	f = fopen(status, "r");
+	if (!f)
+		return bret;
+
+	while (getline(&line, &n, f) != -1) {
+		if (!strncmp(line, "SigBlk:\t", 8))
+			if (sscanf(line + 8, "%lx", &sigblk) != 1)
+				goto out;
+	}
+
+	if (sigblk & signal)
+		bret = true;
+
+out:
+	free(line);
+	fclose(f);
+	return bret;
+}

src/lxc/utils.h

@@ -296,4 +296,7 @@ int open_devnull(void);
 int set_stdfds(int fd);
 int null_stdfds(void);
 int lxc_count_file_lines(const char *fn);
+
+/* Check whether a signal is blocked by a process. */
+bool task_blocking_signal(pid_t pid, int signal);
 #endif /* __LXC_UTILS_H */