Move cgroup dbus requests to the child to avoid a race where a spawned child

that quickly forks will have only the parent process moved to the newly created
cgroup. From Daniel De Graaf, GitHub issue 4435.

compat.h

@@ -450,7 +450,7 @@ void		*recallocarray(void *, size_t, size_t, size_t);
 /* systemd.c */
 int		 systemd_activated(void);
 int		 systemd_create_socket(int, char **);
-int		 systemd_move_pid_to_new_cgroup(pid_t, char **);
+int		 systemd_move_to_new_cgroup(char **);
 #endif
 
 #ifdef HAVE_UTF8PROC

compat/systemd.c

@@ -75,16 +75,49 @@ fail:
 	return (-1);
 }
 
+struct systemd_job_watch {
+	const char	*path;
+	int		 done;
+};
+
+static int
+job_removed_handler(sd_bus_message *m, void *userdata, sd_bus_error *ret_error)
+{
+	struct systemd_job_watch *watch = userdata;
+	const char		 *path = NULL;
+	uint32_t		 id;
+	int			 r;
+
+	/* This handler could be called during the sd_bus_call. */
+	if (watch->path == NULL)
+		return 0;
+
+	r = sd_bus_message_read(m, "uo", &id, &path);
+	if (r < 0)
+		return (r);
+
+	if (strcmp(path, watch->path) == 0)
+		watch->done = 1;
+
+	return (0);
+}
+
 int
-systemd_move_pid_to_new_cgroup(pid_t pid, char **cause)
+systemd_move_to_new_cgroup(char **cause)
 {
 	sd_bus_error		 error = SD_BUS_ERROR_NULL;
 	sd_bus_message		*m = NULL, *reply = NULL;
 	sd_bus 			*bus = NULL;
+	sd_bus_slot		*slot = NULL;
 	char			*name, *desc, *slice;
 	sd_id128_t		 uuid;
 	int			 r;
-	pid_t		 parent_pid;
+	uint64_t		 elapsed_usec;
+	pid_t			 pid, parent_pid;
+	struct timeval		 start, now;
+	struct systemd_job_watch watch = {};
+
+	gettimeofday(&start, NULL);
 
 	/* Connect to the session bus. */
 	r = sd_bus_default_user(&bus);
@@ -94,6 +127,20 @@ systemd_move_pid_to_new_cgroup(pid_t pid, char **cause)
 		goto finish;
 	}
 
+	/* Start watching for JobRemoved events */
+	r = sd_bus_match_signal(bus, &slot,
+	    "org.freedesktop.systemd1",
+	    "/org/freedesktop/systemd1",
+	    "org.freedesktop.systemd1.Manager",
+	    "JobRemoved",
+	    job_removed_handler,
+	    &watch);
+	if (r < 0) {
+		xasprintf(cause, "failed to create match signal: %s",
+		    strerror(-r));
+		goto finish;
+	}
+
 	/* Start building the method call. */
 	r = sd_bus_message_new_method_call(bus, &m,
 	    "org.freedesktop.systemd1",
@@ -138,7 +185,8 @@ systemd_move_pid_to_new_cgroup(pid_t pid, char **cause)
 		goto finish;
 	}
 
-	parent_pid = getpid();
+	pid = getpid();
+	parent_pid = getppid();
 	xasprintf(&desc, "tmux child pane %ld launched by process %ld",
 	    (long)pid, (long)parent_pid);
 	r = sd_bus_message_append(m, "(sv)", "Description", "s", desc);
@@ -223,10 +271,55 @@ systemd_move_pid_to_new_cgroup(pid_t pid, char **cause)
 		goto finish;
 	}
 
+	/* Get the job (object path) from the reply */
+	r = sd_bus_message_read(reply, "o", &watch.path);
+	if (r < 0) {
+		xasprintf(cause, "failed to parse method reply: %s",
+		    strerror(-r));
+		goto finish;
+	}
+
+	while (!watch.done) {
+		/* Process events including callbacks. */
+		r = sd_bus_process(bus, NULL);
+		if (r < 0) {
+			xasprintf(cause,
+			    "failed waiting for cgroup allocation: %s",
+			    strerror(-r));
+			goto finish;
+		}
+
+		/*
+		 * A positive return means we handled an event and should keep
+		 * processing; zero indicates no events available, so wait.
+		 */
+		if (r > 0)
+			continue;
+
+		gettimeofday(&now, NULL);
+		elapsed_usec = (now.tv_sec - start.tv_sec) * 1000000 +
+		    now.tv_usec - start.tv_usec;
+
+		if (elapsed_usec >= 1000000) {
+			xasprintf(cause,
+			    "timeout waiting for cgroup allocation");
+			goto finish;
+		}
+
+		r = sd_bus_wait(bus, 1000000 - elapsed_usec);
+		if (r < 0) {
+			xasprintf(cause,
+			    "failed waiting for cgroup allocation: %s",
+			    strerror(-r));
+			goto finish;
+		}
+	}
+
 finish:
 	sd_bus_error_free(&error);
 	sd_bus_message_unref(m);
 	sd_bus_message_unref(reply);
+	sd_bus_slot_unref(slot);
 	sd_bus_unref(bus);
 
 	return (r);

spawn.c

@@ -382,20 +382,19 @@ spawn_pane(struct spawn_context *sc, char **cause)
 
 	/* In the parent process, everything is done now. */
 	if (new_wp->pid != 0) {
+		goto complete;
+	}
+
 #if defined(HAVE_SYSTEMD) && defined(ENABLE_CGROUPS)
 	/*
-		 * Move the child process into a new cgroup for systemd-oomd
-		 * isolation.
+	 * Move the child process into a new cgroup for systemd-oomd isolation.
 	 */
-		if (systemd_move_pid_to_new_cgroup(new_wp->pid, cause) < 0) {
+	if (systemd_move_to_new_cgroup(cause) < 0) {
 		log_debug("%s: moving pane to new cgroup failed: %s",
 		    __func__, *cause);
 		free (*cause);
 	}
 #endif
-		goto complete;
-	}
-
 	/*
 	 * Child process. Change to the working directory or home if that
 	 * fails.