[dm-devel] [PATCH v2] multipathd: fix client response for socket activation

[Martin Wilck]

When a client wakes up multipathd through the socket, it is likely that the
ux listener responds to client requests before multipathd startup has
completed. This means that client commands such as "show paths" or "show
topology" return success with an empty result, which is quite confusing.

Therefore, in the ux listener, don't start answering client requests while
the daemon is configuring. Rather, wait for some other daemon state. We
can't wait hard, because the ux listener must also handle signals. So just
wait for some short time, and poll again.

This has the side effect that command responses for commands that don't
require full initialization, such as "show wildcards", "show config" or
"shutdown", are also delayed until the configuration stage has completed.
But that seems to be a relatively cheap price to pay for getting the
expected response for other commands. To avoid this side effect, the client
handling would need to be rewritten such that the uxlsnr thread would have
a means to "know" which commands require the configuration stage to
complete and which do not.

v2: Removed an unrelated, unnecessary hunk in child().

Signed-off-by: Martin Wilck <mwilck at suse.com>
---
 multipathd/main.c   | 27 +++++++++++++++++++++++++++
 multipathd/main.h   |  2 ++
 multipathd/uxlsnr.c | 12 ++++++++++++
 3 files changed, 41 insertions(+)

diff --git a/multipathd/main.c b/multipathd/main.c
index f203d77f..ad818320 100644
--- a/multipathd/main.c
+++ b/multipathd/main.c
@@ -220,6 +220,33 @@ static void config_cleanup(void *arg)
 	pthread_mutex_unlock(&config_lock);
 }
 
+/*
+ * If the current status is @oldstate, wait for at most @ms milliseconds
+ * for the state to change, and return the new state, which may still be
+ * @oldstate.
+ */
+enum daemon_status wait_for_state_change_if(enum daemon_status oldstate,
+					    unsigned long ms)
+{
+	enum daemon_status st;
+	struct timespec tmo;
+
+	if (oldstate == DAEMON_SHUTDOWN)
+		return DAEMON_SHUTDOWN;
+
+	pthread_mutex_lock(&config_lock);
+	pthread_cleanup_push(config_cleanup, NULL);
+	st = running_state;
+	if (st == oldstate && clock_gettime(CLOCK_MONOTONIC, &tmo) == 0) {
+		tmo.tv_nsec += ms * 1000 * 1000;
+		normalize_timespec(&tmo);
+		(void)pthread_cond_timedwait(&config_cond, &config_lock, &tmo);
+		st = running_state;
+	}
+	pthread_cleanup_pop(1);
+	return st;
+}
+
 /* must be called with config_lock held */
 static void __post_config_state(enum daemon_status state)
 {
diff --git a/multipathd/main.h b/multipathd/main.h
index e5c1398f..7bb8463f 100644
--- a/multipathd/main.h
+++ b/multipathd/main.h
@@ -20,6 +20,8 @@ extern int uxsock_timeout;
 
 void exit_daemon(void);
 const char * daemon_status(void);
+enum daemon_status wait_for_state_change_if(enum daemon_status oldstate,
+					    unsigned long ms);
 int need_to_delay_reconfig (struct vectors *);
 int reconfigure (struct vectors *);
 int ev_add_path (struct path *, struct vectors *, int);
diff --git a/multipathd/uxlsnr.c b/multipathd/uxlsnr.c
index 773bc878..04cbb7c7 100644
--- a/multipathd/uxlsnr.c
+++ b/multipathd/uxlsnr.c
@@ -249,6 +249,18 @@ void * uxsock_listen(uxsock_trigger_fn uxsock_trigger, long ux_sock,
 			continue;
 		}
 
+		/*
+		 * Client connection. We shouldn't answer while we're
+		 * configuring - nothing may be configured yet.
+		 * But we can't wait forever either, because this thread
+		 * must handle signals. So wait a short while only.
+		 */
+		if (wait_for_state_change_if(DAEMON_CONFIGURE, 10)
+		    == DAEMON_CONFIGURE) {
+			handle_signals(false);
+			continue;
+		}
+
 		/* see if a client wants to speak to us */
 		for (i = 1; i < num_clients + 1; i++) {
 			if (polls[i].revents & POLLIN) {
-- 
2.21.0