[lttng-dev] libust "Error: Error opening shm" every ~4 seconds if HOME variable is not set

[David OShea]

Hi all,

I have an application that works as expected when I run it from a shell, but when I start it as a daemon from an init script, it outputs a message like this every 4 or 5 seconds:

libust[15229/15231]: Error: Error opening shm  (in get_wait_shm() at lttng-ust-comm.c:480)

I did a little debugging to try and work out what was going on.  I didn't actually end up seeing get_wait_shm() being called, but I noticed that ust_listener_thread() was entering the block of code containing:

DBG("Info: sessiond not accepting connections to %s apps socket", sock_info->name);

It appeared that this was because the sock_path was empty.  From looking at sock_info, the wait_shm_path is also empty as shown below, as confirmed in the error message above where "Error opening shm" was meant to be followed by the path that was being opened but instead is just followed by two spaces:

(gdb) p * (struct sock_info *) arg
$7 = {name = 0x2b2721bd8b48 "local", ust_listener = 1110300992, root_handle = -1, constructor_sem_posted = 1, allowed = 1, global = 0, sock_path = '\000' <repeats 4095 times>, socket = -1, wait_shm_path = '\000' <repeats 4095 times>, wait_shm_mmap = 0x0}

setup_local_apps() appears to not fill in the sock_path and wait_shm_path if the HOME environment variable is not set, which is the case for my daemon.  Looking back, I can see that this message appeared during my application's startup:

libust[15229/15229]: Error: Error setting up to local apps (in lttng_ust_init() at lttng-ust-comm.c:895)

which is the indicator that setup_local_apps() failed.

I assume that lttng-ust is attempting to connect to a per-user session daemon, but can't since HOME is not set.  In my case, the application is running as root and connecting to the global session daemon, so I don't mind that it can't connect to a per-user session daemon.  It would be nice if I didn't have to get all of those error messages, although I can certainly ignore them for now and could consider working around them by just making sure HOME is set to something :)

I'm using lttng-ust-2.0.5 which I have patched with commit 009745db "Cache the procname per-thread rather than per-process to take into account that prctl() can be used to set thread names."

Thanks in advance,
David

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