[lldb-dev] The lit test driver gets killed because of SIGHUP

[Pavel Labath via lldb-dev]

On 05/12/2018 03:49, Jonas Devlieghere via lldb-dev wrote:
> Hi everyone,
> 
> Since we switched to lit as the test driver we've been seeing it getting killed as the result of a SIGHUP signal. The problem doesn't reproduce on every machine and there seems to be a correlation between number of occurrences and thread count.
> 
> Davide and Raphael spent some time narrowing down what particular test is causing this and it seems that TestChangeProcessGroup.py is always involved. However it never reproduces when running just this test. I was able to reproduce pretty consistently with the following filter:
> 
> ./bin/llvm-lit ../llvm/tools/lldb/lit/Suite/ --filter="process"
> 
> Bisecting the test itself didn't help much, the problem reproduces as soon as we attach to the inferior.
> 
> At this point it is still not clear who is sending the SIGHUP and why it's reaching the lit test driver. Fred suggested that it might have something to do with process groups (which would be an interesting coincidence given the previously mentioned test) and he suggested having the test run in different process groups. Indeed, adding a call to os.setpgrp() in lit's executeCommand and having a different process group per test prevent us from seeing this. Regardless of this issue I think it's reasonable to have tests run in their process group, so if nobody objects I propose adding this to lit in llvm.
> 
> Still, I'd like to understand where the signal is coming from and fix the root cause in addition to the symptom. Maybe someone here has an idea of what might be going on?
> 
> Thanks,
> Jonas
> 
> PS
>   
> 1. There's two places where we send a SIGHUP ourself, with that code removed we still receive the signal, which suggests that it might be coming from Python or the OS.
> 2. If you're able to reproduce you'll see that adding an early return before the attach in TestChangeProcessGroup.py hides/prevents the problem. Moving the return down one line and it pops up again.
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Hi Jonas,

Sounds like you have found an interesting issue to debug. I've tried 
running the command you mention locally, and I didn't see any failures 
in 100 runs.

There doesn't seem to be anything in the TestChangeProcessGroup which 
sends a signal, though I can imagine that the act of changing a process 
group mid-debug could be enough to confuse someone to send it. However, 
I am having trouble reconciling this with your PS #2, because if 
attaching is sufficient to trigger this (i.e., no group changing takes 
place), then this test is not much different than any other test where 
we spawn an inferior and then attach to it.

I am aware of one other instance where we send a spurious signal, though 
it's SIGINT in this case 
<https://github.com/llvm-mirror/lldb/blob/master/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp#L3645>. 
The issue there is that we don't check whether the debug server has 
exited before we send SIGINT to it (which it normally does on its own at 
the end of debug session). So if the debug server does exit and its pid 
gets recycled before we get a chance to send this signal, we can end up 
killing a random process.

Now this may seem unrelated to your issue, but SIGHUP is sent 
automatically as a result of a process losing its controlling tty. So, 
if that SIGINT ends up killing the process holding the master end of a 
pty, this could result in some SIGHUPs being sent too. Unfortunately, 
this doesn't fully stack up either, because the process holding the 
master pty is probably a long-lived one, so its pid is unlikely to match 
one of the transient debugserver pids. Nevertheless, it could be worth 
just commenting out that line and seeing what happens.

For debugging, maybe you could try installing a SIGHUP handler into the 
lit process, which would dump the received siginfo_t structure. Decoding 
that may provide some insight into who is sending that signal (si_pid) 
and why (si_code).

As for adding process group support into lit, I think that having each 
test run (*not* each executed command) in it's own group is reasonable. 
However, be aware that this changes the behaviour of how all signals (in 
particular the SIGINT you get when typing ^C) get delivered. AFAIK, lit 
doesn't have any special code for cleaning up the spawned processes and 
relies on the fact that ^C will send a SIGINT to the entire "foreground 
process group" and terminate stuff. If you start creating a bunch of 
process groups, you may need to add more explicit termination logic too.

cheers,
pl