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Date: Thu, 5 Oct 2023 05:37:41 +0200
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Subject: Re: Hung processes with althttpd web server
Am Wed, Oct 04, 2023 at 09:41:41PM -0400 schrieb Carl Chave:
> Hello, I'm running the althttpd web server on Alpine Linux using a Ramnode VPS.
>
> I've been having issues for quite a while with "hung" processes. There
> is a long lived parent process and then a short lived forked process
> for each http request. What I've been seeing is that the forked
> processes will sometimes get stuck:
>
> sod01:/srv/www/log$ sudo strace -p 11329
> strace: Process 11329 attached
> futex(0x7f5bdcd77900, FUTEX_WAIT_PRIVATE, 4294967295, NULL
>
I often see this system call hung when signal handlers are doing
signal-unsafe things. Looking at the source code, that is exactly what
happens if the process catches a signal at the wrong time. Try removing
all calls to signal(); that should do what the designers intended
better (namely quit the process). If you want to log when a process dies
of unnatural causes, that's something the parent process can do.
The signal handler will call MakeLogEntry(), and that will do
signal-unsafe things such as call free(), localtime(), or fopen(). If
the main process is currently using malloc() when that happens, you will
get precisely this hang.
> Please see this forum thread for additional information:
> https://sqlite.org/althttpd/forumpost/4dc31619341ce947
>
Seems like they haven't yet found the trail of the signal handler.
Ciao,
Markus
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