Signals and the haskell runtime

[jeff-davis]

I haven't looked deeply into this, but on my platform (x86_64 ubuntu 22.04), based on output of strace -p $PID -e signal, it seems that plhaskell makes some changes related to signals due to the haskell runtime.

I see calls to rt_sigaction, rt_sigreturn, and rt_sigprocmask involving SIGVTALRM and SIGURG. It seems that Postgres doesn't use SIGVTARLM, but it does seem to use SIGURG for latches. I'm slightly worried that there are edge cases where a plhaskell function can end up interfering with Postgres.

Is there a way to get the haskell runtime to not mess with the signal handlers?

[ed-o-saurus]

I turned off signal handlers by passing the RTS the --install-signal-handlers=no option. (see here)

It looks like I should have also added -V0. Does the behavior go away when you add it at line 559 of src/plhaskell.c?

[ed-o-saurus]

I ran strace against a postgres backend and was able to observe SIGVALRM calls. How were you able to observe SIGURG calls?

[jeff-davis]

Without -V0, strace -p $PID -e -signal shows:

rt_sigaction(SIGVTALRM, {sa_handler=0x7ff2c31e2470, sa_mask=[], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7ff2ce501520}, NULL, 8) = 0
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=2, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 6
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283469926400
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 1
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 2
rt_sigprocmask(SIG_BLOCK, [], [URG], 8) = 0
rt_sigprocmask(SIG_SETMASK, [URG], NULL, 8) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=670031, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
rt_sigprocmask(SIG_BLOCK, [], [URG], 8) = 0
rt_sigprocmask(SIG_SETMASK, [URG], NULL, 8) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=670032, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
rt_sigprocmask(SIG_BLOCK, [], [URG], 8) = 0
rt_sigprocmask(SIG_SETMASK, [URG], NULL, 8) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=670033, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283474141184
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 3
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283473293312
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 0
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 0
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 100
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283469096257
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283468393954
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283468386920
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = -1 ENOENT (No such file or directory)
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 283476414856
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 13
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 140680609455626
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 3674937295934324844
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = 0
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = -1 EINTR (Interrupted system call)
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = -1 EINTR (Interrupted system call)
--- SIGVTALRM {si_signo=SIGVTALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_int=0, si_ptr=NULL} ---
rt_sigreturn({mask=[URG]})              = -1 EINTR (Interrupted system call)
...

With -V0, it shows just:

rt_sigprocmask(SIG_BLOCK, [], [URG], 8) = 0
rt_sigprocmask(SIG_SETMASK, [URG], NULL, 8) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=670555, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
rt_sigprocmask(SIG_BLOCK, [], [URG], 8) = 0
rt_sigprocmask(SIG_SETMASK, [URG], NULL, 8) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=670556, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
rt_sigprocmask(SIG_BLOCK, [], [URG], 8) = 0
rt_sigprocmask(SIG_SETMASK, [URG], NULL, 8) = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=670557, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---

The syscalls involving SIGURG are coming from:

#0  __GI___pthread_sigmask (how=0, newmask=<optimized out>, oldmask=0x7f1d54d837e0) at ./nptl/pthread_sigmask.c:43
#1  0x00007f1d6006171d in __GI___sigprocmask (how=<optimized out>, set=<optimized out>, oset=<optimized out>) at ../sysdeps/unix/sysv/linux/sigprocmask.c:25
#2  0x00007f1d4ef577e5 in runInteractiveProcess () from /usr/lib/ghc/process-1.6.9.0/libHSprocess-1.6.9.0-ghc8.8.4.so
#3  0x00007f1d4ef50781 in ?? () from /usr/lib/ghc/process-1.6.9.0/libHSprocess-1.6.9.0-ghc8.8.4.so
#4  0x00000042000c3bc8 in ?? ()
#5  0x00000042000c3be0 in ?? ()
#6  0x00000042000c3bf8 in ?? ()
#7  0x0000000000000000 in ?? ()

This is Postgres v14, PL/Haskell 1.0, Ubuntu 22.04.1 on x86_64.