Building Julia aborts when `JULIA_NUM_THREADS=4,1` is set

[oscardssmith]

This appears to have regressed on #56409. Specifically, wehn building Base, Julia aborts at

Compiling the compiler. This may take several minutes ...
Base.Compiler ──── 280.354 seconds
flparse.jl
    JULIA usr/lib/julia/sys.ji
Aborted (core dumped)
*** This error might be fixed by running `make clean`. If the error persists, try `make cleanall`. ***
make[1]: *** [sysimage.mk:71: /home/oscardssmith/julia/usr/lib/julia/sys.ji] Error 1
make: *** [Makefile:114: julia-sysimg-ji] Error 2

GDB ing the process shows

Thread 5 "julia" received signal SIGABRT, Aborted.
[Switching to Thread 0x7fffe499d640 (LWP 292041)]
__pthread_kill_implementation (no_tid=0, signo=6, threadid=140737028675136) at ./nptl/pthread_kill.c:44
44	./nptl/pthread_kill.c: No such file or directory.
(gdb) bt
#0  __pthread_kill_implementation (no_tid=0, signo=6, threadid=140737028675136) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (signo=6, threadid=140737028675136) at ./nptl/pthread_kill.c:78
#2  __GI___pthread_kill (threadid=140737028675136, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3  0x00007ffff7dbd476 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007ffff7da37f3 in __GI_abort () at ./stdlib/abort.c:79
#5  0x00007ffff7144313 in jl_finish_task (ct=ct@entry=0x7fffee934010) at /home/oscardssmith/julia/src/task.c:345
#6  0x00007ffff7185829 in jl_threadfun (arg=0x5555556d5210) at /home/oscardssmith/julia/src/scheduler.c:122
#7  0x00007ffff7e0fac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#8  0x00007ffff7ea1850 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

[DilumAluthge]

@oscardssmith @gbaraldi Should we add a Buildkite job that tests this, so we can make sure that this doesn't regress again in the future?

Alternatively, should we just have our build system override the value of JULIA_NUM_THREADS and set JULIA_NUM_THREADS=1 when building?

[oscardssmith]

IMO the medium term fix to this is that --threads=default should be our actual default which would mean that buildkite would detect these issues. Our build system is supposed to override the number of threads we launch with (which recently broke, hence the issue)

[giordano]

I don't see any segmentation fault.

[oscardssmith]

updated initial issue to be clearer.

[KristofferC]

Should we add a Buildkite job that tests this, so we can make sure that this doesn't regress again in the future?

Seems excessive to me when it is only a build issue.