Segfault when calling back from a thread, created in C environment

[KlausC]

I want to use the main processing loop of a C-library, which implements specific functionality in callbacks.
The callbacks shall be implemented in Julia. The following MWE demonstrates the difficulties, when the callback functions are invoked from POSIX-threads, which have been launched in C.

callmtcb.jl

module Testcb
    f1() = println("called f1()")
    f2() = exit(99)
    cf1 = @cfunction f1 Cvoid ()
    cf2 = @cfunction f2 Cvoid ()

    push!(Base.DL_LOAD_PATH, pwd())
    dl = Base.Libc.dlopen("libmtcb")
    dls = Base.Libc.Libdl.dlsym(dl, :cb_setup)
    ccall(dls, Cvoid, (Ptr{Cvoid}, Ptr{Cvoid}), cf1, cf2)
end

libmtcb.c

#include <pthread.h>

pthread_t threadid;
void(*savedf2)(void);

void* cb_thread(void* arg);

void cb_setup(void(*f1)(void), void(*f2)(void)) {

    (*f1)();
    savedf2 = f2;

    pthread_create(&threadid, NULL, cb_thread, (void*)NULL);

    pthread_join(threadid, NULL);
    pthread_exit((void*)0);
}

void* cb_thread(void* arg) {
    (*savedf2)();
    return (void*)0;
}

Result of execution:

 make run
/home/crusius/julia/julia --startup-file=no callmtcb.jl
called f1()

signal (11): Segmentation fault
in expression starting at /home/crusius/dev/FuseApi/test/embed/callmtcb.jl:11
Allocations: 2749 (Pool: 2739; Big: 10); GC: 0
make: *** [Makefile:18: run] Segmentation fault (core dumped)

A gdb session revealed, that the callback to f2 propagates into the call to exit(99) and then fails.
Actually the threads local variable jl_pgcstack is not initialized, so the Julia runtime has no chance to succeed.

The test files and the setup for a debug session are found here: https://github.com/KlausC/FuseApi.jl/blob/main/test/embed/
There is a Makefile with run and debug targets to explore the example under Linux and Gnome.

[vchuravy]

This is a duplicated of #17573. It is a known limitation that Julia functions can't be executed on a foreign thread.

You can attempt to do some minimal interactions an as an example call ccall(:uv_async_send, ...). To trigger an AsyncCondition on a Julia thread.