Report the top of the exception stack when there's no handler.

[maleadt]

This happens with PackageCompiler and modules that error during __init__ (which gets called way earlier if said modules are part of the system image). Before:

fatal: error thrown and no exception handler available.
#<null>
error at ./error.jl:33
__init__ at /home/tim/Julia/pkg/DummyPackage/src/DummyPackage.jl:4
jl_sysimg_fvars_base at /tmp/sys.so (unknown line)
_jl_invoke at /home/tim/Julia/src/julia/src/gf.c:2205 [inlined]
jl_apply_generic at /home/tim/Julia/src/julia/src/gf.c:2406
jl_apply at /home/tim/Julia/src/julia/src/julia.h:1701 [inlined]
jl_module_run_initializer at /home/tim/Julia/src/julia/src/toplevel.c:74
_julia_init at /home/tim/Julia/src/julia/src/init.c:796
repl_entrypoint at /home/tim/Julia/src/julia/src/jlapi.c:682
main at julia (unknown line)
__libc_start_main at /nix/store/a6rnjp15qgp8a699dlffqj94hzy1nldg-glibc-2.32/lib/libc.so.6 (unknown line)
_start at julia (unknown line)

After:

fatal: error thrown and no exception handler available.
InitError(mod=:DummyPackage, error=ErrorException("true"))
error at ./error.jl:33
__init__ at /home/tim/Julia/pkg/DummyPackage/src/DummyPackage.jl:4
jl_sysimg_fvars_base at /tmp/sys.so (unknown line)
_jl_invoke at /home/tim/Julia/src/julia/src/gf.c:2205 [inlined]
jl_apply_generic at /home/tim/Julia/src/julia/src/gf.c:2406
jl_apply at /home/tim/Julia/src/julia/src/julia.h:1701 [inlined]
jl_module_run_initializer at /home/tim/Julia/src/julia/src/toplevel.c:74
_julia_init at /home/tim/Julia/src/julia/src/init.c:796
repl_entrypoint at /home/tim/Julia/src/julia/src/jlapi.c:682
main at julia (unknown line)
__libc_start_main at /nix/store/a6rnjp15qgp8a699dlffqj94hzy1nldg-glibc-2.32/lib/libc.so.6 (unknown line)
_start at julia (unknown line)

At least the exception is there now.

cc @KristofferC

[c42f]

This looks useful, I'm only confused that it's necessary — we seem to only call jl_no_exc_handler with either the current exception in task.c, or using jl_current_exception(). Or does PackageCompiler call jl_no_exc_handler?

After this change, it'll be unnecessary to pass jl_current_exception here (as this will be the default) so perhaps you could change those other couple of call sites too.

[maleadt]

we seem to only call jl_no_exc_handler with either the current exception in task.c, or using jl_current_exception()

During jl_module_run_initializer, we call jl_rethrow_other with jl_current_exception indeed, but that just puts the exception on the top of the stack and continues to call throw_internal(NULL) which then ends up in jl_no_exc_handler with a NULL exception.

[c42f]

Oh I see. Yes it appears the use of NULL to signify "this is a rethrow" has led to confusion here.

Well this code LGTM (other than the comment about NULL denoting the top of the stack. That's either wrong, or I'm confused by it :-) )

[maleadt]

Thanks! I updated the comment to reflect NULL exceptions are only used for rethrowing.

[c42f]

Looks great 👍