mac: Produce coredumps on segfault

[Keno]

This changes the mach exception server to ignore fatal SIGSEGVs,
letting regular kernel processing handle it (by performing POSIX
signal delivery and then subsequently coredumping), rather than
quitting the process directly. There's probably some way to
induce the kernel to perform core dumping directly from the
exception server, but I think it'll be less confusing all around
to just have segfaults take the standard path.

Hoping this will help debug #46152.

[DilumAluthge]

Want to stick a ccall(:raise, Cint, (Cint,), 11) somewhere in one of the test sets? (We'll remove it before merging this PR.)

[Keno]

I've verified that this produces core dumps locally. That doesn't mean the buildkite version works, but I'd rather merge this and then do that experiment in a separate PR.

[DilumAluthge]

FWIW, it seems like this doesn't currently work on Buildkite, because there are no artifacts uploaded for https://buildkite.com/julialang/julia-master/builds/14263#018232a5-5eb0-4f3e-adb3-2a1ec111f781.

But I agree that let's first merge this PR, and then work separately to get this working on Buildkite.

[vtjnash]

Will this break the default stacktrace printer?

[DilumAluthge]

I've verified that this produces core dumps locally.

@Keno:

1. On your machine, what is the output of sysctl -n kern.corefile?
2. In what directory were the coredump files created?

[Keno]

Will this break the default stacktrace printer?

No. The POSIX signal handling will print the stack trace:

               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.9.0-DEV.1040 (2022-07-23)
 _/ |\__'_|_|_|\__'_|  |  Commit cee90db76a* (1 day old master)
|__/                   |

julia> unsafe_load(Ptr{Cint}(C_NULL))

[13573] signal (11): Segmentation fault: 11
in expression starting at REPL[1]:1
unsafe_load at ./pointer.jl:110 [inlined]
unsafe_load at ./pointer.jl:110
unknown function (ip: 0x10099001f)
Allocations: 2875 (Pool: 2864; Big: 11); GC: 0
zsh: segmentation fault (core dumped)  ~/julia/julia

[Keno]

• On your machine, what is the output of sysctl -n kern.corefile?

% sysctl -n kern.corefile
/cores/core.%P

• In what directory were the coredump files created?

/cores