[Issue 16856] D does not work on FreeBSD current (what will eventually be 12) due to libunwind

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16856

--- Comment #10 from Nemanja Boric <4burgos at gmail.com> ---
Ok, I finally got some time to get back to this issue.

There's a sigbus really running, but this is caused by GC, because the runtime
asserts in the shared library finalizers, so it seems that the instance is no
longer there.

I've traced the failing druntime assert (so far, who knows
what else is waiting for us) to this particular commit in rtld.c:

https://github.com/freebsd/freebsd/commit/3ff2e66ecba2094f5c1c1efe7f2d009649527195

So, this is the problem. At the end of the program, fini()s are called for the
shared library and the main executable. Then they call `_do_global_dtors_aux`,
and at that point they will call _d_dso_registry, which will (the problem is
here:) call dlopen (albeit with RTLD_NOLOAD) to obtain the handle for the
object by name.

However, since this particular commit, this doesn't work anymore (and it's
questionable if it should work) - you can't bump a reference count of an object
that's just going to die (dlopen still bumps reference count, even with
RTLD_LOAD passed).

I would guess somehow skipping dlopen calls in this scenario should be figured
out.
Maybe skipping just for the current object, or maybe caching the handles when
first obtained (not sure if they can change on their own; I don't think so, but
still). I'll see to submit a PR tomorrow, now I know where the problem is.

It was quite a ride finding this out. Because first call to dlopen was failing
for the
main executable, so documentation says: "use NULL if you want the main
executable instead", so after doing this - I got it working, so I thought
there's something
special with this path. What's interesting is that my confusion is caused by
the bug
in FreeBSD's code - if the current limitation apply - don't reference "doomed"
object,
one shouldn't be able to work around it by passing NULL. I'll see into sending
a patch there as well.

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