This fixes a crash that looks like:

```
Thread 1 "nix-build" received signal SIGSEGV, Segmentation fault.
0x00007ffff7ad22a0 in GC_push_all_eager () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
(gdb) bt
0  0x00007ffff7ad22a0 in GC_push_all_eager () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
1  0x00007ffff7adeefb in GC_push_all_stacks () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
2  0x00007ffff7ad5ac7 in GC_mark_some () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
3  0x00007ffff7ad77bd in GC_stopped_mark () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
4  0x00007ffff7adbe3a in GC_try_to_collect_inner.part.0 () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
5  0x00007ffff7adc2a2 in GC_collect_or_expand () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
6  0x00007ffff7adc4f8 in GC_allocobj () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
7  0x00007ffff7adc88f in GC_generic_malloc_inner () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
8  0x00007ffff7ae1a04 in GC_generic_malloc_many () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
9  0x00007ffff7ae1c72 in GC_malloc_kind () from /nix/store/p1z58l18klf88iijpd0qi8yd2n9lhlk4-boehm-gc-8.0.4/lib/libgc.so.1
10 0x00007ffff7e003d6 in nix::EvalState::allocValue() () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
11 0x00007ffff7e04b9c in nix::EvalState::callPrimOp(nix::Value&, nix::Value&, nix::Value&, nix::Pos const&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
12 0x00007ffff7e0a773 in nix::EvalState::callFunction(nix::Value&, nix::Value&, nix::Value&, nix::Pos const&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
13 0x00007ffff7e0a91d in nix::ExprApp::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
14 0x00007ffff7e0a8f8 in nix::ExprApp::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
15 0x00007ffff7e0e0e8 in nix::ExprOpNEq::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
16 0x00007ffff7e0d708 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
17 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
18 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
19 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
20 0x00007ffff7e0d695 in nix::ExprOpOr::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
21 0x00007ffff7e09e19 in nix::ExprOpNot::eval(nix::EvalState&, nix::Env&, nix::Value&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
22 0x00007ffff7e0a792 in nix::EvalState::callFunction(nix::Value&, nix::Value&, nix::Value&, nix::Pos const&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
23 0x00007ffff7e8cba0 in nix::addPath(nix::EvalState&, nix::Pos const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Value*, nix::FileIngestionMethod, std::optional<nix::Hash>, nix::Value&)::{lambda(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)#1}::operator()(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixexpr.so
24 0x00007ffff752e6f9 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
25 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
26 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
27 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
28 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
29 0x00007ffff752e8e2 in nix::dump(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, nix::Sink&, std::function<bool (std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)>&) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
30 0x00007ffff757f8c0 in void boost::context::detail::fiber_entry<boost::context::detail::fiber_record<boost::context::fiber, nix::VirtualStackAllocator, boost::coroutines2::detail::pull_coroutine<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::control_block::control_block<nix::VirtualStackAllocator, nix::sinkToSource(std::function<void (nix::Sink&)>, std::function<void ()>)::SinkToSource::read(char*, unsigned long)::{lambda(boost::coroutines2::detail::push_coroutine<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&)#1}>(boost::context::preallocated, nix::VirtualStackAllocator&&, nix::sinkToSource(std::function<void (nix::Sink&)>, std::function<void ()>)::SinkToSource::read(char*, unsigned long)::{lambda(boost::coroutines2::detail::push_coroutine<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >&)#1}&&)::{lambda(boost::context::fiber&&)#1}> >(boost::context::detail::transfer_t) () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libnixutil.so
31 0x00007ffff6f331ef in make_fcontext () from /nix/store/hzdzcv9d3bc8rlsaphh7x54zsf0x8nx6-nix-2.4pre20210601_5985b8b/lib/libboost_context.so.1.69.0
32 0x0000000000000000 in ?? ()
```

When recursive Nix is enabled, NIX_REMOTE is set to
unix:///build/.nix-socket, which doesn't work outside of the sandbox.

configure.ac: fix use of unread LIBS variable

This fixes both the SunOS/Solaris check, and the libatomic check, which
reference $LIBS, which has not been used since automake was stripped
out of the code.

Install zsh completion script

Fix error: 'optional' in namespace 'std' does not name a template type

Flake input changes:

* Updated 'nixpkgs': 'github:NixOS/nixpkgs/3a2e0c36e79cecaf196cbea23e75e74710140ea4' -> 'github:NixOS/nixpkgs/bb8a5e54845012ed1375ffd5f317d2fdf434b20e'

Let nix flake check keep going when keep-going is set

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

This function might or might not throw depending on the value of
`keepGoing`, so naming it `throw_` was a bit confusing

When the `keep-going` option is set to `true`, make `nix flake check`
continue as much as it can before failing.

The UI isn’t perfect as-it-is as all the lines currently start with a
mostly useless `error (ignored): error:` prefix, but I’m not sure what
the best output would be, so I’ll leave it as-it-is for the time being

(This is a bit hijacking the `keep-going` flag as it’s supposed to be a
build-time only thing. But I think it’s faire to reuse it here).

Fix https://github.com/NixOS/nix/issues/4450

When adding a path to the local store (via `LocalStore::addToStore`),
ensure that the `ca` field of the provided `ValidPathInfo` does indeed
correspond to the content of the path.
Otherwise any untrusted user (or any binary cache) can add arbitrary
content-addressed paths to the store (as content-addressed paths don’t
need a signature).

Only link with libdl on Linux

Fix typo in documentation

unified macro style for ENABLE_S3

Linux is (as far as I know) the only mainstream operating system that
requires linking with libdl for dlopen.  On BSD, libdl doesn't exist,
so on non-FreeBSD BSDs linking will currently fail.  On macOS, it's
apparently just a symlink to libSystem (macOS libc), presumably
present for compatibility with things that assume Linux.

So the right thing to do here is to only add -ldl on Linux, not to add
it for everything that isn't FreeBSD.

Co-authored-by: Matthew Bauer <mjbauer95@gmail.com>

disable lowdown sandbox on aarch64-darwin

Add .tar.zst support for TarballInputScheme

Make the Nar hash non modulo

Throw freenode down the memory hole

It makes much more sense to have the Nar hash be a plain straight hash
rather than a hash modulo

fix doc: nix profile info -> nix profile list

Signed-off-by: Michael Adler <therisen06@gmail.com>

Extract a generic `computeClosure` function

Move the `closure` logic of `computeFSClosure` to its own (templated) function.

This doesn’t bring much by itself (except for the ability to properly
test the “closure” functionality independently from the rest), but it
allows reusing it (in particular for the realisations which will require
a very similar closure computation)

Fix extra slash in canonPath output

When you have a symlink like:

  /tmp -> ./private/tmp

you need to resolve ./private/tmp relative to /tmp’s dir: ‘/’. Unlike
any other path output by dirOf, / ends with a slash. We don’t want
trailing slashes here since we will append another slash in the next
comoponent, so clear s like we would if it was a symlink to an absoute
path.

This should fix at least part of the issue in
https://github.com/NixOS/nix/issues/4822, will need confirmation that
it actually fixes the problem to close though.

Introduced in f3f22870.

Restore an accidentally suppressed negation

Accidentally removed in ca96f521. This
caused `nix run` to systematically fail with

```
error: app program '/nix/store/…' is not in the Nix store
```