Fixes https://github.com/NixOS/nixpkgs/issues/237637 and its entire
class of problems by making nix-doc save the version it was built for in
the library and then bail harmlessly if it is mismatched.
```
dev/nixpkgs2 » nix-build -A nix-doc
/nix/store/wv9nm47lplyz4b0pa4549zwrnsp3zvaf-nix-doc-0.5.10
dev/nixpkgs2 » nix-build -A nixVersions.nix_2_14 -o result2
/nix/store/ka0ygdzl9jd0j77y7ls6shngdz9vvqpn-nix-2.14.1
dev/nixpkgs2 » ./result2/bin/nix --plugin-files ./result/lib/libnix_doc_plugin.so repl
nix-doc warning: mismatched nix version, not loading
Welcome to Nix 2.14.1. Type :? for help.
nix-repl>
```
### Summary
This PR completely and finally solves the gcc<->glibc circular
`buildInputs` problem, for cross compilation. The same technique
can be applied to native builds in the future.
Closes#213453
### Motivation
Prior to this PR, we had the following circular `buildInputs` problem:
1. gcc has glibc in its `buildInputs`
- a compiled copy of glibc must be present before building gcc;
if it isn't, gcc cripples itself (`inhibit_libc`) and refuses
to build libgcc_s.so
2. glibc has libgcc_s.so in its `buildInputs`
- glibc `dlopen()`s libgcc_s.so in order to implement POSIX
thread cancellation. For security reasons `glibc` requires
that the path to `libgcc_s.so` is [hardwired] into `glibc` at
compile time, so it's technically not a true dynamic link -- it
just pretends to be one.
3. libgcc_s.so is built in the same derivation as gcc
- libgcc_s.so is built as part of the gcc build process
We must cut one of these three links in the loop.
### Previous Attempts
Previously https://github.com/NixOS/nixpkgs/pull/238154 had
attempted to cut link (1) by building `gcc` without `glibc`, and
using the `libgcc_s` which emerges from that build. Unfortunately
this just doesn't work. GCC's configure script extracts quite a lot
of information from the glibc headers (which are a build artifact --
you can't just copy them out of the source tarball) and various
`./configure`-driven linking attempts. If `glibc` isn't around at
build time you wind up with a `libgcc_s.so` that is missing various
unwinder features (see https://github.com/NixOS/nixpkgs/issues/213453
for the most problematic one).
Musl "cuts" link (2), or rather never creates it in the first place.
["Cancellation cleanup handling in musl has no relationship to C++
exceptions and unwinding... glibc implements cancellation as an
exception"](https://wiki.musl-libc.org/functional-differences-from-glibc.html#Thread-cancellation).
IMHO Musl made the smarter decision here. It is incredibly rare to
find a codebase that uses both POSIX thread cancellation *and* C++
exceptions. I have never seen a codebase that uses both *and*
expects them to be aware of each other, and I would be astonished if
one existed. Glibc paid an immense cost in complexity for something
nobody has ever used.
### Changes Made
This PR cuts link (3): instead of building libgcc_s.so as part of
gcc, we build it separately from gcc. Now there is a strict acyclic
graph of `buildInputs`:
```
gccWithoutTargetLibc
|
+--->glibc-nolibgcc
| |
| v
+--->libgcc
| |
| v
+--->glibc
| |
| v
+--->gcc
```
In other words, there's a simple linear `buildInputs` chain
`glibc-nolibgcc` `->` `libgcc` `->` `glibc` `->` `gcc` where all
four packages are compiled by (and therefore have as a
`(native)BuildInput`) `gccWithoutTargetLibc`.
`gccWithoutTargetLibc` and `glibc-nolibgcc` are strictly
bootstrapping artifacts; nothing else has them as a `buildInput` and
they shouldn't appear in the closure of any final deployment
packages. `glibc-nolibgcc` lacks `libgcc_s.so`, so it will segfault
if you try to use it with POSIX thread cancellation. Fortunately
all we need from it is (a) its headers (`lib.getDev`) and (b) to use
it in the `./configure` script for `libgcc`.
When translated over to the native bootstrap, `xgcc` takes the place
of `gccWithoutTargetLibc`, and the "first `glibc`" (we build two of
them) takes the place of `glibc-nolibgcc`. At that point our native
and cross bootstrap have the same overall architecture, and it
becomes possible to merge them (at last!)
[213453]: https://github.com/NixOS/nixpkgs/issues/213453
[238154]: https://github.com/NixOS/nixpkgs/pull/238154
[hardwired]: 7553d0fe29/pkgs/development/libraries/glibc/default.nix (L69-L88)
