When running `nix develop` for a package, Nix records the stdenv
environment with NIX_LOG_FD set. That is, when the actual development
shell runs, it uses the functions that attempt to log to NIX_LOG_FD, but
this variable is not actually set.
As a workaround, check whether NIX_LOG_FD is set at runtime.
Example (before this change):
```console
$ nix develop --file . bash
$ echo "${NIX_LOG_FD-unset}"
unset
$ runPhase unpackPhase
bash: "$NIX_LOG_FD": Bad file descriptor
Running phase: unpackPhase
unpacking source archive /nix/store/v28dv6l0qk3j382kp40bksa1v6h7dx9p-bash-5.2.tar.gz
bash: "$NIX_LOG_FD": Bad file descriptor
source root is bash-5.2
bash: "$NIX_LOG_FD": Bad file descriptor
setting SOURCE_DATE_EPOCH to timestamp 1663942708 of file bash-5.2/y.tab.h
```
After this change:
```console
$ nix develop --file . bash
$ runPhase unpackPhase
Running phase: unpackPhase
unpacking source archive /nix/store/v28dv6l0qk3j382kp40bksa1v6h7dx9p-bash-5.2.tar.gz
source root is bash-5.2
setting SOURCE_DATE_EPOCH to timestamp 1663942708 of file bash-5.2/y.tab.h
```
This reduces the number of Python builds in the bootstrap to two: a minimal build and a normal build. Both have LTO disabled, which is required due to missing LLVM LTO libraries. This is necessary to correctly enable LTO builds in Python because it needs `llvm-ar` from `stdenv.cc.cc.libllvm`, which does not exist in the bootstrap.
The old stdenv didn't work, and was also impure. The new one works, and
is pure. Presently, the bootstrap tools are cross compiled into one small
nar and one large tar, which is then unpacked, patched, and split into
smaller derivations. Efforts were made to make the boot process as short
as possible - there are only two clangs built, and as many packages are
propagated between stages as possible while leaving the bootstrap tools
out of the final stdenv's closure.
Fetchers can use the `curl` binary from the bootstrap tools. Allowing packages in the Darwin bootstrap to link curl makes any curl update cause a full rebuild on Darwin, which is undesirable.
After https://github.com/NixOS/nixpkgs/pull/322388, darwin.cctools is just Apple’s cctools again. The replacement for what the bootstrap tools wants is darwin.binutils-unwrapped.
GNU binutils is not preferred on Darwin, and newer versions have issues building. Make it an evaluation error to use it in the Darwin stdenv bootstrap.
- Only link binaries that exist for stage 0 cctools and LLVM bintools;
- Drop cctools-llvm in favor of the updated darwin.binutils; and
- Update llvm-manages Python overrides (needed for newer versions of LLVM).
