Co-authored-by: Seeker <meaningseeking@protonmail.com>

Try to re-exec the Homebrew binary to get the environment variables we
need and give up if that doesn't work.

We don’t really have a better solution than this because of how the auto-updater’s Bash code worked on the version they are updating from. Users will never hit this more than once.

Fixes #5513

This means that any new environment variables or changes to `bin/brew`
or `brew.sh` will be used in the new process. This also allows the
removal of various fallbacks from autoupdates from old versions.

Consolidate the handling of which directories need to exist and which
need to be writable. Additionally, add a fatal check for formula
installations to ensure that any directories that need to be writable
are so before attempting an installation.

Fixes #4626.

This can happen when updating from a previous version of Homebrew.

- Ensure that `HOMEBREW_TEMP` is only displayed in `brew config` when
  it's non-default.
- Attempt to create a missing `HOMEBREW_TEMP` directory rather than
  failing to `realpath`. Note this will still fail on permissions errors
  which is to be expected.

We provide a /private/tmp fallback in bin/brew, so
this is no longer necessary.

Fix various circular requirements, method redefinitions, etc.

This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

This reverts commit 23728729, reversing
changes made to 3e4547f5.

If people have `HOMEBREW_RUBY_PATH` set then things explode in a rather
confusing fashion. Instead, run `bundle` for them with the arguments
that they'd want.

Also, move `macho` requires into the module itself; it's a pain having
to do everything for Bundler before requiring `pathname` which is a core
Ruby class.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Move the `LinkedKegs` migration into `utils.rb` so it can also be called
from `brew.rb` on startup.

Ensure that `brew update` always runs the LinkedKegs migration if needed
as it may not have been run by `brew update` if it was using `--preinstall` or
a `git pull` etc.

Also, if the old paths still exist: just use them instead.

Finally, always try to unlink/unpin before link/pin.

Fixes https://github.com/Homebrew/homebrew-core/issues/4918.

These don't make sense to be tied to the `HOMEBREW_REPOSITORY` but
instead should live in the `HOMEBREW_PREFIX` as they all relate to its
state.

See discussion in Homebrew/brew#800.

Print a better exception message and handle this in more cases rather
than producing confusing errors when it’s unset.

TMPDIR, TEMP and TMP (when set) are not whitelisted for writing in
sandbox.rb, which could result in sandbox violations when programs
attempt to write to these locations.

Setting TMPDIR, TEMP and TMP to HOMEBREW_TEMP (which defaults to /tmp
when not set) works around the aforementioned problem and also improves
uniformity in the locations of tempfiles created during Homebrew
operations.

Caveat: Non-matching HOMEBREW_TEMP and TMPDIR could lead to undesirable
side effects in certain cases, e.g., emacsclient not being able to find
an existing server (whose socket lives in $TMPDIR/emacs$UID/) when
launched through brew edit.

Move some stuff formerly in `Library/ENV` around:
- Move `Library/ENV/$XCODE_VERSION` to `Library/Homebrew/env/super` as they are
  all superenv wrappers and all symlinks to the same version. We never needed
  the "separate shims for separate versions" functionality and it just adds
  confusion.
- Move `Library/ENV/pkgconfig` to `Library/Homebrew/env/pkgconfig` to get more
  things under `Library/Homebrew`
- Move `Library/ENV/scm` to `Library/scm` as these wrappers are not actually
  used by or related to superenv (or stdenv) in any way.

Since #292, HOMEBREW_CACHE was moved to a per-user directory. This makes
it unsuitable to store global lock files on multiple users environment.

Therefore, introducing a global lock directory `/Library/Lock.d` to
store lock files from formula lockers as well as `brew update`.

* cleanup: accept cache as an argument.

* config: move default HOMEBREW_CACHE to ~/Library.

* brew.1: document new default Homebrew cache.

* update-report: migrate legacy Homebrew cache.

`Library/ENV` like `Library/Homebrew` is part of Homebrew basecode. It
should be able to be accessed during the `brew tests`.

By adding HOMEBREW_ENV_PATH variable, we allow test suit to locate these
codes.

The default HOMEBREW_LIBRARY_PATH should be
  /usr/local/Library/Homebrew
not
  /usr/local/Library/Homebrew/Homebrew

Closes Homebrew/homebrew#49384.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Currently HOMEBREW_BREW_FILE is a String, while other of HOMEBREW_*
variables are all Pathname. This commit unifies them all as Pathname,
so it will not cause any confusion.

Closes Homebrew/homebrew#48872.

Signed-off-by: Xu Cheng <xucheng@me.com>