When determining whether to remove a symlink during unlinking, we check
three things:
(a) Is the destination a symlink?
(b) Does the destination exist?
(c) Does the destination resolve to the source path?
However, since we know that the source path exists, (b) is guaranteed if
(a) and (c) are true. Thus checking (b) is unnecessary.
Similarly, when creating a new symlink during linking, we first check to
see if the link already exists by checking the same three criteria.
Again, checking (b) is unnecessary here.
See also the expanded test coverage in b52b579b.
Addendum: although we know that the source path exists during unlinking,
it doesn't matter. If the source path does not exist, then we still know
we have a broken symlink pointing into the keg we are unlinking, and
removing that symlink is still safe.
Symlinks in opt and LinkedKegs point directly at a keg in the cellar, so
only resolving one symlink should suffice, and make it clear what path
we are actually interested in.
In order to allow kegs built with the same version but differing formula
revisions to coexist, we must encode the revision as part of the keg's
name. This is necessary to actually perform an upgrade, as we cannot
upgrade a keg in-place, and temporarily moving it pending the result of
the upgrade is error-prone and potentially slow.
To accomplish this, we introduce a new Formula#pkg_version method that
concatenates the active_spec version with the formula revision. An
exception is made for a formula that has no revision: the tag is
omitted. This preserves compatibility with existing installations.
Still in alpha state.
Handles defaults and merging changes with new versions.
Enable by setting the HOMEBREW_GIT_ETC environment variable.
ClosesHomebrew/homebrew#15751.
ClosesHomebrew/homebrew#17713.
