formula_installer: restrict use of Formula from Keg

[nandahkrishna]

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Restrict usage of the formula file from the bottle. Should fix #11468 (comment).

Formulae which require files from lib in the Tap could probably not get post_install to work if installing them without tapping the Tap.

[BrewTestBot]

Review period will end on 2021-06-15 at 12:21:00 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[nandahkrishna]

Just tweaked this since according to #11532 (comment), the formula will always exist in the prefix unless deleted, so the condition was redundant. We haven't seen the error mentioned there (yet), so I haven't tweaked any messaging.

[nandahkrishna]

With the latest push, the formula from the installation prefix is used only if:

• Installing from a local bottle, or
• The tap isn't installed or the formula doesn't exist in the tap, or
• The tap contains a different pkg_version of the formula (effectively applies to brew postinstall only).

I'm using formula.opt_prefix because if the tap has a newer version, formula.prefix will point to the newer version but we want to use the older version for brew postinstall. Only using the older version in this case will allow us to access the formula file from the keg, so the easiest way is to just use opt_prefix.

The formula from the tap is used in all other cases, including:

• The tap formula's pkg_version equals the keg formula's pkg_version (whether or not the user has edited the formula in the tap).
  • If the formula has been edited in such a way that the pkg_version differs, brew postinstall will use the formula from the keg but brew install will use the formula from the tap.
• The formula requires some libraries from a tap, making it unreadable from the keg.

I think this does what we want in all possible cases, except when the formula from the keg is unreadable and an older version is installed – we'd like to use the keg formula but it's not possible (as of now).

It took me a while to wrap my head around this, so hopefully I've got it right 😅.

[MikeMcQuaid]

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