brew.sh: don't require /usr/local/Cellar creation.

[MikeMcQuaid]

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If you're using e.g. a /usr/local/homebrew prefix then don't require the /usr/local/Cellar to be manually created to avoid e.g. /usr/local/homebrew/Cellar being used. Let's do all we can to let
people use this Cellar location as it means they can put their repository wherever they like and still use all our bottles.

CC @Homebrew/maintainers for thoughts as it's a pretty big change and @chdiza as this was part of your neat /usr/local/homebrew installation hack that I'm enjoying.

[MikeMcQuaid]

Note: this is probably something we may need a smarter migration to handle that this but thought I'd open the easy PR to get some thoughts.

[chdiza]

As long as this doesn't prevent someone from making (say) /usr/local/homebrew be both PREFIX and REPOSITORY, it seems fine.

[apjanke]

I'm a little surprised that the Cellar could be located underneath HOMEBREW_REPOSITORY instead of HOMEBREW_PREFIX in the first place. Built packages may depend on the state of stuff in the prefix. If nothing else, on the set of installed Homebrew-managed packages. But since we're not fully sanitizing the environment and PATH yet, they might pick up other unbrewed things located under the prefix. So I'd think that the Cellar would have to be prefix-specific, not per-repo.

As long as this doesn't prevent someone from making (say) /usr/local/homebrew be both PREFIX and REPOSITORY, it seems fine.

When they're the same value, the Cellar directory should have been created as part of setting up the prefix, so it'll still work. (And it's a degenerate case, so both branches of the HOMEBREW_CELLAR initialization logic would come up with the same value.)

[MikeMcQuaid]

As long as this doesn't prevent someone from making (say) /usr/local/homebrew be both PREFIX and REPOSITORY, it seems fine.

@chdiza It would not.

[MikeMcQuaid]

Note: this is probably something we may need a smarter migration to handle that this but thought I'd open the easy PR to get some thoughts.

I've changed the approach here so this will only affect people who don't have $HOMEBREW_REPOSITORY/Cellar already existing so it's tweaking the defaults for new Cellars in /usr/local and /usr/local/bin/brew rather than moving any existing Cellar.

[UniqMartin]

Please use a single = for string comparison in shell code for consistency with existing code.

[UniqMartin]

How about instead of further complicating the logic, we simply change the default (if no Cellar directory exists yet) for all users to a Cellar in HOMEBREW_PREFIX instead of HOMEBREW_REPOSITORY? The following code would accomplish this:

# Where we store built products; a Cellar in HOMEBREW_PREFIX (often /usr/local)
# unless there's already a Cellar in HOMEBREW_REPOSITORY.
if [[ -d "$HOMEBREW_REPOSITORY/Cellar" ]]
then
  HOMEBREW_CELLAR="$HOMEBREW_REPOSITORY/Cellar"
else
  HOMEBREW_CELLAR="$HOMEBREW_PREFIX/Cellar"
fi

The only users for which this might cause trouble are users that have HOMEBREW_PREFIX different from HOMEBREW_REPOSITORY and that (unexpectedly) have a Cellar directory in both. (We could address this potential problem by checking for the existence of both in a brew doctor check if we think this is a likely enough scenario.)

[MikeMcQuaid]

The only users for which this might cause trouble are users that have HOMEBREW_PREFIX different from HOMEBREW_REPOSITORY and that (unexpectedly) have a Cellar directory in both. (We could address this potential problem by checking for the existence of both in a brew doctor check if we think this is a likely enough scenario.)

@UniqMartin That makes sense to me and seems suitably non-disruptive 👍

[UniqMartin]

🎉