fix _brew completion default cache path

[bosr]

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Context

When we introduced the completion cache mechanism for zsh about 2 years ago, we needed the path to the installed Taps for checking if the current completion cache should be invalidated. This is the relevant code:

brew/completions/zsh/_brew

Line 50 in 0ec1f04

tmp=( ${HOMEBREW_REPOSITORY:-/usr/local/Homebrew}/Library/Taps/*/*/.git/index(om[1]N) )

But as you can see, the fallback value is hardcoded and often incorrect in more and more cases as users move to /opt/homebrew.

Why it matters

The HOMEBREW_REPOSITORY variable is automatically defined in Homebrew scripts, but not outside of those scripts, in particular during execution of shell autocompletion scripts. As a result, the fallback is incorrect if a user has moved to /opt/homebrew, and the cache is invalidated and recreated on each invocation, which is slow and defeats its purpose.

Proposed solution

After discussing with @Rylan12 in #11023 (thanks again), we settled on the following:

on line 50, replace the fallback /usr/local/Homebrew with $(brew --repository)

This was chosen because it always resolves to the correct path (/usr/local/Homebrew or /opt/homebrew), which no available env var like HOMEBREW_PREFIX is able to.

This PR implements this solution on both the _brew file and the erb template file. (edited)

[Bo98]

This file is generated from zsh.erb so you should modify that .erb file too.

[bosr]

I'll now rebase on the latest master to avoid the codecov failure.

[bosr]

Rebased on the latest master.

[Rylan12]

Looks good to me!

Thanks, @bosr!

[bosr]

Thanks for doing this together