linkage_checker: deprecate linkage to libcrypt.so.1

[Bo98]

This isn't something to be merged yet (there's not even a libxcrypt in homebrew-core yet), but this more formally proposes what I discussed in Slack which is that we should follow what other Linux distributions have done and stop depending on Glibc's libcrypt and replace with libxcrypt (which we'll provide as libcrypt.so.2).

[BrewTestBot]

Review period will end on 2022-04-18 at 19:45:48 UTC.

[MikeMcQuaid]

Suggested change

	opoo "Linkage to libcrypt.so.1 is deprecated and will fail linkage checks in Homebrew 3.??.0.\n" \
	"Migrate to libcrypt.so.2 in the libxcrypt formula instead."
	opoo "Linkage to libcrypt.so.1 is deprecated! \n" \
	"Migrate to libcrypt.so.2 in the libxcrypt formula instead."

or ideally just:

Suggested change

	opoo "Linkage to libcrypt.so.1 is deprecated and will fail linkage checks in Homebrew 3.??.0.\n" \
	"Migrate to libcrypt.so.2 in the libxcrypt formula instead."
	odeprecated "linkage to libcrypt.so.1", "link to libcrypt.so.2 from the libxcrypt formula"

[Bo98]

I intentionally do not want odeprecated's raise-on-HOMEBREW_DEVELOPER behaviour. I'd like the warning to appear in our CI but not actually fail linkage yet. The opt-in raise-to-error behaviour in this case is handled by --strict (see broken_library_linkage?).

I can see us going straight from this to removing libcrypt.so.1 from the allowed library whitelist. Or maybe actually have a stepping stone of some opt-in raise-to-error env once we migrate homebrew-core but haven't removed it here yet (but not as global as HOMEBREW_DEVELOPER which affects everyone using test bot which defeats the point of the warning grace period really).

[Bo98]

Or maybe actually have a stepping stone of some opt-in raise-to-error

Modified it now with that.

[MikeMcQuaid]

Could this use odeprecated when --strict is passed? Feels a bit weird to have a deprecation that's not using it. Alternatively, a flag for odeprecated to make it require more than HOMEBREW_DEVELOPER in this case?

[Bo98]

Adjusted this now. The English is a bit weird since it says "Calling linkage", but other than that it works.

[BrewTestBot]

Review period ended.