Fast and functional replacement for __fish_describe_command for Macos

[folke]

Description

Macos no longer comes with a whatis database, which makes __fish_describe_command and __fish_complete_man command very slow, since apropos rebuilds the database every time it is invoked.

Trying to update the whatis database as root also doesn't work, because it's on a read only file system.

The solution I implemented, simply creates a whatis database in the user directory at ~/.cache/fish/whatis

The whatis cache is being updated at most once a day.

The result of these changes on macos are:

• instant command completion
• instant man completion
• correctly working command descriptions

Fixes issues #6270, #6611, #6615 and possibly others.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[folke]

If needed in other parts of the codebase, the whatis command can be replaced with a grep like so:

/usr/bin/grep -i "\b$argv\b" ~/.whatis.db

[faho]

If needed in other parts of the codebase, the whatis command can be replaced with a grep like so:

Are you sure? Is the format actually line-based like that, or is that just a coincidence? What if $argv occurs in other parts of the line?

I'd rather pass the db to whatis, if at all possible

[folke]

If needed in other parts of the codebase, the whatis command can be replaced with a grep like so:

Are you sure? Is the format actually line-based like that, or is that just a coincidence? What if $argv occurs in other parts of the line?

I'd rather pass the db to whatis, if at all possible

I actually found a way where we can use the system apropos script directly (and possibly whatis, using

[faho]

Looks alright to me, if someone on macOS could confirm that it works that would be lovely.

[ridiculousfish]

This is great! The only issue I see is that the makewhatis call is visibly slow. I think I will make it run in the background instead, if the database already exists.

[ridiculousfish]

Merged as a6a0d43 with my changes to run makewhatis in the bg as a6686ca. Thank you, awesome contribution!!

[folke]

@ridiculousfish that makes a lot of sense. Thanks!