Allow state directory based in $HOME.

[carlocab]

The reasoning for this is already outlined well in #1092.

This would be useful for us in Homebrew, and would probably limit the
number of bug reports you get from Homebrew users that get tripped up by
our hard-coded WATCHMAN_STATE_DIR. (See, e.g., #963.)

It should also reduce the number of users who end up with brew
building Watchman from source because they're using a non-default
prefix, and hopefully issues from them too (e.g. #1132).

Closes #1092.

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[carlocab]

CC @andrewhamon for thoughts, since you might be interested.

[genevievehelsel]

hi @carlocab - this looks like a nice change to have, thanks for the PR! Would you mind sharing some manual test output/examples to verify the behavior and that it works?

Also, instead of gating this at build time in the CMake file, could we put this behind a watchmanconfig (and default it to off)? https://facebook.github.io/watchman/docs/config

[carlocab]

Thanks for having a look!

Would you mind sharing some manual test output/examples to verify the behavior and that it works?

Sure, here you go:

Test command output

❯ _build/bin/watchman; echo $?
0

❯ _build/bin/watchman get-sockname
{
    "sockname": "/Users/carlocab/.local/state/watchman/carlocab-state/sock",
    "version": "20240803.002435.0",
    "unix_domain": "/Users/carlocab/.local/state/watchman/carlocab-state/sock"
}

❯ _build/bin/watchman watch .
{
    "version": "20240803.002435.0",
    "watcher": "fsevents",
    "watch": "/Users/carlocab/github/watchman"
}

❯ _build/bin/watchman watch-list
{
    "version": "20240803.002435.0",
    "roots": [
        "/Users/carlocab/github/watchman"
    ]
}

❯ _build/bin/watchman shutdown-server
{
    "version": "20240803.002435.0",
    "shutdown-server": true
}

Let me know if there are any other commands you'd like me to try.

Also, instead of gating this at build time in the CMake file, could we put this behind a watchmanconfig (and default it to off)? facebook.github.io/watchman/docs/config

Sure, this makes sense to me. Does an option like use_xdg_state_dir or similar sound ok?