Add fallback for Path.home on Unix

[HertzDevil]

Defaults to the home directory of the real user (as opposed to effective) when $HOME is empty or null. This is equivalent to:

require "system/user"

lib LibC
  fun getuid : UidT
end

Path.new(System::User.find_by(id: LibC.getuid.to_s).home_directory)

but without constructing a System::User.

[straight-shoota]

Why real instead of effective? When I'm operating as an effective user, I would expect the effective user's home path.

[HertzDevil]

But why? I wouldn't expect the home directory to suddenly become that of the Crystal program's owner depending on whether the program is setuid or not.

For reference, Ruby uses getpwnam(getlogin()) followed by getpwuid(getuid()).

[HertzDevil]

Another reason: starting a setuid program alone will not change the HOME environment variable, thus $HOME is still associated with the real user, so the fallback should be consistent with this default behaviour.

[straight-shoota]

Yeah, you're probably right. Seeing a different home directory would be weir. You are often not even aware that a program runs with a different effective user.

I was thinking that file ownership goes to the effective user, so it would be weird if they wouldn't have write permission in the home directory. But considering that effective user typically has elevated rights, it's not that relevant.

[straight-shoota]

I think this is not exactly using the API correctly. getpwuid_r returns 0 and sets the result to a null pointer if not matching password record was found. That would result in showing a runtime error "success".
We should handle the case ret.zero? and pwd_pointer.nil? to generate an appropriate error message.

Most actual error codes are already handled by retry_with_buffer. I'm not sure if getpwuid_r can even return any of the ones that are ignored (and make it return nil): ENOENT, ESRCH, EBADF, EPERM. But having a generic fallback in cause anyone of these happens doesn't hurt.

[HertzDevil]

What is a situation where getpwuid(getuid()) fails because no matching UID is found?

[straight-shoota]

I understand that would be ret.zero? && pwd_pointer.nil?.

[HertzDevil]

That is not what I am asking. If I pass an arbitrary user ID, then it can happen that no matching record is found. What is a situation where the current real user ID has no matching record?

[straight-shoota]

I don't know.

[beta-ziliani]

LGTM 👍