getHomeDirectory on POSIX systems is unreliable

[hasufell]

https://github.com/haskell/directory/blob/b3184cabb8c6a90eec7e921d6ca5058d57da3d82/System/Directory/Internal/Posix.hsc#L283-L284

strikes me as a naive implementation. POSIX doesn't say that $HOME is mandatory to be set. The wording is:

If the variables in the following two sections are present in the environment during the execution of an application or utility, they shall be given the meaning described below.

Even worse, getHomeDirectory will error when this optional environment variable is not set.

A better way of doing this would be:

• if $HOME is set in the environment, return it
• if $HOME is not set in the environment, look up getpwuid(getuid())->pw_dir, see getpwuid (or rather getpwuid_r for thread-safety)
• if none are set, raise an error

Inspecting other popular standard library language functions, they seem to do exactly that:

• https://doc.rust-lang.org/std/env/fn.home_dir.html
• https://docs.python.org/3/library/os.path.html#os.path.expanduser

The only thing that is debatable about this is how an empty and set $HOME will be treated.

[Rufflewind]

This was resolved by https://github.com/haskell/directory/pull/103.