Proposal unix. Change to `IO (Maybe String)`: getLoginName, getUserEntry* and getGroupEntry*

[Johan Tibell]

I think we should add the saner versions (that return Maybe) under new
names to avoid unnecessarily breaking backwards compatibility.

On Fri, Sep 26, 2014 at 6:23 PM, Thomas Miedema <thomasmiedema at gmail.com>
wrote:

> In the unix package, in System.Posix.User, we have the following functions
> [1]:
>
> * getLoginName (getlogin)
> * getUserEntryForID (getpwuid_r)
> * getGroupEntryForID (getgrgid_r)
> * getUserEntryForName (getpwnam_r)
> * getGroupEntryForName (getgrnam_r)
>
> They have signature `IO String`, and use `throwErrnoIfNull` to call the c
> functions listed in parenthesis.
>
>
> ## Proposal
> Change the signature of the functions listed above to `IO (Maybe String)`.
>
> The new semantics would be:
> * If the c function returns a NULL pointer with errno set to ENOENT,
> meaning the given user or group could not be found, the result is Nothing.
> * If another error occured, an error is thrown (no change).
> * Otherwise the result is Just "result".
>
>
> ## Motivation
> At least `getlogin` and `getgrgid` are unreliable on Linux. It is possible
> for them to return NULL pointers [2] even when the user is logged in and
> has an associated user entry in /etc/passwd and group entry in /etc/group.
>
> Examples:
> * when `getLoginName` is called using a terminal emulator that doesn't
> write login records to /var/run/utmp, or inside screen/tmux, `getlogin`
> tends to return NULL pointers [3].
> * `getGroupEntryForID` can throw a NULL pointer exception inside chroots
> or other environments where the information in /etc/groups is not to be
> considered reliable [4].
>
> Since we can, let's give the above Haskell functions a type safe(r) api.
>
> If the proposal gets rejected, I will just add a warning message to the
> docstrings of the functions in question and permanently disable their tests
> from the unix testsuite, since this is currently causing problems [5,6].
>
> Discussion period: 2 weeks.
>
>
> ## Links
> [1] https://github.com/haskell/unix/blob/master/System/Posix/User.hsc
> [2] http://linux.die.net/man/3/getlogin
> [3]
> https://github.com/haskell/unix/blob/cad1ef27bb0a51bc68ebadb1297de1ae05bee9db/tests/all.T#L14-L18
> [4] https://ghc.haskell.org/trac/ghc/ticket/8293
> [5] http://www.haskell.org/pipermail/ghc-devs/2014-September/006420.html
> [6] https://ghc.haskell.org/trac/ghc/ticket/1487#comment:23
>
>
> ## Details
> * Another function in the same module that needs to change is
> `getEffectiveUserName`, because it calls the function `getUserEntryForID`
> mentioned before.
>
> * The following functions don't need to change, since they return the
> empty list if no entries are found:
>   - getGroups (getgroups)
>   - getAllGroupEntries (getgrent)
>   - getAllUserEntries (getpwent)
>
> * The rest of the functions in the module don't need to change either,
> since they are not expected to return NULL pointers, according to their
> respective man pages:
>   - getRealUserID (getuid)
>   - getRealGroupID (getgid)
>   - getEffectiveUserID (geteuid)
>   - getEffectiveGroupID (getegid)
>
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