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

[Greg Weber]

Agreed with Johan. If there are meaningful error messages that get
produced, Either is better than Maybe.

On Fri, Sep 26, 2014 at 9:47 AM, Johan Tibell <johan.tibell at gmail.com>
wrote:

> 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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>>
>
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