bug#33195: 27.0.50; user-login-name has no group-login-name

[Jules Tamagnan]

First off I want to say thank you for your review and your time

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Jules Tamagnan <jtamagnan@gmail.com>
>> Date: Mon, 29 Oct 2018 11:52:17 -0700
>>
>> I've found it difficult to go from an integer group id to the name of
>> that group. I've looked at the source for `user-login-name` and
>> `system-groups` and could not find a way to do what I needed.
>>
>> As such I think it would be great to add a function to do that.
>
> Thanks.  Please allow me a few comments:
>
> Please always provide a ChangeLog-style commit log message describing
> your changes.  The file CONTRIBUTE has more about that.
>
>> I'm unsure how to make an argument not be optional.
>
> You want the argument NOT to be optional?  If so, make this:
>
>> +DEFUN ("group-name", Fgroup_name, Sgroup_name, 0, 1, 0,
>
> say this instead:
>
>> +DEFUN ("group-name", Fgroup_name, Sgroup_name, 1, 1, 0,
>
> See the node "Writing Emacs Primitives" in the ELisp manual, which
> describes how to use the DEFUN macro.

Turns out there is a word for not optional... required :)

>> +DEFUN ("group-name", Fgroup_name, Sgroup_name, 0, 1, 0,
>> +       doc: /* If argument GID is an integer or a float, return the login 
>> name
>> +of the group with that GID, or nil if there is no such GID.  */)
>> +  (Lisp_Object gid)
>> +{
>> +  struct group *gr;
>> +  gid_t id;
>> +
>> +  if (NILP (gid))
>> +    return Qnil;
>
> Wouldn't it be more useful to return user's group name when the
> argument is nil or omitted?

I always thought that user could have many groups, for example if I run

    $ groups $(id -un)

I get a long list of groups. I'm not sure if I misunderstood your
statement or the desired behavior but for the moment I think a separate
function would be best for this. Or maybe the `system-groups` function
could be modified to take in a UID and then list groups for that UID,
then if no argument is passed in, it would return the full list of
system-groups as it does now.

> Also, would you please write a couple of simple tests for this new
> function?

I've tried my hand at writing a test for this but am dubious it will
pass on all machines, I believe it should work on gnu/linux

> Finally, this function should be announced in NEWS and documented in
> the ELisp manual.

I've added a short entry in NEWS under "Lisp Changes in Emacs 27.1" and
in os.texi

>From 671c8b55d80743a13d6fda99f5181558cd56eea6 Mon Sep 17 00:00:00 2001
From: Jules Tamagnan <jtamagnan@gmail.com>
Date: Tue, 30 Oct 2018 10:22:03 -0700
Subject: [PATCH] src/editfns.c (group-name): New function.

---
 doc/lispref/os.texi       |  5 +++++
 etc/NEWS                  |  3 +++
 src/editfns.c             | 16 ++++++++++++++++
 test/src/editfns-tests.el |  7 +++++++
 4 files changed, 31 insertions(+)

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index cb33757..dc32958 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1230,6 +1230,11 @@ User Identification
 return value is @code{nil}.
 @end defun
 
+@defun user-login-name &optional gid
+This runction returns the group name that corresponds to @var{gid},
+or @code{nil} if there is no such group.
+@end defun
+
 
 @node Time of Day
 @section Time of Day
diff --git a/etc/NEWS b/etc/NEWS
index 226ae1e..a0bcbb5 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1228,6 +1228,9 @@ where there's no better alternative.  We believe that the 
incorrect
 uses of this function all but disappeared by now, so we are
 un-obsoleting it.
 
++++
+** New function 'group-name' returns a group name based on a group-GID
+
 
 * Changes in Emacs 27.1 on Non-Free Operating Systems
 
diff --git a/src/editfns.c b/src/editfns.c
index e995b38..15a0fa7 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -1143,6 +1143,21 @@ of the user with that uid, or nil if there is no such 
user.  */)
   return (pw ? build_string (pw->pw_name) : Qnil);
 }
 
+DEFUN ("group-name", Fgroup_name, Sgroup_name, 1, 1, 0,
+       doc: /* If argument GID is an integer or a float, return the login name
+of the group with that gid, or nil if there is no such GID.  */)
+  (Lisp_Object gid)
+{
+  struct group *gr;
+  gid_t id;
+
+  CONS_TO_INTEGER (gid, gid_t, id);
+  block_input ();
+  gr = getgrgid (id);
+  unblock_input ();
+  return (gr ? build_string (gr->gr_name) : Qnil);
+}
+
 DEFUN ("user-real-login-name", Fuser_real_login_name, Suser_real_login_name,
        0, 0, 0,
        doc: /* Return the name of the user's real uid, as a string.
@@ -4487,6 +4502,7 @@ it to be non-nil.  */);
   defsubr (&Sinsert_byte);
 
   defsubr (&Suser_login_name);
+  defsubr (&Sgroup_name);
   defsubr (&Suser_real_login_name);
   defsubr (&Suser_uid);
   defsubr (&Suser_real_uid);
diff --git a/test/src/editfns-tests.el b/test/src/editfns-tests.el
index 17b2c51..6ee0ab0 100644
--- a/test/src/editfns-tests.el
+++ b/test/src/editfns-tests.el
@@ -351,4 +351,11 @@ transpose-test-get-byte-positions
     (should (equal (format "%-#50.40x" v3)
                    "-0x000000003ffffffffffffffe000000000000000        "))))
 
+(ert-deftest group-name ()
+  (let ((list `((0 . "root")
+                (1000 . ,(user-login-name 1000))
+                (1212345 . nil))))
+    (dolist (test list)
+      (should (equal (group-name (car test)) (cdr test))))))
+
 ;;; editfns-tests.el ends here
-- 
2.7.4