Fix last change

* src/editfns.c (Fgroup_name): Fix the doc string.  Move
closer to the "group" functions.
* src/w32.c (getgrgid): Return NULL if GID is not the group ID
of the user of this Emacs session

* test/src/editfns-tests.el (test-group-name): Rename from
'group-name'.  Add tests for non-Posix hosts.  Test error when
the argument to group-name is invalid.

* etc/NEWS: Fix wording of last added entry.

4 files changed, 35 insertions, 23 deletions

diff --git a/etc/NEWS b/etc/NEWS
index c11b9988e44..7f3e74457da 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1264,7 +1264,7 @@ 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
+** New function 'group-name' returns a group name corresponding to GID.
 
 
 * Changes in Emacs 27.1 on Non-Free Operating Systems
diff --git a/src/editfns.c b/src/editfns.c
index 15a0fa76597..8df4ed107e9 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -1143,21 +1143,6 @@ 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.
@@ -1191,6 +1176,24 @@ Value is a fixnum, if it's small enough, otherwise a bignum.  */)
   return INT_TO_INTEGER (uid);
 }
 
+DEFUN ("group-name", Fgroup_name, Sgroup_name, 1, 1, 0,
+       doc: /* Return the name of the group whose numeric group ID is GID.
+The argument GID should be an integer or a float.
+Return nil if a group with such GID does not exists or is not known.  */)
+  (Lisp_Object gid)
+{
+  struct group *gr;
+  gid_t id;
+
+  if (!NUMBERP (gid) && !CONSP (gid))
+    error ("Invalid GID specification");
+  CONS_TO_INTEGER (gid, gid_t, id);
+  block_input ();
+  gr = getgrgid (id);
+  unblock_input ();
+  return gr ? build_string (gr->gr_name) : Qnil;
+}
+
 DEFUN ("group-gid", Fgroup_gid, Sgroup_gid, 0, 0, 0,
        doc: /* Return the effective gid of Emacs.
 Value is a fixnum, if it's small enough, otherwise a bignum.  */)
diff --git a/src/w32.c b/src/w32.c
index e643c421506..3eaa1279dd6 100644
--- a/src/w32.c
+++ b/src/w32.c
@@ -2043,7 +2043,9 @@ getpwuid (unsigned uid)
 struct group *
 getgrgid (gid_t gid)
 {
-  return &dflt_group;
+  if (gid == dflt_passwd.pw_gid)
+    return &dflt_group;
+  return NULL;
 }
 
 struct passwd *
diff --git a/test/src/editfns-tests.el b/test/src/editfns-tests.el
index 6ee0ab09f7b..7b6c990f350 100644
--- a/test/src/editfns-tests.el
+++ b/test/src/editfns-tests.el
@@ -351,11 +351,18 @@
     (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))))))
+(ert-deftest test-group-name ()
+  (cond
+   ((memq system-type '(windows-nt ms-dos))
+    (should (stringp (group-name (group-gid))))
+    (should-not (group-name 123456789))
+    (should-error (group-name 'foo)))
+   (t
+    (let ((list `((0 . "root")
+                  (1000 . ,(user-login-name 1000))
+                  (1212345 . nil))))
+      (dolist (test list)
+        (should (equal (group-name (car test)) (cdr test)))))
+    (should-error (group-name 'foo)))))
 
 ;;; editfns-tests.el ends here