backup-by-copying-when-privileged-mismatch applies to file gid, too.

* lisp/files.el (backup-by-copying-when-privileged-mismatch):  In addition
to checking the file uid, a second test is added: if the file gid is not
greater than backup-by-copying-when-privileged-mismatch,
backup-by-copying-when-mismatch will also be forced on.

* doc/emacs/files.texi, doc/lispref/backups.texi: Updated documentation.

Also fixed a typo in the Emacs reference manual, changing "higher" to
"no greater" so that the limit is no longer documented reversed.

2 files changed, 6 insertions, 5 deletions

diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index 7221edcc1b2..f5dd408cc9f 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -690,8 +690,9 @@ non-@code{nil} (the default is @code{t}), and renaming would change
 the file's owner or group, use copying.
 
 If you change @code{backup-by-copying-when-mismatch} to @code{nil},
-Emacs checks the numeric user-id of the file's owner.  If this is
-higher than @code{backup-by-copying-when-privileged-mismatch}, then it
+Emacs checks the numeric user-id of the file's owner and the numeric
+group-id of the file's group.  If either is
+no greater than @code{backup-by-copying-when-privileged-mismatch}, then it
 behaves as though @code{backup-by-copying-when-mismatch} is
 non-@code{nil} anyway.
 
diff --git a/doc/lispref/backups.texi b/doc/lispref/backups.texi
index 6a5b6d1661d..c17d98eb190 100644
--- a/doc/lispref/backups.texi
+++ b/doc/lispref/backups.texi
@@ -232,11 +232,11 @@ non-@code{nil}.
 @defopt backup-by-copying-when-privileged-mismatch
 This variable, if non-@code{nil}, specifies the same behavior as
 @code{backup-by-copying-when-mismatch}, but only for certain user-id
-values: namely, those less than or equal to a certain number.  You set
-this variable to that number.
+and group-id values: namely, those less than or equal to a certain number.
+You set this variable to that number.
 
 Thus, if you set @code{backup-by-copying-when-privileged-mismatch}
-to 0, backup by copying is done for the superuser only,
+to 0, backup by copying is done for the superuser and group 0 only,
 when necessary to prevent a change in the owner of the file.
 
 The default is 200.