fileio: set group of backup file

Restores code removed in #9709.

uv_fs_copyfile() copies the perm bits but not the group name.
https://github.com/libuv/libuv/pull/1547

ref #9709
ref #8288

2 files changed, 33 insertions, 4 deletions

diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 55463bdf30..15d7a885d5 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -2868,6 +2868,23 @@ buf_write (
           /* remove old backup, if present */
           os_remove((char *)backup);
 
+          // set file protection same as original file, but
+          // strip s-bit.
+          (void)os_setperm((const char *)backup, perm & 0777);
+
+#ifdef UNIX
+          //
+          // Try to set the group of the backup same as the original file. If
+          // this fails, set the protection bits for the group same as the
+          // protection bits for others.
+          //
+          if (file_info_new.stat.st_gid != file_info_old.stat.st_gid
+              && os_chown((char *)backup, -1, file_info_old.stat.st_gid) != 0) {
+            os_setperm((const char *)backup,
+                       (perm & 0707) | ((perm & 07) << 3));
+          }
+#endif
+
           // copy the file
           if (os_copy((char *)fname, (char *)backup, UV_FS_COPYFILE_FICLONE)
               != 0) {
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 8c7dfcdee7..d1d088cf24 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -712,12 +712,24 @@ int os_setperm(const char *const name, int perm)
   return (r == kLibuvSuccess ? OK : FAIL);
 }
 
-/// Changes the ownership of the file referred to by the open file descriptor.
+/// Changes the owner and group of a file, like chown(2).
 ///
-/// @return `0` on success, a libuv error code on failure.
+/// @return 0 on success, or libuv error code on failure.
+///
+/// @note If `owner` or `group` is -1, then that ID is not changed.
+int os_chown(const char* path, uv_uid_t owner, uv_gid_t group)
+{
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_chown, path, owner, group, NULL);
+  return r;
+}
+
+/// Changes the owner and group of the file referred to by the open file
+/// descriptor, like fchown(2).
+///
+/// @return 0 on success, or libuv error code on failure.
 ///
-/// @note If the `owner` or `group` is specified as `-1`, then that ID is not
-/// changed.
+/// @note If `owner` or `group` is -1, then that ID is not changed.
 int os_fchown(int fd, uv_uid_t owner, uv_gid_t group)
 {
   int r;