Eventually add write permission when setting extended attributes (RsyncProject#212)

* Eventually add write permission when setting extended attributes

When we need to set extended atributes of file which does not
allow write then temporarily add write permission and after
attributes are set, remove it again.

Resolves RsyncProject#208

Co-authored-by: Wayne Davison <wayne@opencoder.net>

Author: TomasKorbar

xattrs.c

@@ -1055,7 +1055,7 @@ int set_xattr(const char *fname, const struct file_struct *file, const char *fna
 {
 	rsync_xa_list *glst = rsync_xal_l.items;
 	item_list *lst;
-	int ndx;
+	int ndx, added_write_perm = 0;
 
 	if (dry_run)
 		return 1; /* FIXME: --dry-run needs to compute this value */
@@ -1084,10 +1084,23 @@ int set_xattr(const char *fname, const struct file_struct *file, const char *fna
 	}
 #endif
 
+	/* If the target file lacks write permission, we try to add it
+	 * temporarily so we can change the extended attributes. */
+	if (!am_root
+#ifdef SUPPORT_LINKS
+	 && !S_ISLNK(sxp->st.st_mode)
+#endif
+	 && access(fname, W_OK) < 0
+	 && do_chmod(fname, (sxp->st.st_mode & CHMOD_BITS) | S_IWUSR) == 0)
+		added_write_perm = 1;
+
 	ndx = F_XATTR(file);
 	glst += ndx;
 	lst = &glst->xa_items;
-	return rsync_xal_set(fname, lst, fnamecmp, sxp);
+	int return_value = rsync_xal_set(fname, lst, fnamecmp, sxp);
+	if (added_write_perm) /* remove the temporary write permission */
+		do_chmod(fname, sxp->st.st_mode);
+	return return_value;
 }
 
 #ifdef SUPPORT_ACLS