(backup-buffer-copy): Check backup directory is writable, to avoid

infloop deleting old backup.

2 files changed, 17 insertions, 0 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index d338b717bb9..2e9bc28b7a7 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,5 +1,8 @@
 2007-08-22  Glenn Morris  <rgm@gnu.org>
 
+        * files.el (backup-buffer-copy): Check backup directory is
+        writable, to avoid infloop deleting old backup.
+
         * mail/rmail.el (rmail-movemail-variant-p): Call on load to set
         movemail related variables.
         (rmail-insert-inbox-text): Use only rmail-movemail-program, which
diff --git a/lisp/files.el b/lisp/files.el
index 94a8c383c5a..dfe976c6749 100644
--- a/lisp/files.el
+++ b/lisp/files.el
@@ -3173,6 +3173,11 @@ BACKUPNAME is the backup file name, which is the old file renamed."
 
 (defun backup-buffer-copy (from-name to-name modes)
   (let ((umask (default-file-modes)))
+        (dir (or (file-name-directory to-name)
+                 default-directory)))
+    ;; Can't delete or create files in a read-only directory.
+    (unless (file-writable-p dir)
+      (signal 'file-error (list "Directory is not writable" dir)))
     (unwind-protect
         (progn
           ;; Create temp files with strict access rights.  It's easy to
@@ -3181,6 +3186,11 @@ BACKUPNAME is the backup file name, which is the old file renamed."
           (set-default-file-modes ?\700)
           (while (condition-case ()
                      (progn
+                       ;; If we allow for the possibility of something
+                       ;; creating the file between delete and copy
+                       ;; (below), we must also allow for the
+                       ;; possibility of something deleting it between
+                       ;; a file-exists-p check and a delete.
                        (condition-case nil
                            (delete-file to-name)
                          (file-error nil))
@@ -3189,6 +3199,10 @@ BACKUPNAME is the backup file name, which is the old file renamed."
                    (file-already-exists t))
             ;; The file was somehow created by someone else between
             ;; `delete-file' and `copy-file', so let's try again.
+            ;; Does that every actually happen in practice?
+            ;; This is a potential infloop, which seems bad...
+            ;; rms says "I think there is also a possible race
+            ;; condition for making backup files" (emacs-devel 20070821).
             nil))
       ;; Reset the umask.
       (set-default-file-modes umask)))