Close bug#4196.

* lisp/emacs-lisp/bytecomp.el (byte-compile-file): Create .elc files
atomically, to avoid parallel build errors.

2 files changed, 21 insertions, 9 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index c47a458f2f7..1088bf432c4 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,8 @@
+2010-09-11  Glenn Morris  <rgm@gnu.org>
+
+        * emacs-lisp/bytecomp.el (byte-compile-file): Create .elc files
+        atomically, to avoid parallel build errors.  (Bug#4196)
+
 2010-09-11  Michael R. Mauger  <mmaug@yahoo.com>
 
         * progmodes/sql.el: Version 2.6
diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index cb1deb9a762..04bc254b319 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -1698,17 +1698,24 @@ The value is non-nil if there were no errors, nil if errors."
           (insert "\n")                 ; aaah, unix.
             (if (file-writable-p target-file)
                 ;; We must disable any code conversion here.
-                (let ((coding-system-for-write 'no-conversion))
+                (let ((coding-system-for-write 'no-conversion)
+                      ;; Write to a tempfile so that if another Emacs
+                      ;; process is trying to load target-file (eg in a
+                      ;; parallel bootstrap), it does not risk getting a
+                      ;; half-finished file.  (Bug#4196)
+                      (tempfile (make-temp-name target-file)))
                   (if (memq system-type '(ms-dos 'windows-nt))
                       (setq buffer-file-type t))
-                  (when (file-exists-p target-file)
-                    ;; Remove the target before writing it, so that any
-                    ;; hard-links continue to point to the old file (this makes
-                    ;; it possible for installed files to share disk space with
-                    ;; the build tree, without causing problems when emacs-lisp
-                    ;; files in the build tree are recompiled).
-                    (delete-file target-file))
-                  (write-region (point-min) (point-max) target-file))
+                  (write-region (point-min) (point-max) tempfile)
+                  ;; This has the intentional side effect that any
+                  ;; hard-links to target-file continue to
+                  ;; point to the old file (this makes it possible
+                  ;; for installed files to share disk space with
+                  ;; the build tree, without causing problems when
+                  ;; emacs-lisp files in the build tree are
+                  ;; recompiled).  Previously this was accomplished by
+                  ;; deleting target-file before writing it.
+                  (rename-file tempfile target-file t))
               ;; This is just to give a better error message than write-region
               (signal 'file-error
                       (list "Opening output file"