Fixing parallel byte-compilation

[Glenn Morris]

Parallel bootstrap sometimes fails if one Emacs process tries to read
an .elc file that another is in the middle of writing. See eg

http://debbugs.gnu.org/cgi/bugreport.cgi?bug=4196
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=6858

and recent mails on this list.

One suggested solution was to compile to a temp-file, then move it in
place. That seems simple to do...?


*** lisp/emacs-lisp/bytecomp.el 2010-09-08 16:02:38 +0000
--- lisp/emacs-lisp/bytecomp.el 2010-09-10 23:42:25 +0000
***************
*** 1698,1714 ****
          (insert "\n")                 ; aaah, unix.
            (if (file-writable-p target-file)
                ;; We must disable any code conversion here.
!               (let ((coding-system-for-write 'no-conversion))
                  (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))
              ;; This is just to give a better error message than write-region
              (signal 'file-error
                      (list "Opening output file"
--- 1698,1720 ----
          (insert "\n")                 ; aaah, unix.
            (if (file-writable-p target-file)
                ;; We must disable any code conversion here.
!               (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))
!                 ;; The old code used to: delete target-file 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).  Renaming works the same way.
!                 (write-region (point-min) (point-max) tempfile)
!                 (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"