Subject: Re: bug#11585: 24.0.50; corrupted byte compiled files

From: Chong Yidong <[email-masked]>
Date: Wed, 30 May 2012 22:45:45 +0800

Pierre Lorenzon <[email-masked]> writes:

> It seems that a part of the .elc file is missing as if a part of the
> character strream was discarded.

This is certainly one of the most interesting bugs I've come across in
quite some time.  Congrats on finding it.

The problem is that `byte-compile-fix-header' tries to insert a message
within a fixed amount of space (in order to preserve file positions of
the actual bytecode), so long file names embedded in the message will
cause it to fail.  Your proposed fix would not preserve file positions,
but here is another way to fix it.

Stefan, I think this problem is serious enough, and the solution
straightforward enough, that we ought to include it in emacs-24 even
though it is not a regression.  WDYT?

=== modified file 'lisp/emacs-lisp/bytecomp.el'
*** lisp/emacs-lisp/bytecomp.el	2012-03-26 19:10:00 +0000
--- lisp/emacs-lisp/bytecomp.el	2012-05-30 14:40:25 +0000
***************
*** 1956,1966 ****
         ;; don't try to check the version number.
         "     (< (aref emacs-version (1- (length emacs-version))) ?A)\n"
         (format "     (string-lessp emacs-version \"%s\")\n" minimum-version)
!        "     (error \"`"
!        ;; prin1-to-string is used to quote backslashes.
!        (substring (prin1-to-string (file-name-nondirectory filename))
! 		  1 -1)
!        (format "' was compiled for Emacs %s or later\"))\n\n"
  	       minimum-version))
        ;; Now compensate for any change in size, to make sure all
        ;; positions in the file remain valid.
--- 1956,1965 ----
         ;; don't try to check the version number.
         "     (< (aref emacs-version (1- (length emacs-version))) ?A)\n"
         (format "     (string-lessp emacs-version \"%s\")\n" minimum-version)
!        ;; Because the header must fit in a fixed width, we cannot
!        ;; insert arbitrary-length file names:
!        (format
! 	"     (error \"Unable to load library compiled for Emacs %s or later\"))\n\n"
  	       minimum-version))
        ;; Now compensate for any change in size, to make sure all
        ;; positions in the file remain valid.