Re: Build failure for Emacs master

[Eli Zaretskii <eliz@gnu.org>]

> From: Andy Moreton <andrewjmoreton@gmail.com>
> Date: Thu, 14 Apr 2016 00:11:42 +0100
> 
> The corrupted loaddefs.el file seems to be the right size, but the last
> 768 bytes are zero instead of the expected content.

What is the total size of the loaddefs.el's text, in bytes?

Also, can you look in GDB at the buffer text, before it is written to
disk, and tell how far, in bytes, is the first null from the beginning
of buffer text?  (Let me know if you need advice for how to do that
with GDB commands.)

> Running that command using "bootstrap-emacs -Q" (i.e. interactively)
> shows that the *autoload-file* buffer contains the 768 zero characters
> at end, so it appears that the corruption occurs before the buffer is
> written to disk.

Yes, of course.  The corruption is caused by inserting and deleting
text into/from the buffer, as part as producing the autoload forms.

> Any hints on how to debug `batch-update-autoloads' to diagnose this
> further would be welcome.

The bug is unlikely to be on the Lisp level, it's something very deep
in managing buffer memory.  Perhaps even on the level of mmap_alloc
and mmap_realloc, as they are implemented in w32heap.c (since the only
people who complain about this are building Emacs on Windows).

If you can somehow establish where exactly in processing of the
autoloads the corruption happens (i.e. what file is Emacs processing
and what is the contents of the *autoload-file* buffer at that point),
that would be a step forward, I think.

Thanks.