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bug#24640: Crashes in 25.1


From: Reuben Thomas
Subject: bug#24640: Crashes in 25.1
Date: Tue, 11 Oct 2016 15:08:45 +0100

On 11 October 2016 at 12:59, Eli Zaretskii <address@hidden> wrote:
Thanks, I've done some initial debugging.  The crash seems to be
related to the variable read_objects, defined and used by lread.c.  It
is an alist of objects read with the #n=object form.  One of its
members is a corrupted Lisp object, which causes the GC crash when
this object is examined.

​Good stuff!
 
read_objects is a global variable, so it could be that some code
invoked in the middle of reading one #n=object form clobbers it by
reading another.  However, I don't immediately see such forms in the
few of your many init files I looked in.
Indeed, I'm not aware of having used such a form myself, nor can I find one by grepping.
 
  Do you have any idea where
​ ​
this could come from?

​No, sorry.​
 
  One place they are abundant is in *.elc files,
so maybe some recursive load together with the timer-based lazy
desktop operation does that?  I don't really have a working hypothesis
for now.

Could it be loading the undo-tree undo history? The crash always seems to happen when loading mit.tex. It tries to load the undo-tree history, fails (because the file has been changed since the history was last saved), then crashes. The undo-tree history is full of #n=object forms.

I can let you have the undo-tree history file if that might help you identify the corrupted data.
 
I'm not an expert on X tricks -- is there any way you can trick Emacs
to start a GUI session when I invoke it via SSH?  Some trick with the
value of DISPLAY in the environment, perhaps?  I don't need to see
what Emacs displays, just run it live under GDB.  The problem that
causes the crash happens before the code I see in the backtrace --
that code just triggers GC.  So it would be beneficial to run Emacs
under GDB and try to see, for example, what code changes read_objects
and how (assuming it is not changed to a non-nil value too many
times).  Can this be arranged?

​If you use "ssh -X", you can get an X connection and Emacs will start a GUI session. That's the simplest thing I can think of; not really a trick at all.​

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