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bug#17510: 24.3.91; Problem with `emacs --daemon' in cygw32 build

From: Ken Brown
Subject: bug#17510: 24.3.91; Problem with `emacs --daemon' in cygw32 build
Date: Sat, 24 May 2014 18:18:47 -0400
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 5/24/2014 3:28 PM, Daniel Colascione wrote:
On 05/24/2014 05:38 AM, Ken Brown wrote:
On 5/19/2014 3:25 PM, Ken Brown wrote:
On 5/19/2014 12:46 PM, Eli Zaretskii wrote:
I guess it's OK for the branch, thanks.  But it strikes me that simply
replacing the car of dpyinfo->name_list_element by something like
"!!!DELETED DISPLAY!!!", or even just an empty string, would serve the
same purpose, and save us the nuisance of an additional list in
cygw32_display_name_list.  After all, all you need is to mark a
display deleted without actually deleting it, right?  IOW, the main
problem is in x_delete_display, and all the rest is just the overhead
you needed to fix that, correct?

I think that's correct, and I agree that there should be a much simpler
fix.  I'll have to look into the code and try to understand better
exactly what happens when emacs is started as a daemon and then a client
frame is opened and closed.

My guess as to the cause of this bug was completely wrong.  What happens
in my recipe is that the pointer dpyinfo->w32_id_name is freed twice.
(This is done in x_delete_display each time the only existing client
frame is deleted.)  An attempt to create a client frame for the third
time then leads to a crash because of malloc corruption.

Thanks for finding that. I wonder whether this double-free also has
something to do with random crashes people have been seeing in 64-bit
Cygwin cygw32 Emacs builds.

I doubt it, because this double-free occurs in both 64-bit and 32-bit Cygwin. Also, I think it can only be triggered by running emacs as a daemon, and none of the people reporting crashes mentioned doing that.


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