Re: Possible issue with mutexes

[Herwig Hochleitner <herwig@bendlas.net>]

On 4/10/19 5:11 PM, Eli Zaretskii wrote:

> [Why private email?]
I hit the wrong reply button, sorry.
>> From: Herwig Hochleitner <herwig@bendlas.net>
>> Date: Wed, 10 Apr 2019 17:02:35 +0200
>>
>>
>>>> Waiting on a condition var or joining a thread on the main thread can
>>>> freeze emacs totally dead. Like kill -9 dead.
>>>>
>>>> Is this a known issue or should I investigate further?
>>> Not sure if I understand correctly the situation you describe.  Would
>>> it be possible to post a simple reproducer?
>>>
>> Try evaluating this snippet (maybe not in your main emacs) and try
>> quitting out of the eval:
>>
>> (let* ((m (make-mutex))
>>          (c (make-condition-variable m)))
>>     (with-mutex m
>>       (condition-wait c)))
> How is the above "joining a thread"?

Joining a thread shows similar behavior, where you can't quit via C-g 
anymore.

(thread-join
  (make-thread
   (lambda ()
     (let* ((m (make-mutex))
            (c (make-condition-variable m)))
       (with-mutex m
         (condition-wait c))))))

> Anyway, I think what you did is wrote code that deliberately
> deadlocks, so why are you surprised it does?

That's because this is a minimal example. I'm not surprised by it 
anymore ;-)

I hit this, when working on somewhat more involved code 
https://github.com/NixOS/nixpkgs/blob/f4d71836af5dd99b5af32ace6d1775b91213858e/pkgs/applications/editors/emacs-modes/update-melpa.el

> Theres no other thread
> to signal the condition variable or signal the waiting thread.
>
> IOW, this is the expected behavior.
Yes, pardon, I'm so used to event-driven programming, I forgot that in 
Unix, everything is pull

What I'm trying to say: It would be nice, if C-g could also release 
pending condition-waits and thread-joins.

I understand, that this might be kind of a hard thing to ask, due to the 
lack of a separate input thread, as you say, but I'm not familiar with 
the internals, so I'm asking it.

kind regards