Re: Threads vs url-http

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 29 Sep 2024 23:53:44 +0000
> From: 813gan <813gan@protonmail.com>
> Cc: emacs-devel@gnu.org
> 
> (defun wiki-read--url-retrieve (url)
>   "Wrapper around url-http that download `URL' without blocking thread."
>   (let* ((parsed-url (if (url-p url) url (url-generic-parse-url url)))
> 	 (buf nil)
> 	 (cb (lambda (&rest _) (setq buf (current-buffer)) )) )
>     (url-https parsed-url cb nil)
>     (while (not buf) (thread-yield))
>     buf))
> (make-thread (apply 'wiki-read--url-retrieve "https://google.com" nil))
> 
> Why thread-yield don't unblock this?

make-thread needs a function as its first argument.  But you pass it
the value returned by 'apply', which is not a function at all.  So
what happens here, I think, is this:

  . Emacs invokes 'apply' to evaluate the argument of make-thread
  . 'apply' invokes your wiki-read--url-retrieve function
  . the function runs in the main thread, not in the thread started by
    make-thread (which is not even called, because the code is stuck
    in 'apply', see below)
  . after url-https returns (which it does immediately), the code
    immediately enters the while-loop, where it calls thread-yield
  . because there's only one running thread, thread-yield grabs the
    global lock immediately after releasing it, and the loop keeps
    looping
  . because the program loops continuously, it doesn't let url-https
    read from the network process (since Emacs only does that when it
    is idle)
  . so the output of the network sub-process is never read, and so the
    sub-process never ends, and so the callback is never called, and
    the loop never ends