[PATCH] make org-attach-url download function as an option

[stardiviner]

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I found when network is bad and slow, or the download file is big, the
org-attach-url will suspend Emacs for a long time. User might have to cancel
downloading, and start again later.

I hope to make "org-attach-url" download file asynchronously. But function
org-attach-attach hardcoded this function for 'url method. Here is a patch to
make it into an option. So user can specify a custom function to download file.

# ==============================================================================

BTW, I'm wandering why function "url-retrieve" download file is not complete and
failed. Bellowing code snippets are my try:

Use function ~url-copy-file~ works fine:

#+begin_src emacs-lisp :eval no
(url-copy-file
 "https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2592521688.webp";
 (expand-file-name "~/kk.webp"))
#+end_src

I check out function ~url-copy-file~ implement, try to implement an async 
version like this:

#+begin_src emacs-lisp
(defun url-copy-file-asynchronous (url newname &optional ok-if-already-exists 
&rest _ignored)
  "Copy URL to NEWNAME.  Both arguments must be strings.
Signal a `file-already-exists' error if file NEWNAME already
exists, unless a third argument OK-IF-ALREADY-EXISTS is supplied
and non-nil.  An integer as third argument means request
confirmation if NEWNAME already exists."
  (and (file-exists-p newname)
       (or (not ok-if-already-exists)
           (and (integerp ok-if-already-exists)
                (not (yes-or-no-p
                      (format "File %s already exists; copy to it anyway? "
                              newname)))))
       (signal 'file-already-exists (list "File already exists" newname)))
  (let* ((buffer (or (url-retrieve url (lambda (&rest ignored)))
                     (signal 'file-missing
                             (list "Opening URL"
                                   "No such file or directory" url))))
         (handle (with-current-buffer buffer
                   (mm-dissect-buffer t))))
    (let ((mm-attachment-file-modes (default-file-modes)))
      (mm-save-part-to-file handle newname))
    (kill-buffer buffer)
    (mm-destroy-parts handle)))

(put 'copy-file 'url-file-handlers #'url-copy-file-asynchronous)
#+end_src

#+begin_src emacs-lisp
(url-copy-file-asynchronous
 "https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2592521688.webp";
 (expand-file-name "~/kk.webp"))
#+end_src

But it download seems incomplete and failed. Don't know why.

Also there are extra async libraries support download file asynchronously. Like
"deferred", "async" etc. Those libraries can be used for user define their own
functions.

Is you have better idea, please tell me, thanks in advance.

- -- 
[ stardiviner ]
       I try to make every word tell the meaning that I want to express.

       Blog: https://stardiviner.github.io/
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