url.el blocks Gnus+nnrss

url.el blocks Gnus+nnrss

[Katsumi Yamaoka]

Hi,

Several people reported that Gnus hangs if they subscribe to
nnrss groups.  nnrss is a back end which uses the url ELisp
package by default and enables to read rss feeds as if they were
newsgroups.

When starting Gnus or typing `g' (gnus-group-get-new-news), it
gets into the infinite loop.  The reason the problem arises is
stated in the `url-retrieve-synchronously' section of the url.el
file as follows:

	;; Quoth Stef:
	;; It turns out that the problem seems to be that the (sit-for
	;; 0.1) below doesn't actually process the data: instead it
	;; returns immediately because there is keyboard input
	;; waiting, so we end up spinning endlessly waiting for the
	;; process to finish while not letting it finish.

	;; However, raman claims that it blocks Emacs with Emacspeak
	;; for unexplained reasons.  Put back for his benefit until
	;; someone can understand it.
	;; (sleep-for 0.1)
	(sit-for 0.1))

I don't know why it sticks to `sleep-for' or `sit-for'.
Although both process data, there are adverse effects as the
comment mentioned.  Isn't the function which should process data
from the network `accept-process-output'?  I tried it there and
confirmed it solves the problem.  Since I don't have Emacspeak,
I'm not sure whether it doesn't trouble Emacspeak users, though.

The patch is here:

*** url.el~	Sun Nov 21 22:23:54 2004
--- url.el	Thu Jan 13 12:15:19 2005
***************
*** 177,194 ****
        (while (not retrieval-done)
  	(url-debug 'retrieval "Spinning in url-retrieve-synchronously: %S (%S)"
  		   retrieval-done asynch-buffer)
! 	;; Quoth Stef:
! 	;; It turns out that the problem seems to be that the (sit-for
! 	;; 0.1) below doesn't actually process the data: instead it
! 	;; returns immediately because there is keyboard input
! 	;; waiting, so we end up spinning endlessly waiting for the
! 	;; process to finish while not letting it finish.
! 
! 	;; However, raman claims that it blocks Emacs with Emacspeak
! 	;; for unexplained reasons.  Put back for his benefit until
! 	;; someone can understand it.
! 	;; (sleep-for 0.1)
! 	(sit-for 0.1))
        asynch-buffer)))
  
  (defun url-mm-callback (&rest ignored)
--- 177,183 ----
        (while (not retrieval-done)
  	(url-debug 'retrieval "Spinning in url-retrieve-synchronously: %S (%S)"
  		   retrieval-done asynch-buffer)
! 	(accept-process-output (get-buffer-process asynch-buffer) 0 100000 1))
        asynch-buffer)))
  
  (defun url-mm-callback (&rest ignored)

Re: url.el blocks Gnus+nnrss

[Stefan Monnier]

> Several people reported that Gnus hangs if they subscribe to
> nnrss groups.  nnrss is a back end which uses the url ELisp
> package by default and enables to read rss feeds as if they were
> newsgroups.

I've myself been using the patch below for a while now (it basically does
the same as yours, except it hoists the get-buffer-process outside the loop
and it removes the timeout since it shouldn't be needed now that Emacs knows
what we're waiting for).

I've just installed it.


        Stefan


--- orig/lisp/url/url.el
+++ mod/lisp/url/url.el
@@ -1,6 +1,7 @@
 ;;; url.el --- Uniform Resource Locator retrieval tool
 
-;; Copyright (c) 1996,1997,1998,1999,2001,2004  Free Software Foundation, Inc.
+;; Copyright (c) 1996, 1997, 1998, 1999, 2001, 2004, 2005
+;;           Free Software Foundation, Inc.
 
 ;; Author: Bill Perry <wmperry@gnu.org>
 ;; Keywords: comm, data, processes, hypermedia
@@ -169,26 +169,24 @@
 			      (url-debug 'retrieval "Synchronous fetching done (%S)" (current-buffer))
 			      (setq retrieval-done t
 				    asynch-buffer (current-buffer)))))
-    (if (not asynch-buffer)
+    (let ((proc (and asynch-buffer (get-buffer-process asynch-buffer))))
+      (if (null proc)
 	;; We do not need to do anything, it was a mailto or something
 	;; similar that takes processing completely outside of the URL
 	;; package.
 	nil
       (while (not retrieval-done)
 	(url-debug 'retrieval "Spinning in url-retrieve-synchronously: %S (%S)"
 		   retrieval-done asynch-buffer)
-	;; Quoth Stef:
-	;; It turns out that the problem seems to be that the (sit-for
-	;; 0.1) below doesn't actually process the data: instead it
-	;; returns immediately because there is keyboard input
-	;; waiting, so we end up spinning endlessly waiting for the
-	;; process to finish while not letting it finish.
-
-	;; However, raman claims that it blocks Emacs with Emacspeak
-	;; for unexplained reasons.  Put back for his benefit until
-	;; someone can understand it.
-	;; (sleep-for 0.1)
-	(sit-for 0.1))
+	  ;; We used to use `sit-for' here, but in some cases it wouldn't
+	  ;; work because apparently pending keyboard input would always
+	  ;; interrupt it before it got a chance to handle process input.
+	  ;; `sleep-for' was tried but it lead to other forms of
+	  ;; hanging.  --Stef
+	  (unless (accept-process-output proc)
+	    ;; accept-process-output returned nil, maybe because the process
+	    ;; exited (and may have been replaced with another).
+	    (setq proc (get-buffer-process asynch-buffer)))))
       asynch-buffer)))
 
 (defun url-mm-callback (&rest ignored)

Re: url.el blocks Gnus+nnrss

[Katsumi Yamaoka]

>>>>> In <87pt09v2m1.fsf-monnier+emacs@gnu.org> Stefan Monnier wrote:

> I've myself been using the patch below for a while now (it basically does
> the same as yours, except it hoists the get-buffer-process outside the loop
> and it removes the timeout since it shouldn't be needed now that Emacs knows
> what we're waiting for).

> I've just installed it.

Excellent.  I confirmed the problem has gone.  Thank you.