GNU bug report logs - #45950
27.1; Certain regexps can cause comint-redirect-results-list-from-process to loop.

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Richard Sharman <rsharman <at> pobox.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 27.1; Certain regexps can cause 
 comint-redirect-results-list-from-process to loop.
Date: Sun, 17 Jan 2021 17:24:40 -0800

Certain regexps can cause comint-redirect-results-list-from-process to loop.

For example, when attempting to match whole lines with the regexp
"^.*$" if the output buffer contains an empty line, then the
re-search-forward expression returns (point) but does not advance point,
and since (eobp) is false the loop never terminates. 

This could avoided by testing if point advances.  If it doesn't, one
could either exit the loop or, preferably, advance a line which would
allow for further matches. 

Function loop.el demonstrates the problem.  It sends to a shell buffer a
command that outputs 4 lines, the 3rd of which is empty.  Trying to
match all lines with "^.*$" causes it to loop. 

(defun loop ()
  (interactive)
  ;; create shell if not done
  (save-window-excursion
    (shell))
  ;; send a command that outputs 4 lines, the 3rd of which is empty
  (let ((p (get-process "shell"))
	(command "(echo a; echo b ; echo ; echo d)\n")
	reply)
    (setq reply (comint-redirect-results-list-from-process p command "^.*$" 0))
    ;; we never get to here!
    (message (car reply))
    reply
    ))

Note:  Starting from emacs -Q it is necessary to run M-x shell before
invoking loop,  otherwise the error "No prompt found or
‘comint-prompt-regexp’ not set properly" is produced.

With this patch correct results are obtained:
--- comint.el.orig	2021-01-17 14:22:17.000000000 -0800
+++ comint.el	2021-01-17 14:23:54.000000000 -0800
@@ -3822,7 +3822,7 @@
Return a list of expressions in the output which match REGEXP.
REGEXP-GROUP is the regular expression group in REGEXP to use."
   (let ((output-buffer " *Comint Redirect Work Buffer*")
-	results)
+	results last-point)
     (with-current-buffer (get-buffer-create output-buffer)
       (erase-buffer)
       (comint-redirect-send-command-to-process command
@@ -3837,13 +3837,18 @@
       ;; Skip past the command, if it was echoed
       (and (looking-at command)
	   (forward-line))
+      (setq last-point (point))
       (while (and (not (eobp))
-		  (re-search-forward regexp nil t))
-	(push (buffer-substring-no-properties
-               (match-beginning regexp-group)
-               (match-end regexp-group))
-              results))
+		  (re-search-forward regexp nil))
+	(if (equal (point) last-point)
+	    (forward-line 1)
+	  (push (buffer-substring-no-properties
+		 (match-beginning regexp-group)
+		 (match-end regexp-group))
+		results))
+	(setq last-point (point)))
       (nreverse results))))
+

;; Converting process modes to use comint mode
;; ===========================================================================



In GNU Emacs 27.1 (build 2, x86_64-apple-darwin17.7.0, NS appkit-1561.61 Version 10.13.6 (Build 17G14033))
of 2020-12-13 built on henry.local
Windowing system distributor 'Apple', version 10.3.1561
System Description:  Mac OS X 10.13.6

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
loop
Quit
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Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS JSON PDUMPER GMP

Important settings:
  value of $LANG: en_CA.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

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None found.

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Memory information:
((conses 16 49326 43672)
(symbols 48 6380 11)
(strings 32 17163 4847)
(string-bytes 1 570096)
(vectors 16 10970)
(vector-slots 8 134400 67778)
(floats 8 19 151)
(intervals 56 211 86)
(buffers 1000 14))

Message #10 received at 45950 <at> debbugs.gnu.org (full text, mbox):

From: Richard Sharman <rsharman <at> pobox.com>
To: 45950 <at> debbugs.gnu.org
Subject: Please see 45952 for correction to patch
Date: Mon, 18 Jan 2021 09:17:45 -0800

Sorry, I found a mistake in the patch soon after submitting and didn’t realize I would get the “acknowlegement” message so sent another report (45952) for the correction to the patch.
Richard

Message #13 received at 45950 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Richard Sharman <rsharman <at> pobox.com>
Cc: 45952 <at> debbugs.gnu.org, 45950 <at> debbugs.gnu.org
Subject: Re: bug#45950: 27.1; Certain regexps can cause
 comint-redirect-results-list-from-process to loop.
Date: Tue, 19 Jan 2021 05:16:02 +0100

Richard Sharman <rsharman <at> pobox.com> writes:

> Sorry, there was a mistake on the patch on my previous message; 
> the (re-search-forward regexp nil t)) was inadvertently changed to
> (re-search-forward regexp nil).
>
> Sorry.
>
> Here’s the corrected version:

[...]

> + (if (equal (point) last-point)
> +    (forward-line 1)
> +  (push (buffer-substring-no-properties
> + (match-beginning regexp-group)
> + (match-end regexp-group))
> + results))
> + (setq last-point (point)))
>        (nreverse results))))

I can reproduce the infloop in Emacs 28, but I don't think this is quite
the correct fix -- if you've asked for "^.*$", then you should get an
empty string in the result set, and not just "a" "b" "d".  So I've
installed a variant of what you suggest in Emacs 28.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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