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:

--- comint.el.orig	2021-01-17 21:35:02.000000000 -0800
+++ comint.el	2021-01-17 21:36:23.000000000 -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))
+	(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
 ;; ===========================================================================



> On Jan 17, 2021, at 5:24 PM, Richard Sharman <rsharman@pobox.com> wrote:
> 
> 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
> Quit
> 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
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> 
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> None found.
> 
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> Memory information:
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