Subject: 28.0.50; Byte-compilation can fail with large fill-column

From: "Basil L. Contovounesios" <[email-masked]>
Date: Tue, 06 Jul 2021 01:16:56 +0100

Tags: patch

As seen on the Dash issue tracker:

https://github.com/magnars/dash.el/issues/383

0. cd "$(mktemp -d)"
1. HOME="$PWD" XDG_CONFIG_HOME="$PWD/.config" emacs -Q
2. (setq-default
    package-archives '(("melpa" . "https://melpa.org/packages/"))
    fill-column #xffff)
3. C-x C-e
4. M-x package-install RET dash RET

Installation fails with the following in *Compile-Log*:

  Compiling file
  /tmp/tmp.IEFN9nE4Fs/.emacs.d/elpa/dash-20210704.1302/dash.el
  at Tue Jul 6 00:44:46 2021
  Entering directory
  ‘/tmp/tmp.IEFN9nE4Fs/.emacs.d/elpa/dash-20210704.1302/’
  dash.el:551:1: Error: Invalid regexp: "Invalid content of \\{\\}"

The same error occurs post-installation with 'M-x byte-compile-file' on
dash.el.  Either way, the error confusingly only shows up when Dash is
installed from MELPA:

  package-archives = '(("melpa" . "https://melpa.org/packages/"))

and not from GNU-devel ELPA:

  package-archives = '(("gnu" . "https://elpa.gnu.org/devel/"))

even though both contain identical versions of the file dash.el.  So I
guess it's some other part of the package .tar that gives rise to the
discrepancy.

Turning on debug-on-error doesn't give a backtrace, presumably because
displaying-byte-compile-warnings or similar demotes errors.

But a breakpoint in signal_or_quit reveals that
byte-compile--wide-docstring-p constructs an invalid regexp whose
repetition exceeds the new (since Emacs 27) RE_DUP_MAX limit of #xffff.

Maybe byte-compile--wide-docstring-p should defend itself against this
as follows?

>From 42de0117547a462ce1f161a3a9cae92fd322e9ec Mon Sep 17 00:00:00 2001
From: "Basil L. Contovounesios" <[email-masked]>
Date: Tue, 6 Jul 2021 01:05:41 +0100
Subject: [PATCH] Avoid invalid regexp in wide docstring check

* lisp/emacs-lisp/bytecomp.el (byte-compile--wide-docstring-p):
Avoid constructing an invalid regexp during byte-compilation by
limiting the number of columns to the current RE_DUP_MAX of 65535.
This protects against pathological values of fill-column, for
example.
---
 lisp/emacs-lisp/bytecomp.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 70999648d4..6970c8a505 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -1627,7 +1627,7 @@ byte-compile--wide-docstring-p
 `byte-compile--wide-docstring-substitution-len'.  Also ignore
 URLs."
   (string-match
-   (format "^.\\{%s,\\}$" (int-to-string (1+ col)))
+   (format "^.\\{%d,\\}$" (min (1+ col) #xffff)) ; Heed RE_DUP_MAX.
    (replace-regexp-in-string
     (rx (or
          ;; Ignore some URLs.
-- 
2.30.2

Thanks,

-- 
Basil

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
 of 2021-07-05 built on tia
Repository revision: 68276f6d30bbdc09cc26fb49d7f0c3aa4bce35f2
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-Og -ggdb' --config-cache
 --prefix=/home/blc/.local --enable-checking=structs
 --with-x-toolkit=lucid --with-file-notification=yes --with-x'

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