bug#7593: 24.0.50; No response from 'occur' (infinite loop when regexp matches empty string?)

[Larry Denenberg]

This bug manifests in all versions of GNU emacs that I have.  The facts:

1) Start with 'emacs -nw -Q'.  We see buffer *scratch* containing the
initial greeting ';; This buffer is for notes . . .'.

2) Enter 'M-x occur<RET>'.  We're now in the minibuffer with prompt
'List lines matching regexp:'.

3) Enter 'a*<RET>'.  Emacs stops responding.

I believe the problem is in function occur-engine (file replace.el),
specifically the section labelled 'Highlight the matches'.

                    ;; Highlight the matches
                    (let ((len (length curstring))
                          (start 0))
                      (while (and (< start len)
                                  (string-match regexp curstring start))
                        (add-text-properties
                         (match-beginning 0) (match-end 0)
                         (append
                          `(occur-match t)
                          (when match-face
                            ;; Use `face' rather than `font-lock-face' here
                            ;; so as to override faces copied from the buffer.
                            `(face ,match-face)))
                         curstring)
                        (setq start (match-end 0))))

When the given regexp matches a null string, the final line of code here
does not change variable start, so we loop infinitely.

/Larry Denenberg
larry@denenberg.com
http://larry.denenberg.com/

In GNU Emacs 24.0.50.1 (i386-apple-darwin9.8.0, NS apple-appkit-949.54)
 of 2010-08-10 on braeburn.aquamacs.org - Aquamacs Distribution 2.1dev
Windowing system distributor `Apple', version 10.3.1038
configured using `configure  '--with-ns' '--without-x' 'CFLAGS=-arch i386 -arch 
ppc' 'LDFLAGS=-arch i386 -arch ppc''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: nil
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default enable-multibyte-characters: t

Major mode: Info

Minor modes in effect:
  TeX-PDF-mode: t
  TeX-source-correlate-mode: t
  display-time-mode: t
  savehist-mode: t
  smart-frame-positioning-mode: t
  aquamacs-autoface-mode: t
  recentf-mode: t
  osx-key-mode: t
  show-paren-mode: t
  delete-selection-mode: t
  pc-selection-mode: t
  cua-mode: t
  tooltip-mode: t
  mouse-wheel-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
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

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/Users/larry/Emacs/Bin/hebrew hides 
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Features:
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