dabbrev can hang in presence of fields

[Roman Belenov]

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In GNU Emacs 21.1.1 (i386-msvc-nt5.0.2195)
 of 2001-10-22 on NNWRBELENOV21
configured using `configure --with-msvc (12.00) --cflags /G6'
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: C
  locale-coding-system: cp1251-dos
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

The symptom is that dabbrev in minibuffer hangs when "inviolable"
minibuffer prompt property is set and inhibit-field-text-motion is nil
(the default value). The reason is the following code in
dabbrev--goto-start-of-abbrev:

          (while (and (looking-at dabbrev--abbrev-char-regexp)
                      (not (bobp)))
            (forward-char -1))

(forward-char -1) does nothing on the field boundary while (bobp) is
nil.

Temporarily binding inhibit-field-text-motion to t in
dabbrev--goto-start-of-abbrev fixes the problem

Recent input:
<down> <down> <down> <down> <down> <down> <down> SPC 
<backspace> <down> <up> <down> <down> <down> <down> 
<down> <next> <up> <up> <up> <up> <up> <up> <up> <up> 
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up> 
<up> <up> <up> <prior> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <up> <up> <up> <up> C-x k <return> <return> 
M-x f o M-/ C-g <up> <up> <home> ; <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<end> <left> C-x C-e M-x f o M-/ M-x <up> <return>

Recent messages:
dabbrev--goto-start-of-abbrev
Wrote d:/apps/emacs-21/lisp/dabbrev.el
cond: No further dynamic expansion for `tv' found
call-interactively: Quit [2 times]
Loading emacsbug...done
Fontifying *mail to bug-gnu-emacs@gnu.org*... (regexps...........)
Auto-saving...done
call-interactively: Quit
dabbrev--goto-start-of-abbrev
minibuffer-avoid-prompt: Quit
call-interactively: Quit
-- 
                                                        With regards, Roman.