Re: fill-paragraph bug

[Chong Yidong]

The problem is in fill-region-as-paragraph, and stems from the
following behavior of `let':

M-: (setq foo 1) RET
M-: (let ((foo 2)) (error "foo")) RET
M-: foo   ---> 2

After an error, the `let' form gets exited too, so the global variable
isn't rebound to its original value.

In this case, fill.el is full of

  (let ((fill-prefix fill-prefix))
       BODY)

If this ends up callin fill-delete-prefix, which signals an error,
then fill-prefix can get screwed up.

One way to fix this is to replace every such construct with

  (let ((old-fill-prefix fill-prefix))
    (unwind-protect
        BODY
      (setq fill-prefix old-fill-prefix)))

OK?

> There is an error condition somewhere in fill-paragraph that makes
> subsequent calls to that command unusable.
>
> Insert the following text in a LaTeX buffer, and call M-q:
>
> AAAAAAAAAAAAAA AAAAAAAAAAAAAAAAA AAAAAAAAAAAAAA 
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAA%AAAAAAAAAAAAAA 
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
> AAAAAAAAAAAAAAAAA
>
> the result:
>
> Debugger entered--Lisp error: (error "fill-prefix too long for specified 
> width")
>   signal(error ("fill-prefix too long for specified width"))
>   error("fill-prefix too long for specified width")
>   fill-delete-prefix(...)
>   fill-region-as-paragraph(1 323 nil nil 227)
>   fill-comment-paragraph(nil)
>   fill-paragraph(nil)
>   call-interactively(fill-paragraph)
>
> Now try to fill any other paragraph anywhere else in Emacs.  This
> triggers an error:
>
> "fill-prefix too long for specified width"
>
> Probably some variable somewhere needs to be put in a condition-case.