Re: string-to-unibyte in image-jpeg-p

[Stefan Monnier]

> One of the last ‘string-to-unibyte’ (deprecated for a while)
> calls is in ‘image-jpeg-p’:
>
>  (defun image-jpeg-p (data)
>    "..."
>    (setq data (ignore-errors (string-to-unibyte data)))
>    ...)

To figure out the best fix, we need to know:
- Why are errors ignored?
- Why/when is data a multibyte string?

> We can simulate ‘string-to-unibyte’ (except for error message
> particulars) with ‘FAKE-string-to-unibyte’, which uses the
> modern ‘encode-coding-string’:
>
>  (defun FAKE-string-to-unibyte (s)
>    (let ((tem (encode-coding-string s 'binary)))
>      (when (string-match-p "\xc2" tem)
>        (error "badness"))

You can't really detect a "non-ascii non-rawbyte input" by looking at
the output of encode-coding-string, so this is not the right approach.

You could try something like:

    (unless (unibyte-string-p data)
      (setq data
            ;; \x3fffnn encode "raw bytes" in multibyte strings.
            (if (string-match "[^\x00-\xff\x3fff80-\x3fffff]" data)
                nil
              (encode-coding-string data 'binary))))

> - If so, which branch gets the change, ‘emacs-26’ or ‘master’?

Definitely not emacs-26


        Stefan