bug#28824: 26.0.90; display of pbm images broken?

[Andy Moreton <andrewjmoreton@gmail.com>]

On Sat 14 Oct 2017, Eli Zaretskii wrote:

>> From: Andy Moreton <andrewjmoreton@gmail.com>
>> Date: Sat, 14 Oct 2017 20:03:27 +0100
>> 
>> Further testing with the images from etc/images in emacs 26 shows that
>> this patch appears to fix the problem:
>> 
>> diff --git a/lisp/image.el b/lisp/image.el
>> index 1d0776180b..32df508bc8 100644
>> --- a/lisp/image.el
>> +++ b/lisp/image.el
>> @@ -34,8 +34,8 @@ 'image-refresh
>>  (defconst image-type-header-regexps
>>    `(("\\`/[\t\n\r ]*\\*.*XPM.\\*/" . xpm)
>>      ("\\`P[1-6]\\(?:\
>> -\\(?:\\(?:#[^\r\n]*[\r\n]\\)?[[:space:]]\\)+\
>> -\\(?:\\(?:#[^\r\n]*[\r\n]\\)?[0-9]\\)+\
>> +\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[[:space:]]\\)+\
>> +\\(?:\\(?:#[^\r\n]*[\r\n]\\)*[0-9]\\)+\
>>  \\)\\{2\\}" . pbm)
>>      ("\\`GIF8[79]a" . gif)
>>      ("\\`\x89PNG\r\n\x1a\n" . png)
>
> Thanks.  However, I'm puzzled by the fact that it works here without
> any changes.

Loading the troublesome images from "emacs -Q" also works in unmodified
emacs 26, so something in my init.el changes the behaviour.

> Can you step with a debugger through the code in image.c and tell what
> exactly fails with the unmodified regexp?

The problem is in image.el, where string-match-p in image-type-from-data
fails to match the regexp.

    AndyM