cups/string.c: Add _cups_snprintf()

[zdohnal]

Add the function which will remove the truncated UTF-8 multibyte characters.

[michaelrsweet]

I was actually thinking of adding a public API - cupsFormatString - and also want to do some other filtering of invalid/unsafe chars like we do for logging.

[zdohnal]

@michaelrsweet ok, I made the function public + used _cups_safe_vsnprintf(), which does the job for logging - however it seems there was a bug in the function, which returned shorter string than the maximum possible, and returned higher number of written bytes than it actually wrote.

[michaelrsweet]

@michaelrsweet ok, I made the function public + used _cups_safe_vsnprintf(), which does the job for logging - however it seems there was a bug in the function, which returned shorter string than the maximum possible, and returned higher number of written bytes than it actually wrote.

Actually, that is the correct behavior. (v)snprintf return the number of bytes that would have been written, not the number of bytes written.

[michaelrsweet]

See comments...

[michaelrsweet]

This is needed to avoid buffer overflow.

[zdohnal]

The decrement is there to prevent buffer overflow if we add an escaping character, right?

Shouldn't we check it in the specific 'ifs'? Otherwise this way we will lose one more character from string to additional null?

[zdohnal]

@michaelrsweet I've updated checks in other ifs to prevent buffer overflows, but use up all possible space for string. Did I understand it correctly that bufend decrement was to protect from overflow in characters which we need to escape?

[michaelrsweet]

Thanks, yes but I ended up doing things a little differently and pushed the results to CUPS 2.5 and libcups v3.

[michaelrsweet]

Fixes are in master.

[zdohnal]

No problem, I hope I helped in some way in the end.

[michaelrsweet]

@zdohnal For sure, aside from fixing the bug in this function, it also lead to related fixes in the cupsCopy/ConcatString. And we also promoted the old _cups_safe_snprintf function to API.