pistache: Accept header parsing is locale-dependent, fails for valid values

My default Firefox Accept header looks like this: Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8.

I suspect it’s similar for a lot of people.

Pistache fails to parse the header, because of the q=0.9 (and q=0.8 for the matter) quality factor.

Stepping through the debugger, I traced the problem here: https://github.com/pistacheio/pistache/blob/master/src/common/mime.cc#L243

match_double internally calls strtod, which is locale dependent. My locale has , as a decimal separator, which means strtod only consumes the “0” of “0.9”, leaving the “.” as the next token, and thus breaking the parser, resulting in the “Unfinished Media Type parameter”.

The solution to this would be parsing the quality factor in a locale-independent way, as seems correct (see https://httpwg.org/specs/rfc7231.html#rfc.section.5.3.1).

To do so, I propose getting inspiration from g++ stdlib here: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B17/floating_from_chars.cc#L331, the underlying implementation of C++17’s from_chars (i.e. temporarily changing the locale before parsing, and restoring it after).

I’d be happy to provide the fix if the solution seems ok.