Replace usage of `cgi.parse_header()` (slated for removal in 3.13)

[vytas7]

PEP 594 specifies removal of "dead batteries" in CPython 3.13, with deprecation announced in 3.11.

Apparently, cgi is also on the chopping block, so we'll have to replace cgi.parse_header(). At first glance, the suggested replacement (instantiating an email.message.Message) looks clunky and likely to perform worse. Maybe we can reimplement it in Cython instead?

[mikael-epigram]

As an alternative why not just use the cgi sourcecode. It seems quite standalone?

https://github.com/python/cpython/blob/3.11/Lib/cgi.py#L238

def _parseparam(s):
    while s[:1] == ';':
        s = s[1:]
        end = s.find(';')
        while end > 0 and (s.count('"', 0, end) - s.count('\\"', 0, end)) % 2:
            end = s.find(';', end + 1)
        if end < 0:
            end = len(s)
        f = s[:end]
        yield f.strip()
        s = s[end:]

def parse_header(line):
    """Parse a Content-type like header.
    Return the main content-type and a dictionary of options.
    """
    parts = _parseparam(';' + line)
    key = parts.__next__()
    pdict = {}
    for p in parts:
        i = p.find('=')
        if i >= 0:
            name = p[:i].strip().lower()
            value = p[i+1:].strip()
            if len(value) >= 2 and value[0] == value[-1] == '"':
                value = value[1:-1]
                value = value.replace('\\\\', '\\').replace('\\"', '"')
            pdict[name] = value
    return key, pdict

[vytas7]

Hi @mikael-epigram, and thanks for reaching out! At a glance this doesn't look very performant, we'll try to write a faster version in both Python (for PyPy) and Cython. But yes, in a pinch, just vendoring this snippet would do.

[mikael-epigram]

I agree that it does not seem that performant, but that is what you are already using on CPython