[3.9] gh-121650: Encode newlines in headers, and verify headers are sound (GH-122233) by ambv · Pull Request #122610 · python/cpython

… are sound (pythonGH-122233)

Per RFC 2047:

> [...] these encoding schemes allow the
> encoding of arbitrary octet values, mail readers that implement this
> decoding should also ensure that display of the decoded data on the
> recipient's terminal will not cause unwanted side-effects

It seems that the "quoted-word" scheme is a valid way to include
a newline character in a header value, just like we already allow
undecodable bytes or control characters.
They do need to be properly quoted when serialized to text, though.

This should fail for custom fold() implementations that aren't careful
about newlines.

(cherry picked from commit 0976339)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Bas Bloemsaat <bas@bloemsaat.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

This was referenced

ambv deleted the backport-0976339-3.9 branch