[PATCH] BUG/MEDIUM: h2: match absolute-path not path-absolute for :path

[PATCH] BUG/MEDIUM: h2: match absolute-path not path-absolute for :path

RFC7540 states that :path follows RFC3986's path-absolute. However
that was a bug introduced in the spec between draft 04 and draft 05
of the spec, which implicitly causes paths starting with "//" to be
forbidden. HTTP/1 (and now HTTP core semantics) made it explicit
that the request-target in origin-form follows a purposely defined
absolute-path defined as 1*(/ segment) to explicitly allow "//".

http2bis now fixes this by relying on absolute-path so that "//"
becomes valid and matches other versions. Full discussion here:

  https://lists.w3.org/Archives/Public/ietf-http-wg/2021JulSep/0245.html

This issue appeared in haproxy with commit 4b8852c70 ("BUG/MAJOR: h2:
verify that :path starts with a '/' before concatenating it") when
making the checks on :path fully comply with the spec, and was backported
as far as 2.0, so this fix must be backported there as well to allow
"//" in H2 again.

(cherry picked from commit 46b7dff8f08cb6c5c3004d8874d6c5bc689a4c51)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 512cee88df5c40f1d3901a82cf6643fe9f74229e)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 65b9cf31a1975eb32e6696059c2bf9f0cfca2dff)
Signed-off-by: Willy Tarreau <w@1wt.eu>
Gbp-Pq: Name 0001-BUG-MEDIUM-h2-match-absolute-path-not-path-absolute-.patch