Consecutive "/" at the beginning of a path causes an H2 protocol error.

[xcir]

Expected Behavior

If "/" is consecutively at the beginning of a path, it does not cause an error.

Current Behavior

22f666f

In this commit, if the first two characters of the path header are "//", an H2 protocol error will now occur.

Possible Solution

I believe that the "/" check on the second character should be removed.

https://github.com/varnishcache/varnish-cache/blob/master/bin/varnishd/http2/cache_http2_hpack.c#L139-L140

139,140c139
<                       if (((len > 0 && *b != '/') ||
<                           (len > 1 && *(b+1) == '/')) &&
---
>                       if ((len > 0 && *b != '/') &&

Steps to Reproduce (for bugs)

1. Access with a path starting with "//" at http2 ( https://varnish-cache.org//index.html etc..)

Context

RFC7540#8.1.2.3 in the comment says "the "path-absolute" production and optionally~", which has been corrected to "absolute-path" in the revised RFC9113#8.3.1.

absolute-path is defined in RFC9110#4.1 and "//" does not appear to be prohibited.

This modification was made in

httpwg/http2-spec#906
httpwg/http2-spec#910

It is discussed in the above issue, but states that it was an unintentional mistake to prohibit "//".
Therefore, I believe it is a mistake to assume it is a protocol error.

However, multiple slashes are not desirable and it would be nice and kind to include them in the documentation. (or default.vcl)

Your Environment

• Version used: 7.3.0
• Operating System and version: Ubuntu 22.04