Fixes a failure in the web server tests caused by d23b296627.
Formerly, `(string->url "http://racket-lang.org")`, with no trailing
slash, would produce a `url` structure with `path-absolute?` as #f.
That doesn't exactly make sense, because a URL with a host must always
have an absolute path component. Claiming a relative path component
interacts badly with extending a URL with a path later. (Although
`combine-url/relative` compenstate, a similar function in the web
server doesn't.) The revised `url->string` always sets `path-absolute?`
to #t when a host is present, and whether the path is empty or contains
an empty string still records whether a trailing "/" was present.
The `url->string` function, meanwhile, now needs to use whether the
path is empty to determine whether a "/" should be added after
the host name, not whether `path-absolute?` is true.
Since an IPv6 literal address includes ":"s, it must be written
between "[" and "]" as a host name.
Based on a patch by @Phlosioneer and comments by @Blaisorblade,
with additional changes to make `url->string` work.
Closes#980Closes#1243
History of the parsing of "file:" URLs for Windows:
* In response to PR 8060 (April 2006): special handling added to
support ill-formed URLs that were (are?) commonly used for
filesystem paths.
* Follow-up to PR 8060 (April 2008): added `path->url` and
`url->path`.
* In response to #1086 (October 2015, the commit): changed
Windows-specific handling to be more constrained and added support
for the proper encoding of UNC paths --- where "proper" means
"according to a blog post from late 2006", which appears to be as
close as we get to documentation of the URL encoding for Windows
paths.
`net/url` provides functions for both converting strings
and paths to and from URLs.
`net/url` also includes functions for creating (pure and import)
network ports. This functionality `require` the HTTP client stack
which is unnecessary when URLs simple need parsing for their
"bits".
New library: `net/url-strings` handles `url->string` and `string->url`
(and also the related `path->url` and `url->path` functions). This is
required by net/url for compatability.
`net/url-exception.rkt` is factored out for use by both libraries.
- See also racket/net changes for T&D
url-string.rkt changes requested by mflatt
url-strings.rkt is now called url-string.rkt
identifiers from url-string.rkt are reprovided by url.rkt
using (all-from-out "url-string.rkt") instead of explicit
exports
