We're accumulating a significant number of bugs related to cross-linking
between the content and the CGI not being as relative as we would like.
This is an attempt to design a solution for them all in a unified way,
rather than solving one bug at the cost of exacerbating another.
--[[smcv]]

# Terminology

* Absolute: starts with a scheme, like
  `http://example.com/ikiwiki.cgi`, `https://www.example.org/`

* Protocol-relative: starts with `//` like `//example.com/ikiwiki.cgi`

* Host-relative: starts with `/` like `/ikiwiki.cgi`

* Relative: starts with neither `/` nor a scheme, like `../ikiwiki.cgi`

# What we need

* Static content must be able to link to other static content

* Static content must be able to link to the CGI

* CGI-generated content must be able to link to arbitrary
  static content (it is sufficient for it to be able to link
  to the "root" of the `destdir`)

* CGI-generated content must be able to link to the CGI

# Constraints

* URIs in RSS feeds must be absolute, because feed readers do not have
  any consistent semantics for the base of relative links

* If we have a `<base href>` then HTML 4.01 says it must be
  absolute, although HTML 5 does relax this by defining semantics
  for a relative `<base href>` - it is interpreted relative to the
  "fallback base URL" which is the URL of the page being viewed
  ([[bugs/trouble_with_base_in_search]],
  [[bugs/preview_base_url_should_be_absolute]])

* It is currently possible for the static content and the CGI
  to be on different domains, e.g. `www.example.com`
  vs. `cgi.example.com`; this should be preserved

* It is currently possible to serve static content "mostly over
  HTTP" (i.e. advertise a http URI to readers, and use a http
  URI in RSS feeds etc.) but use HTTPS for the CGI

* If the static content is served over HTTPS, it must refer
  to other static content and the CGI via HTTPS (to avoid
  mixed content, which is a vulnerability); this may be
  either absolute, protocol-relative, host-relative or relative

* If the CGI is served over HTTPS, it must refer to static
  content and the CGI via HTTPS; again, this may be either
  either absolute, protocol-relative, host-relative or relative
  ([[todo/Protocol_relative_urls_for_stylesheet_linking]])

* Because reverse proxies and `w3mmode` exist, it must be
  possible to configure ikiwiki to not believe the `HTTPS`, etc.,
  CGI variables, and force a particular scheme or host
  ([[bugs/W3MMode_still_uses_http:__47____47__localhost__63__]],
  [[forum/Using_reverse_proxy__59___base_URL_is_http_instead_of_https]],
  [[forum/Dot_CGI_pointing_to_localhost._What_happened__63__]])

* For relative links in page-previews to work correctly without
  having to have global state or thread state through every use of
  `htmllink` etc., `cgitemplate` needs to make links in the page body
  work as if we were on the page being previewed.

# "Would be nice"

* In general, the more relative the better

* [[schmonz]] wants to direct all CGI pageviews to https
  even if the visitor comes from http (but this can be done
  at the webserver level by making http://example.com/ikiwiki.cgi
  a redirect to https://example.com/ikiwiki.cgi, so is not
  necessarily mandatory)

* [[smcv]] has some sites that have non-CA-cartel-approved
  certificates, with a limited number of editors who can be taught
  to add SSL policy exceptions and log in via https;
  anonymous/read-only actions like `do=goto` should
  not go via HTTPS, since random readers would get scary SSL
  warnings
  ([[todo/want_to_avoid_ikiwiki_using_http_or_https_in_urls_to_allow_serving_both]],
  [[forum/CGI_script_and_HTTPS]])

* It would be nice if the CGI did not need to use a `<base>` so that
  we could use host-relative URI references (`/sandbox/`) or scheme-relative
  URI references (`//static.example.com/sandbox/`)
  (see [[bugs/trouble_with_base_in_search]])

As a consequence of the "no mixed content" constraint, I think we can
make some assumptions:

* if the `cgiurl` is http but the CGI discovers at runtime that it has
  been reached via https, we can assume that the https equivalent,
  or a host- or protocol-relative URI reference to itself, would work;

* if the `url` is http but the CGI discovers at runtime that it has been
  reached via https, we can assume that the https equivalent of the `url`
  would work

In other words, best-practice would be to list your `url` and `cgiurl`
in the setup file as http if you intend that they will most commonly
be accessed via http (e.g. the "my cert is not CA-cartel approved"
use-case), or as https if you intend to force accesses into
being via https (the "my wiki is secret" use-case).

# Regression test

I've added a regression test in `t/relativity.t`. We might want to
consider dropping some of it or skipping it unless a special environment
variable is set once this is all working, since it's a bit slow.
--[[smcv]]