file_create_url() does not support protocol-relative nor root-relative file URLs

Problem

file_create_url() currently does not allow protocol-relative (//files/image.jpg or even ://files/image.jpg) nor root-relative (/files/image.jpg) file URLs to be created.

Protocol-relative URLs

Protocol-relative URLs are a necessity to be able to transparently switch between the HTTP and HTTPs version of a site, i.e. without generating different HTML.

What is gained:

• This would allow the Secure Pages module in Drupal 7 to automatically serve all resources on the page through HTTPS when HTTPS is used for the page and through HTTP when HTTP is used for the page, thanks to the power of hook_file_url_alter() (contributed by me in #499156: CDN integration: allow file URLs to be rewritten by hook_file_url_alter(), documentation).

  See http://carloamoroso.it/blog/web-standard/protocol-relative-urls-and-mixe... and http://www.stevesouders.com/blog/2010/02/10/5a-missing-schema-double-dow... for more details.
  (That last post also details a problem with IE7 and IE8 when it comes to protocol-relative URLs though: it causes CSS files to be downloaded twice. Likely, this will be fixed in the next version of IE. This won't prevent us from using protocol-relative URLs for resources other than CSS of course.)

• This would allow the same cached content to be served on both HTTP and HTTPS pages.
• This would allow Drupal 7 ports of modules such as Parallel to remain protocol-agnostic, CDN integration to remain flexible and Pathologic to continue to offer full flexibility in configuring preferred URL styles.

Root-relative URLs

I personally don't use root-relative URLs, but there's no good reason that I know of to prevent these URLs from being created.

What is gained:

• This would allow Drupal 7 ports of modules such as Pathologic to continue to offer full flexibility in configuring preferred URL styles.

Solution

Patch attached that makes this possible. Comes with updated unit tests, that pass.