silverstripe-framework: BASE_URL / Director.alternate_base_url issues & inconsistency

Affects 3.6.3. See also https://github.com/silverstripe/silverstripe-framework/issues/5109.

Issue 1:

Director:
  alternate_base_url: 'http://mysite.test'

This results in “Undefined index: path” here: https://github.com/silverstripe/silverstripe-framework/blob/3.6.3/control/Session.php#L358

Adding a trailing slash fixes the issue. I can’t find any documentation for Director.alternate_base_url which indicates whether or not a trailing slash should be present.

Issue 2:

define('BASE_URL', 'http://mysite.test');

Visiting http://mysite.test?flush results in a redirection to the wrong URL: http://mysite.test/http:/mysite.test/?flush=&flushtoken=1234

This is because ParameterConfirmationToken treats the BASE_URL constant as a relative path, not an absolute URL. BASE_URL appears to be treated like a relative path in most cases, though the code docs for it suggest that it should be an absolute URL. The code to calculate BASE_PATH if it’s not defined will also always return a directory name.

Issue 3:

BASE_URL is used as a fallback for Director.alternate_base_url when calling Director::baseURL(), so they should be consistent. Adding a trailing slash to BASE_URL results in double-slashes in URLs, and removing the trailing slash from Director.alternate_base_url results in issue 1 above.

Suggested action:

To save a massive refactor to support relative/absolute urls and trailing/non-trailing slashes for both, I’m advocating mostly documentation changes and simple patches.

• Update the code docs for BASE_URL to indicate that it should be a relative path to the document root from the domain name (with no trailing slash);
• Add documentation for Director.alternate_base_url to indicate that a trailing slash should be present;
• Bonus round: patch Director::baseURL() to force a trailing slash