twisted.web should allow getChild to return a Deferred.

[radix]

This potentially requires changes to a number of public APIs:

• resource.IResource.getChildWithDefault
• resource.getChildForRequest
• resource.Resource.getChildWithDefault
• resource.Resource.getChild
• server.Site.getChildWithDefault
• server.Site.getResourceFor

Given that these are all public, we mustn't break compatibility with code that invokes them with existing resource objects, as long as those resource objects return resources synchronously. So we could either do a bunch of conditionals to make sure we return resource objects synchronously when no deferreds are returned, or we could just come up with a separate implementation of Request.process that uses a different codepath entirely. If we do the latter, we must make sure we maintain compatibility with users utilizing the various resource hooks.

Related: #3711

[glyph]

I take issue with the summary "allow getChild to return a Deferred". I think the real idea here is to allow Deferreds to be returned during resource traversal, regardless of what method is used :). One of the options here to maintain compatibility — my preferred option, actually — is to come up with a new method name and deprecate getChildWithDefault().

Another option would be to have getChildWithDefault always return a DeferredResource when getChild returns a Deferred.