Lesson 36 · Resolve the Location - Build an HTTP Client

Resolve the Location

A redirect's Location is often a relative path, not a full URL. Today you resolve it against the current URL so the client always ends up with an absolute target to dial.

The goal

Resolve a Location against the current URL, handling both absolute URLs and absolute-path references.

Start here - the target

TO DO

Scenario: Resolving a redirect target

Giventhe current URL "http://example.com/a/b" and a Location of "/new"

Whenthe Location is resolved

Thenthe resolved URL is "http://example.com/new" (an absolute-path Location keeps the current scheme, host, and port)

Anda Location that is already absolute, "http://other.com/x", resolves to itself unchanged

Background

The Location header may be a full URL (http://other.com/x) or a relative reference - most commonly an absolute path like /new. A client has to turn whichever it gets into a concrete absolute URL it can connect to. If the Location already has a scheme, it is absolute and you just parse it. If it starts with /, it is an absolute path on the same server, so you keep the current URL’s scheme, host, and port and swap in the new path.

Full relative-reference resolution (../, paths relative to the current directory) is a corner of the URI standard we deliberately skip - absolute URLs and absolute paths cover the overwhelming majority of real redirects, and stopping there keeps the lesson to one clear idea. With the target resolved, the last redirect question is whether to reuse the original method, which is next.

Make it work

// if the Location has a scheme ("http://"...), it is absolute:
// parse and use it as-is. otherwise, if it starts with "/", it
// is an absolute path: keep the current scheme+host+port, replace
// the path (and query) with the Location.
func resolveLocation(current *URL, location string) *URL { /* ... */ }

CheckpointDONE

The client resolves a Location to an absolute URL to follow. Commit and stop here.