Route.URL creates incorrect URL when PathPrefix is in use about mux

[eastwood]

I believe this is correct behavior. Though I can understand the confusion. The PathPrefix doesn't prefix all the set Path routes per se, but in fact behaves just like Path does. However, with the exception that it matches any path appended to the value. That may be a little verbose so, a little explanation!

In the source, the Route type uses Matchers to determine whether a request is matching a given Handler. Think of a matcher like a regular expression and the route your creating is in essence, building up a regular expression.

In your code, the first statement:

r.Path( "/testpath")

You've essentially create a route that matches on specifically /testpath. Continuing on...

r.Path("/testpath").PathPrefix("/prefix/")

When you add another matcher to the stack (it's sequential), it's says to match the criteria of /prefix/.* after you've matched "/testpath". Therefore, you've essentially created a route that says match any request with the Matcher (or regex):
/testpath/prefix/.*

In lieu, think of the PathPrefix Matcher, just like Path in that it chains Matchers. However, instead of matching strictly on the path given, it matches on anything beginning with the path given.

[endophage]

Yeah, I worked it out eventually. PathPrefix seems like a generally poorly named and probably redundant function IMO. If a developer knows only one regex, it's probably .*