url.Parse incorrectly unescapes "%2F" back into a literal slash

[gopherbot]

by Jens.Alfke:

url.Parse changes escaped “/“ characters (“%2F”) back into literal slashes. This
alters the meaning of the URL, and will cause trouble with HTTP handlers if they need to
accept path components containing slashes.

Here’s a playground example: http://play.golang.org/p/3q4yN7QsGf
It demonstrates that calling
    url.Parse("http://example.com/AC%2FDC/Back%20In%20Black”)
results in the URL
    http://example.com/AC/DC/Back%20In%20Black
where the %2F has incorrectly been unescaped into a “/“. 

This will cause trouble when an HTTP handler breaks the path apart — imagine a REST
server that returns info about a record album given a GET to the path
“/artist/albumname”. Works great until you look up AC/DC — you encode the artist
as “AC%2FDC” then plug that into a URL, but the Go side unescapes the %2F, and the
path components end up as [“AC”, “DC”, “Back In Black”] which of course
doesn’t work.

As a more serious example, CouchDB's REST API allows document IDs to contain slash
characters, which then have to be escaped in URLs. I'm implementing a CouchDB-compatible
server (https://github.com/couchbase/sync_gateway) which cannot correctly handle such
documents.

I can't think of a way for an HTTP handler to work around this, because the URL that it
gets in the Request object has already been damaged, and I don't think it can find the
original raw URL string from the client.

What steps will reproduce the problem?
Run http://play.golang.org/p/3q4yN7QsGf

What is the expected output?
Path components decode as ["AC/DC", "Back In Black"]

What do you see instead?
Path components decode as ["AC", "DC", "Back In Black"].
The example panics.

Which compiler are you using (5g, 6g, 8g, gccgo)?
go build

Which operating system are you using?
Mac OS X 10.9

Which version are you using?  (run 'go version')
1.1.2

[adg]

Comment 1:

Sadly I don't think we can do anything about this. See issue #3659 for a workaround.

Status changed to Duplicate.

Merged into issue #3659.