go: [go1.8beta2] Investigating bad bad break in docker test suite

For the record, 7db996ee77b20b68ee583c65d59be1d81cef9090 (CL 29852) changed shouldRedirectPost from only redirecting StatusFound, StatusSeeOther (302 & 303) to also including StatusMovedPermanently (301) in the new func redirectBehavior, to fix #9348 and other bugs.

https://tools.ietf.org/html/rfc7231#section-6.4.2 says:

6.4.2.  301 Moved Permanently

   The 301 (Moved Permanently) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource ought to use one of the enclosed URIs.
   Clients with link-editing capabilities ought to automatically re-link
   references to the effective request URI to one or more of the new
   references sent by the server, where possible.

   The server SHOULD generate a Location header field in the response
   containing a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location field value for automatic
   redirection.  The server's response payload usually contains a short
   hypertext note with a hyperlink to the new URI(s).

      Note: For historical reasons, a user agent MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, the 307 (Temporary Redirect) status code
      can be used instead.

And from the discussion in #9348, it seems the new behavior is the desired and correct behavior.

This program changed from Go 1.7 to Go 1.8 and is (I believe) the source of docker’s problems.

package main

import (
	"bytes"
	"fmt"
	"io"
	"log"
	"net/http"
	"net/http/httptest"
	"os"
)

func main() {
	http.DefaultClient.CheckRedirect = func(req *http.Request, via []*http.Request) error {
		fmt.Printf("redirect %s", req.URL)
		for _, v := range via {
			fmt.Printf(" via %s", v.URL)
		}
		fmt.Printf("\n")
		return nil
	}
	http.HandleFunc("/", show)
	srv := httptest.NewServer(nil)
	r, err := http.Post(srv.URL+"/x//z", "text/plain", bytes.NewReader(nil))
	if err != nil {
		log.Fatal(err)
	}
	if r.StatusCode != 200 {
		log.Fatal("status: " + r.Status)
	}
	io.Copy(os.Stdout, r.Body)
}

func show(w http.ResponseWriter, req *http.Request) {
	fmt.Fprintf(w, "%s %s\n", req.Method, req.URL)
}

Before the POST got a redirect and returned that error. Now it follows the redirect, translates to GET, and gets 404. I guess Docker was swallowing the 301 but is now choking on the 404.

r$ go1.7 run /tmp/x.go
2016/12/20 10:29:59 status: 301 Moved Permanently
exit status 1
r$ go run /tmp/x.go
redirect http://127.0.0.1:52118/x/z via http://127.0.0.1:52118/x//z
GET /x/z
r$