http: Transport hook to register non-http(s) protocols

This permits external packages implementing e.g.
FTP or gopher to register themselves with the
http.DefaultClient:

package ftp
func init() {
    http.DefaultTransport.RegisterProtocol("ftp", &ftp{})
}

Client code would look like:

import (
    _ "github.com/exampleuser/go/gopher"
    _ "github.com/exampleuser/go/ftp"
)

func main() {
    resp, err := http.Get("ftp://example.com/path.txt")
    ...
}

R=dsymonds, rsc
CC=golang-dev
https://golang.org/cl/4526077

src/pkg/http/transport.go

@@ -36,6 +36,7 @@ const DefaultMaxIdleConnsPerHost = 2
 type Transport struct {
 	lk       sync.Mutex
 	idleConn map[string][]*persistConn
+	altProto map[string]RoundTripper // nil or map of URI scheme => RoundTripper
 
 	// TODO: tunable on global max cached connections
 	// TODO: tunable on timeout on cached connections
@@ -97,8 +98,17 @@ func (t *Transport) RoundTrip(req *Request) (resp *Response, err os.Error) {
 		}
 	}
 	if req.URL.Scheme != "http" && req.URL.Scheme != "https" {
+		t.lk.Lock()
+		var rt RoundTripper
+		if t.altProto != nil {
+			rt = t.altProto[req.URL.Scheme]
+		}
+		t.lk.Unlock()
+		if rt == nil {
 			return nil, &badStringError{"unsupported protocol scheme", req.URL.Scheme}
 		}
+		return rt.RoundTrip(req)
+	}
 
 	cm, err := t.connectMethodForRequest(req)
 	if err != nil {
@@ -117,6 +127,27 @@ func (t *Transport) RoundTrip(req *Request) (resp *Response, err os.Error) {
 	return pconn.roundTrip(req)
 }
 
+// RegisterProtocol registers a new protocol with scheme.
+// The Transport will pass requests using the given scheme to rt.
+// It is rt's responsibility to simulate HTTP request semantics.
+//
+// RegisterProtocol can be used by other packages to provide
+// implementations of protocol schemes like "ftp" or "file".
+func (t *Transport) RegisterProtocol(scheme string, rt RoundTripper) {
+	if scheme == "http" || scheme == "https" {
+		panic("protocol " + scheme + " already registered")
+	}
+	t.lk.Lock()
+	defer t.lk.Unlock()
+	if t.altProto == nil {
+		t.altProto = make(map[string]RoundTripper)
+	}
+	if _, exists := t.altProto[scheme]; exists {
+		panic("protocol " + scheme + " already registered")
+	}
+	t.altProto[scheme] = rt
+}
+
 // CloseIdleConnections closes any connections which were previously
 // connected from previous requests but are now sitting idle in
 // a "keep-alive" state. It does not interrupt any connections currently

src/pkg/http/transport_test.go

@@ -17,6 +17,7 @@ import (
 	"io/ioutil"
 	"os"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 )
@@ -531,6 +532,36 @@ func TestTransportGzipRecursive(t *testing.T) {
 	}
 }
 
+type fooProto struct{}
+
+func (fooProto) RoundTrip(req *Request) (*Response, os.Error) {
+	res := &Response{
+		Status:     "200 OK",
+		StatusCode: 200,
+		Header:     make(Header),
+		Body:       ioutil.NopCloser(strings.NewReader("You wanted " + req.URL.String())),
+	}
+	return res, nil
+}
+
+func TestTransportAltProto(t *testing.T) {
+	tr := &Transport{}
+	c := &Client{Transport: tr}
+	tr.RegisterProtocol("foo", fooProto{})
+	res, err := c.Get("foo://bar.com/path")
+	if err != nil {
+		t.Fatal(err)
+	}
+	bodyb, err := ioutil.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	body := string(bodyb)
+	if e := "You wanted foo://bar.com/path"; body != e {
+		t.Errorf("got response %q, want %q", body, e)
+	}
+}
+
 // rgz is a gzip quine that uncompresses to itself.
 var rgz = []byte{
 	0x1f, 0x8b, 0x08, 0x08, 0x00, 0x00, 0x00, 0x00,