proxy: Reuse a bytes buffer as proxy request body.

The call to transport.RoundTrip closes the request body regardless of
the value of request.Closed. This causes subsequent calls to RoundTrip
using the same request body to fail.

Fixes #2895

proxy/reverse.go

@@ -15,8 +15,10 @@
 package proxy
 
 import (
+	"bytes"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
@@ -55,6 +57,21 @@ func (p *reverseProxy) ServeHTTP(rw http.ResponseWriter, clientreq *http.Request
 	proxyreq := new(http.Request)
 	*proxyreq = *clientreq
 
+	var (
+		proxybody []byte
+		err       error
+	)
+
+	if clientreq.Body != nil {
+		proxybody, err = ioutil.ReadAll(clientreq.Body)
+		if err != nil {
+			msg := fmt.Sprintf("proxy: failed to read request body: %v", err)
+			e := httptypes.NewHTTPError(http.StatusInternalServerError, msg)
+			e.WriteTo(rw)
+			return
+		}
+	}
+
 	// deep-copy the headers, as these will be modified below
 	proxyreq.Header = make(http.Header)
 	copyHeader(proxyreq.Header, clientreq.Header)
@@ -93,9 +110,11 @@ func (p *reverseProxy) ServeHTTP(rw http.ResponseWriter, clientreq *http.Request
 	}
 
 	var res *http.Response
-	var err error
 
 	for _, ep := range endpoints {
+		if proxybody != nil {
+			proxyreq.Body = ioutil.NopCloser(bytes.NewBuffer(proxybody))
+		}
 		redirectRequest(proxyreq, ep.URL)
 
 		res, err = p.transport.RoundTrip(proxyreq)