net/http: Client support for Expect: 100-continue

Current http client doesn't support Expect: 100-continue request
header(RFC2616-8/RFC7231-5.1.1). So even if the client have the header,
the head of the request body is consumed prematurely.

Those are my intentions to avoid premature consuming body in this change.
- If http.Request header contains body and Expect: 100-continue
  header, it blocks sending body until it gets the first response.
- If the first status code to the request were 100, the request
  starts sending body. Otherwise, sending body will be cancelled.
- Tranport.ExpectContinueTimeout specifies the amount of the time to
  wait for the first response.

Fixes #3665

Change-Id: I4c04f7d88573b08cabd146c4e822061764a7cd1f
Reviewed-on: https://go-review.googlesource.com/10091
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 19 insertions, 3 deletions

diff --git a/src/net/http/request.go b/src/net/http/request.go
index 31fe45a4ed..8467decc18 100644
--- a/src/net/http/request.go
+++ b/src/net/http/request.go
@@ -354,7 +354,7 @@ const defaultUserAgent = "Go-http-client/1.1"
 // hasn't been set to "identity", Write adds "Transfer-Encoding:
 // chunked" to the header. Body is closed after it is sent.
 func (r *Request) Write(w io.Writer) error {
-	return r.write(w, false, nil)
+	return r.write(w, false, nil, nil)
 }
 
 // WriteProxy is like Write but writes the request in the form
@@ -364,11 +364,12 @@ func (r *Request) Write(w io.Writer) error {
 // In either case, WriteProxy also writes a Host header, using
 // either r.Host or r.URL.Host.
 func (r *Request) WriteProxy(w io.Writer) error {
-	return r.write(w, true, nil)
+	return r.write(w, true, nil, nil)
 }
 
 // extraHeaders may be nil
-func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) error {
+// waitForContinue may be nil
+func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header, waitForContinue func() bool) error {
 	// Find the target host. Prefer the Host: header, but if that
 	// is not given, use the host from the request URL.
 	//
@@ -458,6 +459,21 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) err
 		return err
 	}
 
+	// Flush and wait for 100-continue if expected.
+	if waitForContinue != nil {
+		if bw, ok := w.(*bufio.Writer); ok {
+			err = bw.Flush()
+			if err != nil {
+				return err
+			}
+		}
+
+		if !waitForContinue() {
+			req.closeBody()
+			return nil
+		}
+	}
+
 	// Write body and trailer
 	err = tw.WriteBody(w)
 	if err != nil {