http: reply to Expect 100-continue requests automatically

This CL replaces my earlier https://golang.org/cl/1640044/show
in which Continue handling was explicit.  Instead, this CL makes
it automatic.  Reading from Body() is an implicit acknowledgement
that the request headers were fine and the body is wanted.  In that
case, the 100 Continue response is written automatically when the
request continues the "Expect: 100-continue" header.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/1610042

src/pkg/http/request.go

@@ -635,3 +635,8 @@ func (r *Request) FormValue(key string) string {
 	}
 	return ""
 }
+
+func (r *Request) expectsContinue() bool {
+	expectation, ok := r.Header["Expect"]
+	return ok && strings.ToLower(expectation) == "100-continue"
+}

src/pkg/http/server.go

@@ -56,6 +56,7 @@ type Conn struct {
 	closeAfterReply bool              // close connection after this reply
 	chunking        bool              // using chunked transfer encoding for reply body
 	wroteHeader     bool              // reply header has been written
+	wroteContinue   bool              // 100 Continue response was written
 	header          map[string]string // reply header parameters
 	written         int64             // number of bytes written in body
 	status          int               // status code passed to WriteHeader
@@ -75,6 +76,28 @@ func newConn(rwc net.Conn, handler Handler) (c *Conn, err os.Error) {
 	return c, nil
 }
 
+// wrapper around io.ReaderCloser which on first read, sends an
+// HTTP/1.1 100 Continue header
+type expectContinueReader struct {
+	conn       *Conn
+	readCloser io.ReadCloser
+}
+
+func (ecr *expectContinueReader) Read(p []byte) (n int, err os.Error) {
+	if !ecr.conn.wroteContinue && !ecr.conn.hijacked {
+		ecr.conn.wroteContinue = true
+		if ecr.conn.Req.ProtoAtLeast(1, 1) {
+			io.WriteString(ecr.conn.buf, "HTTP/1.1 100 Continue\r\n\r\n")
+			ecr.conn.buf.Flush()
+		}
+	}
+	return ecr.readCloser.Read(p)
+}
+
+func (ecr *expectContinueReader) Close() os.Error {
+	return ecr.readCloser.Close()
+}
+
 // Read next request from connection.
 func (c *Conn) readRequest() (req *Request, err os.Error) {
 	if c.hijacked {
@@ -87,8 +110,15 @@ func (c *Conn) readRequest() (req *Request, err os.Error) {
 	// Reset per-request connection state.
 	c.header = make(map[string]string)
 	c.wroteHeader = false
+	c.wroteContinue = false
 	c.Req = req
 
+	// Expect 100 Continue support
+	if req.expectsContinue() {
+		// Wrap the Body reader with one that replies on the connection
+		req.Body = &expectContinueReader{readCloser: req.Body, conn: c}
+	}
+
 	// Default output is HTML encoded in UTF-8.
 	c.SetHeader("Content-Type", "text/html; charset=utf-8")