write out content length for http request (#405)

Signed-off-by: Scott Nichols <snichols@vmware.com>

v2/protocol/http/write_request.go

@@ -1,10 +1,12 @@
 package http
 
 import (
+	"bytes"
 	"context"
 	"io"
 	"io/ioutil"
 	"net/http"
+	"strings"
 
 	"github.com/cloudevents/sdk-go/v2/binding"
 	"github.com/cloudevents/sdk-go/v2/binding/format"
@@ -32,8 +34,7 @@ type httpRequestWriter http.Request
 
 func (b *httpRequestWriter) SetStructuredEvent(ctx context.Context, format format.Format, event io.Reader) error {
 	b.Header.Set(ContentType, format.MediaType())
-	b.Body = ioutil.NopCloser(event)
-	return nil
+	return b.setBody(event)
 }
 
 func (b *httpRequestWriter) Start(ctx context.Context) error {
@@ -44,8 +45,60 @@ func (b *httpRequestWriter) End(ctx context.Context) error {
 	return nil
 }
 
-func (b *httpRequestWriter) SetData(reader io.Reader) error {
-	b.Body = ioutil.NopCloser(reader)
+func (b *httpRequestWriter) SetData(data io.Reader) error {
+	return b.setBody(data)
+}
+
+// setBody is a cherry-pick of the implementation in http.NewRequestWithContext
+func (b *httpRequestWriter) setBody(body io.Reader) error {
+	rc, ok := body.(io.ReadCloser)
+	if !ok && body != nil {
+		rc = ioutil.NopCloser(body)
+	}
+	b.Body = rc
+	if body != nil {
+		switch v := body.(type) {
+		case *bytes.Buffer:
+			b.ContentLength = int64(v.Len())
+			buf := v.Bytes()
+			b.GetBody = func() (io.ReadCloser, error) {
+				r := bytes.NewReader(buf)
+				return ioutil.NopCloser(r), nil
+			}
+		case *bytes.Reader:
+			b.ContentLength = int64(v.Len())
+			snapshot := *v
+			b.GetBody = func() (io.ReadCloser, error) {
+				r := snapshot
+				return ioutil.NopCloser(&r), nil
+			}
+		case *strings.Reader:
+			b.ContentLength = int64(v.Len())
+			snapshot := *v
+			b.GetBody = func() (io.ReadCloser, error) {
+				r := snapshot
+				return ioutil.NopCloser(&r), nil
+			}
+		default:
+			// This is where we'd set it to -1 (at least
+			// if body != NoBody) to mean unknown, but
+			// that broke people during the Go 1.8 testing
+			// period. People depend on it being 0 I
+			// guess. Maybe retry later. See Issue 18117.
+		}
+		// For client requests, Request.ContentLength of 0
+		// means either actually 0, or unknown. The only way
+		// to explicitly say that the ContentLength is zero is
+		// to set the Body to nil. But turns out too much code
+		// depends on NewRequest returning a non-nil Body,
+		// so we use a well-known ReadCloser variable instead
+		// and have the http package also treat that sentinel
+		// variable to mean explicitly zero.
+		if b.GetBody != nil && b.ContentLength == 0 {
+			b.Body = http.NoBody
+			b.GetBody = func() (io.ReadCloser, error) { return http.NoBody, nil }
+		}
+	}
 	return nil
 }