net/http: document and test Client.Post treating Reader body as ReadCloser

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/11542044

src/pkg/net/http/client.go

@@ -74,8 +74,8 @@ type RoundTripper interface {
 	// authentication, or cookies.
 	//
 	// RoundTrip should not modify the request, except for
-	// consuming the Body.  The request's URL and Header fields
-	// are guaranteed to be initialized.
+	// consuming and closing the Body. The request's URL and
+	// Header fields are guaranteed to be initialized.
 	RoundTrip(*Request) (*Response, error)
 }
 
@@ -346,6 +346,9 @@ func Post(url string, bodyType string, body io.Reader) (resp *Response, err erro
 // Post issues a POST to the specified URL.
 //
 // Caller should close resp.Body when done reading from it.
+//
+// If the provided body is also an io.Closer, it is closed after the
+// body is successfully written to the server.
 func (c *Client) Post(url string, bodyType string, body io.Reader) (resp *Response, err error) {
 	req, err := NewRequest("POST", url, body)
 	if err != nil {

src/pkg/net/http/transport_test.go

@@ -15,6 +15,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net"
+	"net/http"
 	. "net/http"
 	"net/http/httptest"
 	"net/url"
@@ -1610,6 +1611,42 @@ func TestIdleConnChannelLeak(t *testing.T) {
 	}
 }
 
+// Verify the status quo: that the Client.Post function coerces its
+// body into a ReadCloser if it's a Closer, and that the Transport
+// then closes it.
+func TestTransportClosesRequestBody(t *testing.T) {
+	defer afterTest(t)
+	ts := httptest.NewServer(http.HandlerFunc(func(w ResponseWriter, r *Request) {
+		io.Copy(ioutil.Discard, r.Body)
+	}))
+	defer ts.Close()
+
+	tr := &Transport{}
+	defer tr.CloseIdleConnections()
+	cl := &Client{Transport: tr}
+
+	closes := 0
+
+	res, err := cl.Post(ts.URL, "text/plain", countCloseReader{&closes, strings.NewReader("hello")})
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	if closes != 1 {
+		t.Errorf("closes = %d; want 1", closes)
+	}
+}
+
+type countCloseReader struct {
+	n *int
+	io.Reader
+}
+
+func (cr countCloseReader) Close() error {
+	(*cr.n)++
+	return nil
+}
+
 // rgz is a gzip quine that uncompresses to itself.
 var rgz = []byte{
 	0x1f, 0x8b, 0x08, 0x08, 0x00, 0x00, 0x00, 0x00,