net/http: allow reuse of http.Request objects

Calling response.Body.Close() early would generarate a race before this.
Since closing would return early before the main code path had a chance
to reset the request canceler. Having a non-nil request canceler at the
start of the next request would cause a "request canceled" error.

Here we simply wait for the eofc channel to be closed before returning
from earlyCloseFn, ensuring that the caller won't be re-using that
Request object before we have a chance to reset the request canceler to
nil.

Fixes #21838

Change-Id: I641815526c6ac63d1816c9b6ad49d73715f7a5cb
Reviewed-on: https://go-review.googlesource.com/62891
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>

src/net/http/transport.go

@@ -1616,6 +1616,7 @@ func (pc *persistConn) readLoop() {
 			body: resp.Body,
 			earlyCloseFn: func() error {
 				waitForBodyRead <- false
+				<-eofc // will be closed by deferred call at the end of the function
 				return nil
 
 			},

src/net/http/transport_test.go

@@ -124,6 +124,34 @@ func (tcs *testConnSet) check(t *testing.T) {
 	}
 }
 
+func TestReuseRequest(t *testing.T) {
+	defer afterTest(t)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Write([]byte("{}"))
+	}))
+	defer ts.Close()
+
+	c := ts.Client()
+	req, _ := NewRequest("GET", ts.URL, nil)
+	res, err := c.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = res.Body.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	res, err = c.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = res.Body.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 // Two subsequent requests and verify their response is the same.
 // The response from the server is our own IP:port
 func TestTransportKeepAlives(t *testing.T) {