http2: mostly test for Transport memory issue

Updates golang/go#20448

Change-Id: I6366b0b0b36d81095ab347ca35e1e72169c05c15
Reviewed-on: https://go-review.googlesource.com/43853
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>

http2/transport_test.go

@@ -2915,3 +2915,58 @@ func TestAuthorityAddr(t *testing.T) {
 		}
 	}
 }
+
+// Issue 20448: stop allocating for DATA frames' payload after
+// Response.Body.Close is called.
+func TestTransportAllocationsAfterResponseBodyClose(t *testing.T) {
+	megabyteZero := make([]byte, 1<<20)
+
+	writeErr := make(chan error, 1)
+
+	st := newServerTester(t, func(w http.ResponseWriter, r *http.Request) {
+		w.(http.Flusher).Flush()
+		var sum int64
+		for i := 0; i < 100; i++ {
+			n, err := w.Write(megabyteZero)
+			sum += int64(n)
+			if err != nil {
+				writeErr <- err
+				return
+			}
+		}
+		t.Logf("wrote all %d bytes", sum)
+		writeErr <- nil
+	}, optOnlyServer)
+	defer st.Close()
+
+	tr := &Transport{TLSClientConfig: tlsConfigInsecure}
+	defer tr.CloseIdleConnections()
+	c := &http.Client{Transport: tr}
+	res, err := c.Get(st.ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var buf [1]byte
+	if _, err := res.Body.Read(buf[:]); err != nil {
+		t.Error(err)
+	}
+	if err := res.Body.Close(); err != nil {
+		t.Error(err)
+	}
+
+	trb, ok := res.Body.(transportResponseBody)
+	if !ok {
+		t.Fatalf("res.Body = %T; want transportResponseBody", res.Body)
+	}
+	if trb.cs.bufPipe.b != nil {
+		// TODO(tombergan,bradfitz): turn this into an error:
+		t.Logf("response body pipe is still open")
+	}
+
+	gotErr := <-writeErr
+	if gotErr == nil {
+		t.Errorf("Handler unexpectedly managed to write its entire response without getting an error")
+	} else if gotErr != errStreamClosed {
+		t.Errorf("Handler Write err = %v; want errStreamClosed", gotErr)
+	}
+}