x/net/http2: investigate client write buffering

[odeke-em]

testClientWrite in http2 mode with CL https://go-review.googlesource.com/#/c/17751/1 fails, but the fails aren't that consistent

Run 1:

$ go test -cover
--- FAIL: TestClientWrites_h2 (0.00s)
    client_test.go:578: Get request did 8 Write calls, want 1
    client_test.go:587: Post request did 3 Write calls, want 1
FAIL
coverage: 78.6% of statements
exit status 1
FAIL    net/http    23.081s

Run 2:

$ go test -cover
--- FAIL: TestClientWrites_h2 (0.01s)
    client_test.go:578: Get request did 8 Write calls, want 1
FAIL
coverage: 78.5% of statements
exit status 1

The diff that produced this is

diff --git a/src/net/http/client_test.go b/src/net/http/client_test.go
index e72f3bc..05a543c 100644
--- a/src/net/http/client_test.go
+++ b/src/net/http/client_test.go
@@ -550,11 +550,14 @@ func (c *writeCountingConn) Write(p []byte) (int, error) {

 // TestClientWrites verifies that client requests are buffered and we
 // don't send a TCP packet per line of the http request + body.
-func TestClientWrites(t *testing.T) {
+func TestClientWrites_h1(t *testing.T) { testClientWrites(t, h1Mode) }
+func TestClientWrites_h2(t *testing.T) { testClientWrites(t, h2Mode) }
+
+func testClientWrites(t *testing.T, h2 bool) {
    defer afterTest(t)
-   ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+   cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
    }))
-   defer ts.Close()
+   defer cst.close()

    writes := 0
    dialer := func(netz string, addr string) (net.Conn, error) {
@@ -564,9 +567,10 @@ func TestClientWrites(t *testing.T) {
        }
        return c, err
    }
-   c := &Client{Transport: &Transport{Dial: dialer}}
+   cst.c.Transport.(*Transport).Dial = dialer

-   _, err := c.Get(ts.URL)
+   c := cst.c
+   _, err := c.Get(cst.ts.URL)
    if err != nil {
        t.Fatal(err)
    }
@@ -575,7 +579,7 @@ func TestClientWrites(t *testing.T) {
    }

    writes = 0
-   _, err = c.PostForm(ts.URL, url.Values{"foo": {"bar"}})
+   _, err = c.PostForm(cst.ts.URL, url.Values{"foo": {"bar"}})
    if err != nil {
        t.Fatal(err)
    }

[bradfitz]

h1 and h2 have different write patterns. This test was locking in a certain behavior for the h1 client.

It's true that the h2 client does more writes than it needs to in some cases (less buffering) for simplicity, but I wasn't planning on investigating whether it's worth fixing until after Go 1.6.

[odeke-em]

Aye aye, sounds like a plan.