http2: avoid Transport hang with Connection: close and AllowHTTP

CL 111835 changed Transport stream ID numbering to start at
stream 3 when AllowHTTP is set. This was based on a
misunderstanding:

When a connection upgrades an HTTP/1.1 request to HTTP/2,
the initial HTTP/1.1 request occupies stream 1.
However, Transport does not perform HTTP protocol upgrades.
When using a Transport to send unencrypted HTTP/2 requests,
the entire connection uses HTTP/2, the first request is
sent as HTTP/2, and there is no reason not to use stream 1
for this request.

Starting from stream 3 is mostly harmless,
but ClientConn.idleStateLocked assumes that client streams
start from 1. This causes it to misidentify new single-use
connections as having already sent a request (when AllowHTTP
is set), and therefore not suitable for use.

Revert to always starting stream IDs at 1.

Fixes golang/go#67671

Change-Id: I97c89de4ae49623d916f9dbd200f8252d2fd4247
Reviewed-on: https://go-review.googlesource.com/c/net/+/591275
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

http2/transport.go

diff --git a/http2/transport.go b/http2/transport.go
index 98a49c6..61f511f 100644
--- a/http2/transport.go
+++ b/http2/transport.go
@@ -827,10 +827,6 @@
 	cc.henc.SetMaxDynamicTableSizeLimit(t.maxEncoderHeaderTableSize())
 	cc.peerMaxHeaderTableSize = initialHeaderTableSize
 
-	if t.AllowHTTP {
-		cc.nextStreamID = 3
-	}
-
 	if cs, ok := c.(connectionStater); ok {
 		state := cs.ConnectionState()
 		cc.tlsState = &state

http2/transport_test.go

diff --git a/http2/transport_test.go b/http2/transport_test.go
index ddeaf61..498e279 100644
--- a/http2/transport_test.go
+++ b/http2/transport_test.go
@@ -5401,3 +5401,23 @@
 
 	<-donec
 }
+
+// Issue 67671: Sending a Connection: close request on a Transport with AllowHTTP
+// set caused a the transport to wedge.
+func TestIssue67671(t *testing.T) {
+	ts := newTestServer(t, func(w http.ResponseWriter, r *http.Request) {})
+	tr := &Transport{
+		TLSClientConfig: tlsConfigInsecure,
+		AllowHTTP:       true,
+	}
+	defer tr.CloseIdleConnections()
+	req, _ := http.NewRequest("GET", ts.URL, nil)
+	req.Close = true
+	for i := 0; i < 2; i++ {
+		res, err := tr.RoundTrip(req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		res.Body.Close()
+	}
+}