quic: fix appendMaxDataFrame erroneously accumulating sentLimit

sentLimit value should be of the last MAX_DATA sent to the peer.
However, we are currently incorrectly incrementing its value rather than
updating it.

Change-Id: Id73ce651158e7c4f031e8a2e5099e9d36a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/778380
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>

Author: nicholashusin

quic/conn_flow.go

@@ -107,7 +107,7 @@ func (c *Conn) appendMaxDataFrame(w *packetWriter, pnum packetNumber, pto bool)
 		if !w.appendMaxDataFrame(c.streams.inflow.newLimit) {
 			return false
 		}
-		c.streams.inflow.sentLimit += c.streams.inflow.newLimit
+		c.streams.inflow.sentLimit = c.streams.inflow.newLimit
 		c.streams.inflow.sent.setSent(pnum)
 	}
 	return true

quic/conn_flow_test.go

@@ -266,6 +266,19 @@ func testConnInflowMultipleStreams(t *testing.T) {
 		packetType1RTT, debugFrameMaxData{
 			max: 128 + 32 + 1 + 32 + 1,
 		})
+
+	// Current limit is 194 (128 + 32 + 1 + 32 + 1).
+	// Available window is 194 - 128 = 66 bytes.
+	// Send 67 bytes on streams[0] to violate flow control.
+	tc.writeFrames(packetType1RTT, debugFrameStream{
+		id:   streams[0].id,
+		off:  32,
+		data: make([]byte, 67),
+	})
+	tc.wantFrame("peer violates MAX_DATA limit",
+		packetType1RTT, debugFrameConnectionCloseTransport{
+			code: errFlowControl,
+		})
 }
 
 func TestConnOutflowBlocked(t *testing.T) {