Fix CurrentConnected counter overflow

The CurrentConnected counter is decremented in multiple places and in function
`maybeFailTimerHandler`, the caller does not make sure the state was one of the
connected states, causing overflow. This change puts the decrement logic in a
single place in `setEndpointState`. Added a test that would fail before this
change, though it is not the only case an overflow can happen.

https://fxbug.dev/105542

PiperOrigin-RevId: 464938313

pkg/tcpip/transport/tcp/connect.go

@@ -130,7 +130,6 @@ func maybeFailTimerHandler(e *endpoint, f func() tcpip.Error) func() {
 			e.lastError = err
 			e.lastErrorMu.Unlock()
 			e.hardError = err
-			e.stack.Stats().TCP.CurrentConnected.Decrement()
 			e.cleanupLocked()
 			e.setEndpointState(StateError)
 			e.mu.Unlock()
@@ -1050,7 +1049,6 @@ func (e *endpoint) transitionToStateCloseLocked() {
 	}
 
 	if s.connected() {
-		e.stack.Stats().TCP.CurrentConnected.Decrement()
 		e.stack.Stats().TCP.EstablishedClosed.Increment()
 	}
 

pkg/tcpip/transport/tcp/dispatcher.go

@@ -196,7 +196,6 @@ func (p *processor) handleConnected(ep *endpoint) {
 		fallthrough
 	case ep.EndpointState() == StateClose:
 		ep.mu.Unlock()
-		ep.stack.Stats().TCP.CurrentConnected.Decrement()
 		ep.drainClosingSegmentQueue()
 		ep.waiterQueue.Notify(waiter.EventHUp | waiter.EventErr | waiter.ReadableEvents | waiter.WritableEvents)
 		return

pkg/tcpip/transport/tcp/endpoint.go

@@ -731,6 +731,9 @@ func (e *endpoint) setEndpointState(state EndpointState) {
 		if oldstate == StateCloseWait || oldstate == StateEstablished {
 			e.stack.Stats().TCP.EstablishedResets.Increment()
 		}
+		if oldstate.connected() {
+			e.stack.Stats().TCP.CurrentConnected.Decrement()
+		}
 		fallthrough
 	default:
 		if oldstate == StateEstablished {
@@ -977,7 +980,6 @@ func (e *endpoint) Abort() {
 	// Reset all connected endpoints.
 	switch state := e.EndpointState(); {
 	case state.connected():
-		e.stack.Stats().TCP.CurrentConnected.Decrement()
 		e.resetConnectionLocked(&tcpip.ErrAborted{})
 		return
 	}

pkg/tcpip/transport/tcp/test/e2e/tcp_test.go

@@ -292,6 +292,37 @@ func TestCloseWithoutConnect(t *testing.T) {
 	}
 }
 
+func TestHandshakeTimeoutConnectedCount(t *testing.T) {
+	c := context.New(t, e2e.DefaultMTU)
+	defer c.Cleanup()
+
+	ep, err := c.Stack().NewEndpoint(tcp.ProtocolNumber, ipv4.ProtocolNumber, &c.WQ)
+	if err != nil {
+		t.Fatalf("NewEndpoint failed: %s", err)
+	}
+	c.EP = ep
+
+	we, ch := waiter.NewChannelEntry(waiter.WritableEvents)
+	c.WQ.EventRegister(&we)
+	defer c.WQ.EventUnregister(&we)
+
+	switch err := c.EP.Connect(tcpip.FullAddress{Addr: context.TestAddr, Port: context.TestPort}).(type) {
+	case *tcpip.ErrConnectStarted:
+	default:
+		t.Fatalf("Connect did not start: %v", err)
+	}
+
+	<-ch
+	switch err := c.EP.LastError().(type) {
+	case *tcpip.ErrTimeout:
+	default:
+		t.Fatalf("Connect didn't timeout: %v", err)
+	}
+	if got, want := c.Stack().Stats().TCP.CurrentConnected.Value(), uint64(0); got != want {
+		t.Fatalf("got stats.TCP.CurrentConnected.Value() = %d, want = %d", got, want)
+	}
+}
+
 func TestTCPSegmentsSentIncrement(t *testing.T) {
 	c := context.New(t, e2e.DefaultMTU)
 	defer c.Cleanup()