wgengine/netstack: correctly proxy half-closed TCP connections

TCP connections are two unidirectional data streams, and if one of these
streams closes, we should not assume the other half is closed as well.
For example, if an HTTP client closes its write half of the connection
early, it may still be expecting to receive data on its read half, so we
should keep the server -> client half of the connection open, while
terminating the client -> server half.

Fixes tailscale/corp#29837.

Signed-off-by: Naman Sood <mail@nsood.in>

wgengine/netstack/netstack.go

@@ -1435,6 +1435,13 @@ func (ns *Impl) acceptTCP(r *tcp.ForwarderRequest) {
 	}
 }
 
+// tcpCloser is an interface to abstract around various TCPConn types that
+// allow closing of the read and write streams independently of each other.
+type tcpCloser interface {
+	CloseRead() error
+	CloseWrite() error
+}
+
 func (ns *Impl) forwardTCP(getClient func(...tcpip.SettableSocketOption) *gonet.TCPConn, clientRemoteIP netip.Addr, wq *waiter.Queue, dialAddr netip.AddrPort) (handled bool) {
 	dialAddrStr := dialAddr.String()
 	if debugNetstack() {
@@ -1501,18 +1508,48 @@ func (ns *Impl) forwardTCP(getClient func(...tcpip.SettableSocketOption) *gonet.
 	}
 	defer client.Close()
 
+	// As of 2025-07-03, backend is always either a net.TCPConn
+	// from stdDialer.DialContext (which has the requisite functions),
+	// or nil from hangDialer in tests (in which case we would have
+	// errored out by now), so this conversion should always succeed.
+	backendTCPCloser, backendIsTCPCloser := backend.(tcpCloser)
 	connClosed := make(chan error, 2)
 	go func() {
 		_, err := io.Copy(backend, client)
+		if err != nil {
+			err = fmt.Errorf("client -> backend: %w", err)
+		}
 		connClosed <- err
+		err = nil
+		if backendIsTCPCloser {
+			err = backendTCPCloser.CloseWrite()
+		}
+		err = errors.Join(err, client.CloseRead())
+		if err != nil {
+			ns.logf("client -> backend close connection: %v", err)
+		}
 	}()
 	go func() {
 		_, err := io.Copy(client, backend)
+		if err != nil {
+			err = fmt.Errorf("backend -> client: %w", err)
+		}
 		connClosed <- err
+		err = nil
+		if backendIsTCPCloser {
+			err = backendTCPCloser.CloseRead()
+		}
+		err = errors.Join(err, client.CloseWrite())
+		if err != nil {
+			ns.logf("backend -> client close connection: %v", err)
+		}
 	}()
-	err = <-connClosed
+	// Wait for both ends of the connection to close.
-	if err != nil {
+	for range 2 {
-		ns.logf("proxy connection closed with error: %v", err)
+		err = <-connClosed
+		if err != nil {
+			ns.logf("proxy connection closed with error: %v", err)
+		}
 	}
 	ns.logf("[v2] netstack: forwarder connection to %s closed", dialAddrStr)
 	return