Implement server as a v2.1 PT Go API

server/lib/http.go

@@ -0,0 +1,211 @@
+package lib
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"time"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/encapsulation"
+	"git.torproject.org/pluggable-transports/snowflake.git/common/turbotunnel"
+	"git.torproject.org/pluggable-transports/snowflake.git/common/websocketconn"
+	"github.com/gorilla/websocket"
+)
+
+const requestTimeout = 10 * time.Second
+
+// How long to remember outgoing packets for a client, when we don't currently
+// have an active WebSocket connection corresponding to that client. Because a
+// client session may span multiple WebSocket connections, we keep packets we
+// aren't able to send immediately in memory, for a little while but not
+// indefinitely.
+const clientMapTimeout = 1 * time.Minute
+
+// How big to make the map of ClientIDs to IP addresses. The map is used in
+// turbotunnelMode to store a reasonable IP address for a client session that
+// may outlive any single WebSocket connection.
+const clientIDAddrMapCapacity = 1024
+
+// How long to wait for ListenAndServe or ListenAndServeTLS to return an error
+// before deciding that it's not going to return.
+const listenAndServeErrorTimeout = 100 * time.Millisecond
+
+var upgrader = websocket.Upgrader{
+	CheckOrigin: func(r *http.Request) bool { return true },
+}
+
+// clientIDAddrMap stores short-term mappings from ClientIDs to IP addresses.
+// When we call pt.DialOr, tor wants us to provide a USERADDR string that
+// represents the remote IP address of the client (for metrics purposes, etc.).
+// This data structure bridges the gap between ServeHTTP, which knows about IP
+// addresses, and handleStream, which is what calls pt.DialOr. The common piece
+// of information linking both ends of the chain is the ClientID, which is
+// attached to the WebSocket connection and every session.
+var clientIDAddrMap = newClientIDMap(clientIDAddrMapCapacity)
+
+// overrideReadConn is a net.Conn with an overridden Read method. Compare to
+// recordingConn at
+// https://dave.cheney.net/2015/05/22/struct-composition-with-go.
+type overrideReadConn struct {
+	net.Conn
+	io.Reader
+}
+
+func (conn *overrideReadConn) Read(p []byte) (int, error) {
+	return conn.Reader.Read(p)
+}
+
+type HTTPHandler struct {
+	// pconn is the adapter layer between stream-oriented WebSocket
+	// connections and the packet-oriented KCP layer.
+	pconn *turbotunnel.QueuePacketConn
+	ln    *SnowflakeListener
+}
+
+func (handler *HTTPHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	ws, err := upgrader.Upgrade(w, r, nil)
+	if err != nil {
+		log.Println(err)
+		return
+	}
+
+	conn := websocketconn.New(ws)
+	defer conn.Close()
+
+	// Pass the address of client as the remote address of incoming connection
+	clientIPParam := r.URL.Query().Get("client_ip")
+	addr := clientAddr(clientIPParam)
+
+	var token [len(turbotunnel.Token)]byte
+	_, err = io.ReadFull(conn, token[:])
+	if err != nil {
+		// Don't bother logging EOF: that happens with an unused
+		// connection, which clients make frequently as they maintain a
+		// pool of proxies.
+		if err != io.EOF {
+			log.Printf("reading token: %v", err)
+		}
+		return
+	}
+
+	switch {
+	case bytes.Equal(token[:], turbotunnel.Token[:]):
+		err = turbotunnelMode(conn, addr, handler.pconn)
+	default:
+		// We didn't find a matching token, which means that we are
+		// dealing with a client that doesn't know about such things.
+		// "Unread" the token by constructing a new Reader and pass it
+		// to the old one-session-per-WebSocket mode.
+		conn2 := &overrideReadConn{Conn: conn, Reader: io.MultiReader(bytes.NewReader(token[:]), conn)}
+		err = oneshotMode(conn2, addr, handler.ln)
+	}
+	if err != nil {
+		log.Println(err)
+		return
+	}
+}
+
+// oneshotMode handles clients that did not send turbotunnel.Token at the start
+// of their stream. These clients use the WebSocket as a raw pipe, and expect
+// their session to begin and end when this single WebSocket does.
+func oneshotMode(conn net.Conn, addr net.Addr, ln *SnowflakeListener) error {
+	return ln.QueueConn(&SnowflakeClientConn{Conn: conn, address: addr})
+}
+
+// turbotunnelMode handles clients that sent turbotunnel.Token at the start of
+// their stream. These clients expect to send and receive encapsulated packets,
+// with a long-lived session identified by ClientID.
+func turbotunnelMode(conn net.Conn, addr net.Addr, pconn *turbotunnel.QueuePacketConn) error {
+	// Read the ClientID prefix. Every packet encapsulated in this WebSocket
+	// connection pertains to the same ClientID.
+	var clientID turbotunnel.ClientID
+	_, err := io.ReadFull(conn, clientID[:])
+	if err != nil {
+		return fmt.Errorf("reading ClientID: %v", err)
+	}
+
+	// Store a a short-term mapping from the ClientID to the client IP
+	// address attached to this WebSocket connection. tor will want us to
+	// provide a client IP address when we call pt.DialOr. But a KCP session
+	// does not necessarily correspond to any single IP address--it's
+	// composed of packets that are carried in possibly multiple WebSocket
+	// streams. We apply the heuristic that the IP address of the most
+	// recent WebSocket connection that has had to do with a session, at the
+	// time the session is established, is the IP address that should be
+	// credited for the entire KCP session.
+	clientIDAddrMap.Set(clientID, addr.String())
+
+	errCh := make(chan error)
+
+	// The remainder of the WebSocket stream consists of encapsulated
+	// packets. We read them one by one and feed them into the
+	// QueuePacketConn on which kcp.ServeConn was set up, which eventually
+	// leads to KCP-level sessions in the acceptSessions function.
+	go func() {
+		for {
+			p, err := encapsulation.ReadData(conn)
+			if err != nil {
+				errCh <- err
+				break
+			}
+			pconn.QueueIncoming(p, clientID)
+		}
+	}()
+
+	// At the same time, grab packets addressed to this ClientID and
+	// encapsulate them into the downstream.
+	go func() {
+		// Buffer encapsulation.WriteData operations to keep length
+		// prefixes in the same send as the data that follows.
+		bw := bufio.NewWriter(conn)
+		for p := range pconn.OutgoingQueue(clientID) {
+			_, err := encapsulation.WriteData(bw, p)
+			if err == nil {
+				err = bw.Flush()
+			}
+			if err != nil {
+				errCh <- err
+				break
+			}
+		}
+	}()
+
+	// Wait until one of the above loops terminates. The closing of the
+	// WebSocket connection will terminate the other one.
+	<-errCh
+
+	return nil
+}
+
+type ClientMapAddr string
+
+func (addr ClientMapAddr) Network() string {
+	return "snowflake"
+}
+
+func (addr ClientMapAddr) String() string {
+	return string(addr)
+}
+
+// Return a client address
+func clientAddr(clientIPParam string) net.Addr {
+	if clientIPParam == "" {
+		return ClientMapAddr("")
+	}
+	// Check if client addr is a valid IP
+	clientIP := net.ParseIP(clientIPParam)
+	if clientIP == nil {
+		return ClientMapAddr("")
+	}
+	// Check if client addr is 0.0.0.0 or [::]. Some proxies erroneously
+	// report an address of 0.0.0.0: https://bugs.torproject.org/33157.
+	if clientIP.IsUnspecified() {
+		return ClientMapAddr("")
+	}
+	// Add a stub port number. USERADDR requires a port number.
+	return ClientMapAddr((&net.TCPAddr{IP: clientIP, Port: 1, Zone: ""}).String())
+}

server/lib/server_test.go

@@ -0,0 +1,55 @@
+package lib
+
+import (
+	"net"
+	"strconv"
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+func TestClientAddr(t *testing.T) {
+	Convey("Testing clientAddr", t, func() {
+		// good tests
+		for _, test := range []struct {
+			input    string
+			expected net.IP
+		}{
+			{"1.2.3.4", net.ParseIP("1.2.3.4")},
+			{"1:2::3:4", net.ParseIP("1:2::3:4")},
+		} {
+			useraddr := clientAddr(test.input).String()
+			host, port, err := net.SplitHostPort(useraddr)
+			if err != nil {
+				t.Errorf("clientAddr(%q) → SplitHostPort error %v", test.input, err)
+				continue
+			}
+			if !test.expected.Equal(net.ParseIP(host)) {
+				t.Errorf("clientAddr(%q) → host %q, not %v", test.input, host, test.expected)
+			}
+			portNo, err := strconv.Atoi(port)
+			if err != nil {
+				t.Errorf("clientAddr(%q) → port %q", test.input, port)
+				continue
+			}
+			if portNo == 0 {
+				t.Errorf("clientAddr(%q) → port %d", test.input, portNo)
+			}
+		}
+
+		// bad tests
+		for _, input := range []string{
+			"",
+			"abc",
+			"1.2.3.4.5",
+			"[12::34]",
+			"0.0.0.0",
+			"[::]",
+		} {
+			useraddr := clientAddr(input).String()
+			if useraddr != "" {
+				t.Errorf("clientAddr(%q) → %q, not %q", input, useraddr, "")
+			}
+		}
+	})
+}

server/lib/snowflake.go

@@ -0,0 +1,242 @@
+package lib
+
+import (
+	"crypto/tls"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"sync"
+	"time"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/turbotunnel"
+	"github.com/xtaci/kcp-go/v5"
+	"github.com/xtaci/smux"
+	"golang.org/x/net/http2"
+)
+
+// Transport is a structure with methods that conform to the Go PT v2.1 API
+// https://github.com/Pluggable-Transports/Pluggable-Transports-spec/blob/master/releases/PTSpecV2.1/Pluggable%20Transport%20Specification%20v2.1%20-%20Go%20Transport%20API.pdf
+type Transport struct {
+	getCertificate func(*tls.ClientHelloInfo) (*tls.Certificate, error)
+}
+
+func NewSnowflakeServer(getCertificate func(*tls.ClientHelloInfo) (*tls.Certificate, error)) *Transport {
+
+	return &Transport{getCertificate: getCertificate}
+}
+
+func (t *Transport) Listen(addr net.Addr) (*SnowflakeListener, error) {
+	listener := &SnowflakeListener{addr: addr, queue: make(chan net.Conn, 65534)}
+
+	handler := HTTPHandler{
+		// pconn is shared among all connections to this server. It
+		// overlays packet-based client sessions on top of ephemeral
+		// WebSocket connections.
+		pconn: turbotunnel.NewQueuePacketConn(addr, clientMapTimeout),
+	}
+	server := &http.Server{
+		Addr:        addr.String(),
+		Handler:     &handler,
+		ReadTimeout: requestTimeout,
+	}
+	// We need to override server.TLSConfig.GetCertificate--but first
+	// server.TLSConfig needs to be non-nil. If we just create our own new
+	// &tls.Config, it will lack the default settings that the net/http
+	// package sets up for things like HTTP/2. Therefore we first call
+	// http2.ConfigureServer for its side effect of initializing
+	// server.TLSConfig properly. An alternative would be to make a dummy
+	// net.Listener, call Serve on it, and let it return.
+	// https://github.com/golang/go/issues/16588#issuecomment-237386446
+	err := http2.ConfigureServer(server, nil)
+	if err != nil {
+		return nil, err
+	}
+	server.TLSConfig.GetCertificate = t.getCertificate
+
+	// Another unfortunate effect of the inseparable net/http ListenAndServe
+	// is that we can't check for Listen errors like "permission denied" and
+	// "address already in use" without potentially entering the infinite
+	// loop of Serve. The hack we apply here is to wait a short time,
+	// listenAndServeErrorTimeout, to see if an error is returned (because
+	// it's better if the error message goes to the tor log through
+	// SMETHOD-ERROR than if it only goes to the snowflake log).
+	errChan := make(chan error)
+	go func() {
+		if t.getCertificate == nil {
+			// TLS is disabled
+			log.Printf("listening with plain HTTP on %s", addr)
+			err := server.ListenAndServe()
+			if err != nil {
+				log.Printf("error in ListenAndServe: %s", err)
+			}
+			errChan <- err
+		} else {
+			log.Printf("listening with HTTPS on %s", addr)
+			err := server.ListenAndServeTLS("", "")
+			if err != nil {
+				log.Printf("error in ListenAndServeTLS: %s", err)
+			}
+			errChan <- err
+		}
+	}()
+
+	select {
+	case err = <-errChan:
+		break
+	case <-time.After(listenAndServeErrorTimeout):
+		break
+	}
+
+	listener.server = server
+
+	// Start a KCP engine, set up to read and write its packets over the
+	// WebSocket connections that arrive at the web server.
+	// handler.ServeHTTP is responsible for encapsulation/decapsulation of
+	// packets on behalf of KCP. KCP takes those packets and turns them into
+	// sessions which appear in the acceptSessions function.
+	ln, err := kcp.ServeConn(nil, 0, 0, handler.pconn)
+	if err != nil {
+		server.Close()
+		return nil, err
+	}
+	go func() {
+		defer ln.Close()
+		err := listener.acceptSessions(ln)
+		if err != nil {
+			log.Printf("acceptSessions: %v", err)
+		}
+	}()
+
+	listener.ln = ln
+
+	return listener, nil
+
+}
+
+type SnowflakeListener struct {
+	addr      net.Addr
+	queue     chan net.Conn
+	server    *http.Server
+	ln        *kcp.Listener
+	closed    chan struct{}
+	closeOnce sync.Once
+}
+
+// Allows the caller to accept incoming Snowflake connections
+// We accept connections from a queue to accommodate both incoming
+// smux Streams and legacy non-turbotunnel connections
+func (l *SnowflakeListener) Accept() (net.Conn, error) {
+	select {
+	case <-l.closed:
+		//channel has been closed, no longer accepting connections
+		return nil, io.ErrClosedPipe
+	case conn := <-l.queue:
+		return conn, nil
+	}
+}
+
+func (l *SnowflakeListener) Addr() net.Addr {
+	return l.addr
+}
+
+func (l *SnowflakeListener) Close() error {
+	// Close our HTTP server and our KCP listener
+	l.closeOnce.Do(func() {
+		close(l.closed)
+		l.server.Close()
+		l.ln.Close()
+	})
+	return nil
+}
+
+// acceptStreams layers an smux.Session on the KCP connection and awaits streams
+// on it. Passes each stream to our SnowflakeListener accept queue.
+func (l *SnowflakeListener) acceptStreams(conn *kcp.UDPSession) error {
+	// Look up the IP address associated with this KCP session, via the
+	// ClientID that is returned by the session's RemoteAddr method.
+	addr, ok := clientIDAddrMap.Get(conn.RemoteAddr().(turbotunnel.ClientID))
+	if !ok {
+		// This means that the map is tending to run over capacity, not
+		// just that there was not client_ip on the incoming connection.
+		// We store "" in the map in the absence of client_ip. This log
+		// message means you should increase clientIDAddrMapCapacity.
+		log.Printf("no address in clientID-to-IP map (capacity %d)", clientIDAddrMapCapacity)
+	}
+
+	smuxConfig := smux.DefaultConfig()
+	smuxConfig.Version = 2
+	smuxConfig.KeepAliveTimeout = 10 * time.Minute
+	sess, err := smux.Server(conn, smuxConfig)
+	if err != nil {
+		return err
+	}
+
+	for {
+		stream, err := sess.AcceptStream()
+		if err != nil {
+			if err, ok := err.(net.Error); ok && err.Temporary() {
+				continue
+			}
+			return err
+		}
+		l.QueueConn(&SnowflakeClientConn{Conn: stream, address: clientAddr(addr)})
+	}
+}
+
+// acceptSessions listens for incoming KCP connections and passes them to
+// acceptStreams. It is handler.ServeHTTP that provides the network interface
+// that drives this function.
+func (l *SnowflakeListener) acceptSessions(ln *kcp.Listener) error {
+	for {
+		conn, err := ln.AcceptKCP()
+		if err != nil {
+			if err, ok := err.(net.Error); ok && err.Temporary() {
+				continue
+			}
+			return err
+		}
+		// Permit coalescing the payloads of consecutive sends.
+		conn.SetStreamMode(true)
+		// Set the maximum send and receive window sizes to a high number
+		// Removes KCP bottlenecks: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40026
+		conn.SetWindowSize(65535, 65535)
+		// Disable the dynamic congestion window (limit only by the
+		// maximum of local and remote static windows).
+		conn.SetNoDelay(
+			0, // default nodelay
+			0, // default interval
+			0, // default resend
+			1, // nc=1 => congestion window off
+		)
+		go func() {
+			defer conn.Close()
+			err := l.acceptStreams(conn)
+			if err != nil && err != io.ErrClosedPipe {
+				log.Printf("acceptStreams: %v", err)
+			}
+		}()
+	}
+}
+
+func (l *SnowflakeListener) QueueConn(conn net.Conn) error {
+	select {
+	case <-l.closed:
+		return fmt.Errorf("accepted connection on closed listener")
+	case l.queue <- conn:
+		return nil
+	}
+}
+
+// A wrapper for the underlying oneshot or turbotunnel conn
+// because we need to reference our mapping to determine the client
+// address
+type SnowflakeClientConn struct {
+	net.Conn
+	address net.Addr
+}
+
+func (conn *SnowflakeClientConn) RemoteAddr() net.Addr {
+	return conn.address
+}

server/lib/turbotunnel.go

@@ -0,0 +1,85 @@
+package lib
+
+import (
+	"sync"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/turbotunnel"
+)
+
+// clientIDMap is a fixed-capacity mapping from ClientIDs to address strings.
+// Adding a new entry using the Set method causes the oldest existing entry to
+// be forgotten.
+//
+// This data type is meant to be used to remember the IP address associated with
+// a ClientID, during the short period of time between when a WebSocket
+// connection with that ClientID began, and when a KCP session is established.
+//
+// The design requirements of this type are that it needs to remember a mapping
+// for only a short time, and old entries should expire so as not to consume
+// unbounded memory. It is not a critical error if an entry is forgotten before
+// it is needed; better to forget entries than to use too much memory.
+type clientIDMap struct {
+	lock sync.Mutex
+	// entries is a circular buffer of (ClientID, addr) pairs.
+	entries []struct {
+		clientID turbotunnel.ClientID
+		addr     string
+	}
+	// oldest is the index of the oldest member of the entries buffer, the
+	// one that will be overwritten at the next call to Set.
+	oldest int
+	// current points to the index of the most recent entry corresponding to
+	// each ClientID.
+	current map[turbotunnel.ClientID]int
+}
+
+// newClientIDMap makes a new clientIDMap with the given capacity.
+func newClientIDMap(capacity int) *clientIDMap {
+	return &clientIDMap{
+		entries: make([]struct {
+			clientID turbotunnel.ClientID
+			addr     string
+		}, capacity),
+		oldest:  0,
+		current: make(map[turbotunnel.ClientID]int),
+	}
+}
+
+// Set adds a mapping from clientID to addr, replacing any previous mapping for
+// clientID. It may also cause the clientIDMap to forget at most one other
+// mapping, the oldest one.
+func (m *clientIDMap) Set(clientID turbotunnel.ClientID, addr string) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	if len(m.entries) == 0 {
+		// The invariant m.oldest < len(m.entries) does not hold in this
+		// special case.
+		return
+	}
+	// m.oldest is the index of the entry we're about to overwrite. If it's
+	// the current entry for any ClientID, we need to delete that clientID
+	// from the current map (that ClientID is now forgotten).
+	if i, ok := m.current[m.entries[m.oldest].clientID]; ok && i == m.oldest {
+		delete(m.current, m.entries[m.oldest].clientID)
+	}
+	// Overwrite the oldest entry.
+	m.entries[m.oldest].clientID = clientID
+	m.entries[m.oldest].addr = addr
+	// Add the overwritten entry to the quick-lookup map.
+	m.current[clientID] = m.oldest
+	// What was the oldest entry is now the newest.
+	m.oldest = (m.oldest + 1) % len(m.entries)
+}
+
+// Get returns a previously stored mapping. The second return value indicates
+// whether clientID was actually present in the map. If it is false, then the
+// returned address string will be "".
+func (m *clientIDMap) Get(clientID turbotunnel.ClientID) (string, bool) {
+	m.lock.Lock()
+	defer m.lock.Unlock()
+	if i, ok := m.current[clientID]; ok {
+		return m.entries[i].addr, true
+	} else {
+		return "", false
+	}
+}

server/lib/turbotunnel_test.go

@@ -0,0 +1,119 @@
+package lib
+
+import (
+	"encoding/binary"
+	"testing"
+
+	"git.torproject.org/pluggable-transports/snowflake.git/common/turbotunnel"
+)
+
+func TestClientIDMap(t *testing.T) {
+	// Convert a uint64 into a ClientID.
+	id := func(n uint64) turbotunnel.ClientID {
+		var clientID turbotunnel.ClientID
+		binary.PutUvarint(clientID[:], n)
+		return clientID
+	}
+
+	// Does m.Get(key) and checks that the output matches what is expected.
+	expectGet := func(m *clientIDMap, clientID turbotunnel.ClientID, expectedAddr string, expectedOK bool) {
+		t.Helper()
+		addr, ok := m.Get(clientID)
+		if addr != expectedAddr || ok != expectedOK {
+			t.Errorf("expected (%+q, %v), got (%+q, %v)", expectedAddr, expectedOK, addr, ok)
+		}
+	}
+
+	// Checks that the len of m.current is as expected.
+	expectSize := func(m *clientIDMap, expectedLen int) {
+		t.Helper()
+		if len(m.current) != expectedLen {
+			t.Errorf("expected map len %d, got %d %+v", expectedLen, len(m.current), m.current)
+		}
+	}
+
+	// Zero-capacity map can't remember anything.
+	{
+		m := newClientIDMap(0)
+		expectSize(m, 0)
+		expectGet(m, id(0), "", false)
+		expectGet(m, id(1234), "", false)
+
+		m.Set(id(0), "A")
+		expectSize(m, 0)
+		expectGet(m, id(0), "", false)
+		expectGet(m, id(1234), "", false)
+
+		m.Set(id(1234), "A")
+		expectSize(m, 0)
+		expectGet(m, id(0), "", false)
+		expectGet(m, id(1234), "", false)
+	}
+
+	{
+		m := newClientIDMap(1)
+		expectSize(m, 0)
+		expectGet(m, id(0), "", false)
+		expectGet(m, id(1), "", false)
+
+		m.Set(id(0), "A")
+		expectSize(m, 1)
+		expectGet(m, id(0), "A", true)
+		expectGet(m, id(1), "", false)
+
+		m.Set(id(1), "B") // forgets the (0, "A") entry
+		expectSize(m, 1)
+		expectGet(m, id(0), "", false)
+		expectGet(m, id(1), "B", true)
+
+		m.Set(id(1), "C") // forgets the (1, "B") entry
+		expectSize(m, 1)
+		expectGet(m, id(0), "", false)
+		expectGet(m, id(1), "C", true)
+	}
+
+	{
+		m := newClientIDMap(5)
+		m.Set(id(0), "A")
+		m.Set(id(1), "B")
+		m.Set(id(2), "C")
+		m.Set(id(0), "D") // shadows the (0, "D") entry
+		m.Set(id(3), "E")
+		expectSize(m, 4)
+		expectGet(m, id(0), "D", true)
+		expectGet(m, id(1), "B", true)
+		expectGet(m, id(2), "C", true)
+		expectGet(m, id(3), "E", true)
+		expectGet(m, id(4), "", false)
+
+		m.Set(id(4), "F") // forgets the (0, "A") entry but should preserve (0, "D")
+		expectSize(m, 5)
+		expectGet(m, id(0), "D", true)
+		expectGet(m, id(1), "B", true)
+		expectGet(m, id(2), "C", true)
+		expectGet(m, id(3), "E", true)
+		expectGet(m, id(4), "F", true)
+
+		m.Set(id(5), "G") // forgets the (1, "B") entry
+		m.Set(id(0), "H") // forgets the (2, "C") entry and shadows (0, "D")
+		expectSize(m, 4)
+		expectGet(m, id(0), "H", true)
+		expectGet(m, id(1), "", false)
+		expectGet(m, id(2), "", false)
+		expectGet(m, id(3), "E", true)
+		expectGet(m, id(4), "F", true)
+		expectGet(m, id(5), "G", true)
+
+		m.Set(id(0), "I") // forgets the (0, "D") entry and shadows (0, "H")
+		m.Set(id(0), "J") // forgets the (3, "E") entry and shadows (0, "I")
+		m.Set(id(0), "K") // forgets the (4, "F") entry and shadows (0, "J")
+		m.Set(id(0), "L") // forgets the (5, "G") entry and shadows (0, "K")
+		expectSize(m, 1)
+		expectGet(m, id(0), "L", true)
+		expectGet(m, id(1), "", false)
+		expectGet(m, id(2), "", false)
+		expectGet(m, id(3), "", false)
+		expectGet(m, id(4), "", false)
+		expectGet(m, id(5), "", false)
+	}
+}