USERADDR support for turbotunnel sessions.

The difficulty here is that the whole point of turbotunnel sessions is
that they are not necessarily tied to a single WebSocket connection, nor
even a single client IP address. We use a heuristic: whenever a
WebSocket connection starts that has a new ClientID, we store a mapping
from that ClientID to the IP address attached to the WebSocket
connection in a lookup table. Later, when enough packets have arrived to
establish a turbotunnel session, we recover the ClientID associated with
the session (which kcp-go has stored in the RemoteAddr field), and look
it up in the table to get an IP address. We introduce a new data type,
clientIDMap, to store the clientID-to-IP mapping during the short time
between when a WebSocket connection starts and handleSession receives a
fully fledged KCP session.

server/turbotunnel_test.go

@@ -0,0 +1,119 @@
+package main
+
+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)
+	}
+}