wgengine/magicsock: add new endpoint type used for discovery-supporting peers

This adds a new magicsock endpoint type only used when both sides
support discovery (that is, are advertising a discovery
key). Otherwise the old code is used.

So far the new code only communicates over DERP as proof that the new
code paths are wired up. None of the actually discovery messaging is
implemented yet.

Support for discovery (generating and advertising a key) are still
behind an environment variable for now.

Updates #483

control/controlclient/direct.go

@@ -451,8 +451,6 @@ func (c *Direct) SetEndpoints(localPort uint16, endpoints []string) (changed boo
 	return c.newEndpoints(localPort, endpoints)
 }
 
-var debugNetmap, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_NETMAP"))
-
 func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkMap)) error {
 	c.mu.Lock()
 	persist := c.persist
@@ -472,7 +470,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	c.logf("PollNetMap: stream=%v :%v %v", maxPolls, localPort, ep)
 
 	vlogf := logger.Discard
-	if debugNetmap {
+	if Debug.NetMap {
 		vlogf = c.logf
 	}
 
@@ -603,6 +601,18 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 		if resp.Debug != nil && resp.Debug.LogHeapPprof {
 			go logheap.LogHeap(resp.Debug.LogHeapURL)
 		}
+		// Temporarily (2020-06-29) support removing all but
+		// discovery-supporting nodes during development, for
+		// less noise.
+		if Debug.OnlyDisco {
+			filtered := resp.Peers[:0]
+			for _, p := range resp.Peers {
+				if !p.DiscoKey.IsZero() {
+					filtered = append(filtered, p)
+				}
+			}
+			resp.Peers = filtered
+		}
 
 		nm := &NetworkMap{
 			NodeKey:      tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
@@ -766,3 +776,33 @@ func loadServerKey(ctx context.Context, httpc *http.Client, serverURL string) (w
 	}
 	return key, nil
 }
+
+// Debug contains temporary internal-only debug knobs.
+// They're unexported to not draw attention to them.
+var Debug = initDebug()
+
+type debug struct {
+	NetMap    bool
+	OnlyDisco bool
+	Disco     bool
+}
+
+func initDebug() debug {
+	return debug{
+		NetMap:    envBool("TS_DEBUG_NETMAP"),
+		OnlyDisco: os.Getenv("TS_DEBUG_USE_DISCO") == "only",
+		Disco:     os.Getenv("TS_DEBUG_USE_DISCO") == "only" || envBool("TS_DEBUG_USE_DISCO"),
+	}
+}
+
+func envBool(k string) bool {
+	e := os.Getenv(k)
+	if e == "" {
+		return false
+	}
+	v, err := strconv.ParseBool(os.Getenv("TS_DEBUG_NETMAP"))
+	if err != nil {
+		panic(fmt.Sprintf("invalid non-bool %q for env var %q", e, k))
+	}
+	return v
+}