netcheck: use best DERP server in past 5 minutes to avoid flip-flopping

Fixes #162

netcheck/netcheck.go

@@ -65,10 +65,14 @@ type Client struct {
 	// Logf optionally specifies where to log to.
 	Logf logger.Logf
 
+	// TimeNow, if non-nil, is used instead of time.Now.
+	TimeNow func() time.Time
+
 	GetSTUNConn4 func() STUNConn
 	GetSTUNConn6 func() STUNConn
 
-	mu          sync.Mutex // guards following
+	mu          sync.Mutex            // guards following
+	prev        map[time.Time]*Report // some previous reports
 	s4          *stunner.Stunner
 	s6          *stunner.Stunner
 	hairTX      stun.TxID
@@ -218,9 +222,8 @@ func (c *Client) GetReport(ctx context.Context) (*Report, error) {
 		ret = &Report{
 			DERPLatency: map[string]time.Duration{},
 		}
-		gotEP           = map[string]string{} // server -> ipPort
-		gotEP4          string
-		bestDerpLatency time.Duration
+		gotEP  = map[string]string{} // server -> ipPort
+		gotEP4 string
 	)
 	anyV6 := func() bool {
 		mu.Lock()
@@ -266,11 +269,6 @@ func (c *Client) GetReport(ctx context.Context) (*Report, error) {
 			}
 		}
 		gotEP[server] = ipPort
-
-		if ret.PreferredDERP == 0 || d < bestDerpLatency {
-			bestDerpLatency = d
-			ret.PreferredDERP = c.DERP.NodeIDOfSTUNServer(server)
-		}
 	}
 
 	var pc4, pc6 STUNConn
@@ -409,5 +407,57 @@ func (c *Client) GetReport(ctx context.Context) (*Report, error) {
 	// the other side of the planet? Or try ICMP? (likely also
 	// blocked?)
 
-	return ret.Clone(), nil
+	report := ret.Clone()
+
+	c.addReportHistoryAndSetPreferredDERP(report)
+
+	return report, nil
+}
+
+func (c *Client) timeNow() time.Time {
+	if c.TimeNow != nil {
+		return c.TimeNow()
+	}
+	return time.Now()
+}
+
+// addReportHistoryAndSetPreferredDERP adds r to the set of recent Reports
+// and mutates r.PreferredDERP to contain the best recent one.
+func (c *Client) addReportHistoryAndSetPreferredDERP(r *Report) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if c.prev == nil {
+		c.prev = map[time.Time]*Report{}
+	}
+	now := c.timeNow()
+	c.prev[now] = r
+
+	const maxAge = 5 * time.Minute
+
+	// STUN host:port => its best recent latency in last maxAge
+	bestRecent := map[string]time.Duration{}
+
+	for t, pr := range c.prev {
+		if now.Sub(t) > maxAge {
+			delete(c.prev, t)
+			continue
+		}
+		for hp, d := range pr.DERPLatency {
+			if bd, ok := bestRecent[hp]; !ok || d < bd {
+				bestRecent[hp] = d
+			}
+		}
+	}
+
+	// Then, pick which currently-alive DERP server from the
+	// current report has the best latency over the past maxAge.
+	var bestAny time.Duration
+	for hp := range r.DERPLatency {
+		best := bestRecent[hp]
+		if r.PreferredDERP == 0 || best < bestAny {
+			bestAny = best
+			r.PreferredDERP = c.DERP.NodeIDOfSTUNServer(hp)
+		}
+	}
 }

netcheck/netcheck_test.go

@@ -6,6 +6,7 @@ package netcheck
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"reflect"
 	"strings"
@@ -98,3 +99,117 @@ func TestWorksWhenUDPBlocked(t *testing.T) {
 		t.Errorf("mismatch\n got: %+v\nwant: %+v\n", r, want)
 	}
 }
+
+func TestAddReportHistoryAndSetPreferredDERP(t *testing.T) {
+	derps := derpmap.NewTestWorldWith(
+		&derpmap.Server{
+			ID:    1,
+			STUN4: "d1",
+		},
+		&derpmap.Server{
+			ID:    2,
+			STUN4: "d2",
+		},
+		&derpmap.Server{
+			ID:    3,
+			STUN4: "d3",
+		},
+	)
+	// report returns a *Report from (DERP host, time.Duration)+ pairs.
+	report := func(a ...interface{}) *Report {
+		r := &Report{DERPLatency: map[string]time.Duration{}}
+		for i := 0; i < len(a); i += 2 {
+			k := a[i].(string)
+			switch v := a[i+1].(type) {
+			case time.Duration:
+				r.DERPLatency[k] = v
+			case int:
+				r.DERPLatency[k] = time.Second * time.Duration(v)
+			default:
+				panic(fmt.Sprintf("unexpected type %T", v))
+			}
+		}
+		return r
+	}
+	type step struct {
+		after time.Duration
+		r     *Report
+	}
+	tests := []struct {
+		name        string
+		steps       []step
+		wantDERP    int // want PreferredDERP on final step
+		wantPrevLen int // wanted len(c.prev)
+	}{
+		{
+			name: "first_reading",
+			steps: []step{
+				{0, report("d1", 2, "d2", 3)},
+			},
+			wantPrevLen: 1,
+			wantDERP:    1,
+		},
+		{
+			name: "with_two",
+			steps: []step{
+				{0, report("d1", 2, "d2", 3)},
+				{1 * time.Second, report("d1", 4, "d2", 3)},
+			},
+			wantPrevLen: 2,
+			wantDERP:    1, // t0's d1 of 2 is still best
+		},
+		{
+			name: "but_now_d1_gone",
+			steps: []step{
+				{0, report("d1", 2, "d2", 3)},
+				{1 * time.Second, report("d1", 4, "d2", 3)},
+				{2 * time.Second, report("d2", 3)},
+			},
+			wantPrevLen: 3,
+			wantDERP:    2, // only option
+		},
+		{
+			name: "d1_is_back",
+			steps: []step{
+				{0, report("d1", 2, "d2", 3)},
+				{1 * time.Second, report("d1", 4, "d2", 3)},
+				{2 * time.Second, report("d2", 3)},
+				{3 * time.Second, report("d1", 4, "d2", 3)}, // same as 2 seconds ago
+			},
+			wantPrevLen: 4,
+			wantDERP:    1, // t0's d1 of 2 is still best
+		},
+		{
+			name: "things_clean_up",
+			steps: []step{
+				{0, report("d1", 1, "d2", 2)},
+				{1 * time.Second, report("d1", 1, "d2", 2)},
+				{2 * time.Second, report("d1", 1, "d2", 2)},
+				{3 * time.Second, report("d1", 1, "d2", 2)},
+				{10 * time.Minute, report("d3", 3)},
+			},
+			wantPrevLen: 1, // t=[0123]s all gone. (too old, older than 10 min)
+			wantDERP:    3, // only option
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fakeTime := time.Unix(123, 0)
+			c := &Client{
+				DERP:    derps,
+				TimeNow: func() time.Time { return fakeTime },
+			}
+			for _, s := range tt.steps {
+				fakeTime = fakeTime.Add(s.after)
+				c.addReportHistoryAndSetPreferredDERP(s.r)
+			}
+			lastReport := tt.steps[len(tt.steps)-1].r
+			if got, want := len(c.prev), tt.wantPrevLen; got != want {
+				t.Errorf("len(prev) = %v; want %v", got, want)
+			}
+			if got, want := lastReport.PreferredDERP, tt.wantDERP; got != want {
+				t.Errorf("PreferredDERP = %v; want %v", got, want)
+			}
+		})
+	}
+}