wgengine: lazily create peer wireguard configs more explicitly

Rather than consider bigs jumps in last-received-from activity as a
signal to possibly reconfigure the set of wireguard peers to have
configured, instead just track the set of peers that are currently
excluded from the configuration. Easier to reason about.

Also adds a bit more logging.

This might fix an error we saw on a machine running a recent unstable
build:

2020-08-26 17:54:11.528033751 +0000 UTC: 8.6M/92.6M magicsock: [unexpected] lazy endpoint not created for [UcppE], d:42a770f678357249
2020-08-26 17:54:13.691305296 +0000 UTC: 8.7M/92.6M magicsock: DERP packet received from idle peer [UcppE]; created=false
2020-08-26 17:54:13.691383687 +0000 UTC: 8.7M/92.6M magicsock: DERP packet from unknown key: [UcppE]

If it does happen again, though, we'll have more logs.

wgengine/magicsock/magicsock.go

@@ -1765,6 +1765,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netaddr.IPPort) bool {
 		needsRecvActivityCall = de.isFirstRecvActivityInAwhile()
 	}
 	if needsRecvActivityCall && c.noteRecvActivity != nil {
+		c.logf("magicsock: got disco message from idle peer, starting lazy conf for %v, %v", peerNode.Key.ShortString(), sender.ShortString())
 		// We can't hold Conn.mu while calling noteRecvActivity.
 		// noteRecvActivity acquires userspaceEngine.wgLock (and per our
 		// lock ordering rules: wgLock must come first), and also calls

wgengine/userspace.go

@@ -105,6 +105,7 @@ type userspaceEngine struct {
 	lastEngineSigFull   string // of full wireguard config
 	lastEngineSigTrim   string // of trimmed wireguard config
 	recvActivityAt      map[tailcfg.DiscoKey]time.Time
+	trimmedDisco        map[tailcfg.DiscoKey]bool // set of disco keys of peers currently excluded from wireguard config
 	sentActivityAt      map[packet.IP]*int64      // value is atomic int64 of unixtime
 	destIPActivityFuncs map[packet.IP]func()
 
@@ -636,9 +637,11 @@ func (e *userspaceEngine) noteReceiveActivity(dk tailcfg.DiscoKey) {
 	e.wgLock.Lock()
 	defer e.wgLock.Unlock()
 
-	was, ok := e.recvActivityAt[dk]
+	if _, ok := e.recvActivityAt[dk]; !ok {
-	if !ok {
 		// Not a trimmable peer we care about tracking. (See isTrimmablePeer)
+		if e.trimmedDisco[dk] {
+			e.logf("wgengine: [unexpected] noteReceiveActivity called on idle discokey %v that's not in recvActivityAt", dk.ShortString())
+		}
 		return
 	}
 	now := e.timeNow()
@@ -650,7 +653,8 @@ func (e *userspaceEngine) noteReceiveActivity(dk tailcfg.DiscoKey) {
 	// lazyPeerIdleThreshold without the divide by 2, but
 	// maybeReconfigWireguardLocked is cheap enough to call every
 	// couple minutes (just not on every packet).
-	if was.IsZero() || now.Sub(was) > lazyPeerIdleThreshold/2 {
+	if e.trimmedDisco[dk] {
+		e.logf("wgengine: idle peer %v now active, reconfiguring wireguard", dk.ShortString())
 		e.maybeReconfigWireguardLocked()
 	}
 }
@@ -718,6 +722,8 @@ func (e *userspaceEngine) maybeReconfigWireguardLocked() error {
 	trackDisco := make([]tailcfg.DiscoKey, 0, len(full.Peers))
 	trackIPs := make([]wgcfg.IP, 0, len(full.Peers))
 
+	trimmedDisco := map[tailcfg.DiscoKey]bool{} // TODO: don't re-alloc this map each time
+
 	for i := range full.Peers {
 		p := &full.Peers[i]
 		if !isTrimmablePeer(p, len(full.Peers)) {
@@ -730,6 +736,8 @@ func (e *userspaceEngine) maybeReconfigWireguardLocked() error {
 		trackIPs = append(trackIPs, tsIP)
 		if e.isActiveSince(dk, tsIP, activeCutoff) {
 			min.Peers = append(min.Peers, *p)
+		} else {
+			trimmedDisco[dk] = true
 		}
 	}
 
@@ -738,6 +746,8 @@ func (e *userspaceEngine) maybeReconfigWireguardLocked() error {
 		return nil
 	}
 
+	e.trimmedDisco = trimmedDisco
+
 	e.updateActivityMapsLocked(trackDisco, trackIPs)
 
 	e.logf("wgengine: Reconfig: configuring userspace wireguard config (with %d/%d peers)", len(min.Peers), len(full.Peers))

wgengine/userspace_test.go

@@ -17,7 +17,6 @@ import (
 
 func TestNoteReceiveActivity(t *testing.T) {
 	now := time.Unix(1, 0)
-	tick := func(d time.Duration) { now = now.Add(d) }
 	var logBuf bytes.Buffer
 
 	confc := make(chan bool, 1)
@@ -37,6 +36,7 @@ func TestNoteReceiveActivity(t *testing.T) {
 		},
 		tundev:                new(tstun.TUN),
 		testMaybeReconfigHook: func() { confc <- true },
+		trimmedDisco:          map[tailcfg.DiscoKey]bool{},
 	}
 	ra := e.recvActivityAt
 
@@ -51,7 +51,7 @@ func TestNoteReceiveActivity(t *testing.T) {
 		t.Fatalf("unexpected log write (and thus activity): %s", logBuf.Bytes())
 	}
 
-	// Now track it and expect updates.
+	// Now track it, but don't mark it trimmed, so shouldn't update.
 	ra[dk] = time.Time{}
 	e.noteReceiveActivity(dk)
 	if len(ra) != 1 {
@@ -60,29 +60,20 @@ func TestNoteReceiveActivity(t *testing.T) {
 	if got := ra[dk]; got != now {
 		t.Fatalf("time in map = %v; want %v", got, now)
 	}
-	if !gotConf() {
+	if gotConf() {
-		t.Fatalf("didn't get expected reconfig")
+		t.Fatalf("unexpected reconfig")
 	}
 
-	// With updates 1 second apart, don't expect a reconfig.
+	// Now mark it trimmed and expect an update.
-	for i := 0; i < 300; i++ {
+	e.trimmedDisco[dk] = true
-		tick(time.Second)
 	e.noteReceiveActivity(dk)
 	if len(ra) != 1 {
-			t.Fatalf("map len = %d; want 1", len(ra))
+		t.Fatalf("unexpected growth in map: now has %d keys; want 1", len(ra))
 	}
 	if got := ra[dk]; got != now {
 		t.Fatalf("time in map = %v; want %v", got, now)
 	}
-		if gotConf() {
-			t.Fatalf("unexpected reconfig")
-		}
-	}
-
-	// But if there's a big jump it should get an update.
-	tick(3 * time.Minute)
-	e.noteReceiveActivity(dk)
 	if !gotConf() {
-		t.Fatalf("expected config")
+		t.Fatalf("didn't get expected reconfig")
 	}
 }