ipn/ipnlocal: move where auto exit node selection happens

In the process, because I needed it for testing, make all
LocalBackend-managed goroutines be accounted for. And then in tests,
verify they're no longer running during LocalBackend.Shutdown.

Updates tailscale/corp#19681

Change-Id: Iad873d4df7d30103a4a7863dfacf9e078c77e6a3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -367,7 +367,7 @@ type LocalBackend struct {
 	allowedSuggestedExitNodes   set.Set[tailcfg.StableNodeID]
 
 	// refreshAutoExitNode indicates if the exit node should be recomputed when the next netcheck report is available.
-	refreshAutoExitNode bool
+	refreshAutoExitNode bool // guarded by mu
 
 	// captiveCtx and captiveCancel are used to control captive portal
 	// detection. They are protected by 'mu' and can be changed during the
@@ -1812,8 +1812,9 @@ func (b *LocalBackend) UpdateNetmapDelta(muts []netmap.NodeMutation) (handled bo
 			b.send(*notify)
 		}
 	}()
-	unlock := b.lockAndGetUnlock()
+	b.mu.Lock()
-	defer unlock()
+	defer b.mu.Unlock()
+
 	if !b.updateNetmapDeltaLocked(muts) {
 		return false
 	}
@@ -1821,14 +1822,8 @@ func (b *LocalBackend) UpdateNetmapDelta(muts []netmap.NodeMutation) (handled bo
 	if b.netMap != nil && mutationsAreWorthyOfTellingIPNBus(muts) {
 		nm := ptr.To(*b.netMap) // shallow clone
 		nm.Peers = make([]tailcfg.NodeView, 0, len(b.peers))
-		shouldAutoExitNode := shouldAutoExitNode()
 		for _, p := range b.peers {
 			nm.Peers = append(nm.Peers, p)
-			// If the auto exit node currently set goes offline, find another auto exit node.
-			if shouldAutoExitNode && b.pm.prefs.ExitNodeID() == p.StableID() && p.Online() != nil && !*p.Online() {
-				b.setAutoExitNodeIDLockedOnEntry(unlock)
-				return false
-			}
 		}
 		slices.SortFunc(nm.Peers, func(a, b tailcfg.NodeView) int {
 			return cmp.Compare(a.ID(), b.ID())
@@ -1859,6 +1854,20 @@ func mutationsAreWorthyOfTellingIPNBus(muts []netmap.NodeMutation) bool {
 	return false
 }
 
+// pickNewAutoExitNode picks a new automatic exit node if needed.
+func (b *LocalBackend) pickNewAutoExitNode() {
+	unlock := b.lockAndGetUnlock()
+	defer unlock()
+
+	newPrefs := b.setAutoExitNodeIDLockedOnEntry(unlock)
+	if !newPrefs.Valid() {
+		// Unchanged.
+		return
+	}
+
+	b.send(ipn.Notify{Prefs: &newPrefs})
+}
+
 func (b *LocalBackend) updateNetmapDeltaLocked(muts []netmap.NodeMutation) (handled bool) {
 	if b.netMap == nil || len(b.peers) == 0 {
 		return false
@@ -1881,6 +1890,12 @@ func (b *LocalBackend) updateNetmapDeltaLocked(muts []netmap.NodeMutation) (hand
 			mak.Set(&mutableNodes, nv.ID(), n)
 		}
 		m.Apply(n)
+
+		// If our exit node went offline, we need to schedule picking
+		// a new one.
+		if mo, ok := m.(netmap.NodeMutationOnline); ok && !mo.Online && n.StableID == b.pm.prefs.ExitNodeID() && shouldAutoExitNode() {
+			b.goTracker.Go(b.pickNewAutoExitNode)
+		}
 	}
 	for nid, n := range mutableNodes {
 		b.peers[nid] = n.View()
@@ -5542,29 +5557,34 @@ func (b *LocalBackend) setNetInfo(ni *tailcfg.NetInfo) {
 	}
 }
 
-func (b *LocalBackend) setAutoExitNodeIDLockedOnEntry(unlock unlockOnce) {
+func (b *LocalBackend) setAutoExitNodeIDLockedOnEntry(unlock unlockOnce) (newPrefs ipn.PrefsView) {
+	var zero ipn.PrefsView
 	defer unlock()
 
 	prefs := b.pm.CurrentPrefs()
 	if !prefs.Valid() {
 		b.logf("[unexpected]: received tailnet exit node ID pref change callback but current prefs are nil")
-		return
+		return zero
 	}
 	prefsClone := prefs.AsStruct()
 	newSuggestion, err := b.suggestExitNodeLocked(nil)
 	if err != nil {
 		b.logf("setAutoExitNodeID: %v", err)
-		return
+		return zero
+	}
+	if prefsClone.ExitNodeID == newSuggestion.ID {
+		return zero
 	}
 	prefsClone.ExitNodeID = newSuggestion.ID
-	_, err = b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
+	newPrefs, err = b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
 		Prefs:         *prefsClone,
 		ExitNodeIDSet: true,
 	}, unlock)
 	if err != nil {
 		b.logf("setAutoExitNodeID: failed to apply exit node ID preference: %v", err)
-		return
+		return zero
 	}
+	return newPrefs
 }
 
 // setNetMapLocked updates the LocalBackend state to reflect the newly

ipn/ipnlocal/local_test.go

@@ -1872,11 +1872,11 @@ func TestUpdateNetmapDeltaAutoExitNode(t *testing.T) {
 		netmap                *netmap.NetworkMap
 		muts                  []*tailcfg.PeerChange
 		exitNodeIDWant        tailcfg.StableNodeID
-		updateNetmapDeltaResponse bool
 		report                *netcheck.Report
 	}{
 		{
-			name:                  "selected auto exit node goes offline",
+			// selected auto exit node goes offline
+			name:                  "exit-node-goes-offline",
 			lastSuggestedExitNode: peer1.StableID(),
 			netmap: &netmap.NetworkMap{
 				Peers: []tailcfg.NodeView{
@@ -1896,11 +1896,11 @@ func TestUpdateNetmapDeltaAutoExitNode(t *testing.T) {
 				},
 			},
 			exitNodeIDWant: peer2.StableID(),
-			updateNetmapDeltaResponse: false,
 			report:         report,
 		},
 		{
-			name:                  "other exit node goes offline doesn't change selected auto exit node that's still online",
+			// other exit node goes offline doesn't change selected auto exit node that's still online
+			name:                  "other-node-goes-offline",
 			lastSuggestedExitNode: peer2.StableID(),
 			netmap: &netmap.NetworkMap{
 				Peers: []tailcfg.NodeView{
@@ -1920,7 +1920,6 @@ func TestUpdateNetmapDeltaAutoExitNode(t *testing.T) {
 				},
 			},
 			exitNodeIDWant: peer2.StableID(),
-			updateNetmapDeltaResponse: true,
 			report:         report,
 		},
 	}
@@ -1939,6 +1938,20 @@ func TestUpdateNetmapDeltaAutoExitNode(t *testing.T) {
 			b.lastSuggestedExitNode = tt.lastSuggestedExitNode
 			b.sys.MagicSock.Get().SetLastNetcheckReportForTest(b.ctx, tt.report)
 			b.SetPrefsForTest(b.pm.CurrentPrefs().AsStruct())
+
+			allDone := make(chan bool, 1)
+			defer b.goTracker.AddDoneCallback(func() {
+				b.mu.Lock()
+				defer b.mu.Unlock()
+				if b.goTracker.RunningGoroutines() > 0 {
+					return
+				}
+				select {
+				case allDone <- true:
+				default:
+				}
+			})()
+
 			someTime := time.Unix(123, 0)
 			muts, ok := netmap.MutationsFromMapResponse(&tailcfg.MapResponse{
 				PeersChangedPatch: tt.muts,
@@ -1946,16 +1959,34 @@ func TestUpdateNetmapDeltaAutoExitNode(t *testing.T) {
 			if !ok {
 				t.Fatal("netmap.MutationsFromMapResponse failed")
 			}
+
 			if b.pm.prefs.ExitNodeID() != tt.lastSuggestedExitNode {
 				t.Fatalf("did not set exit node ID to last suggested exit node despite auto policy")
 			}
 
+			was := b.goTracker.StartedGoroutines()
 			got := b.UpdateNetmapDelta(muts)
-			if got != tt.updateNetmapDeltaResponse {
+			if !got {
-				t.Fatalf("got %v expected %v from UpdateNetmapDelta", got, tt.updateNetmapDeltaResponse)
+				t.Error("got false from UpdateNetmapDelta")
 			}
-			if b.pm.prefs.ExitNodeID() != tt.exitNodeIDWant {
+			startedGoroutine := b.goTracker.StartedGoroutines() != was
-				t.Fatalf("did not get expected exit node id after UpdateNetmapDelta")
+
+			wantChange := tt.exitNodeIDWant != tt.lastSuggestedExitNode
+			if startedGoroutine != wantChange {
+				t.Errorf("got startedGoroutine %v, want %v", startedGoroutine, wantChange)
+			}
+			if startedGoroutine {
+				select {
+				case <-time.After(5 * time.Second):
+					t.Fatal("timed out waiting for goroutine to finish")
+				case <-allDone:
+				}
+			}
+			b.mu.Lock()
+			gotExitNode := b.pm.prefs.ExitNodeID()
+			b.mu.Unlock()
+			if gotExitNode != tt.exitNodeIDWant {
+				t.Fatalf("exit node ID after UpdateNetmapDelta = %v; want %v", gotExitNode, tt.exitNodeIDWant)
 			}
 		})
 	}