ipn/ipnlocal: plumb nodeBackend into suggestExitNode to support delta updates, such as online status changes

Now that (*LocalBackend).suggestExitNodeLocked is never called with a non-current netmap
(the netMap parameter is always nil, indicating that the current netmap should be used),
we can remove the unused parameter.

Additionally, instead of suggestExitNodeLocked passing the most recent full netmap to suggestExitNode,
we now pass the current nodeBackend so it can access peers with delta updates applied.

Finally, with that fixed, we no longer need to skip TestUpdateNetmapDeltaAutoExitNode.

Updates tailscale/corp#29969
Fixes #16455

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -57,6 +57,7 @@ import (
 	"tailscale.com/types/ptr"
 	"tailscale.com/types/views"
 	"tailscale.com/util/dnsname"
+	"tailscale.com/util/eventbus"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
 	"tailscale.com/util/set"
@@ -2327,8 +2328,6 @@ func TestSetExitNodeIDPolicy(t *testing.T) {
 }
 
 func TestUpdateNetmapDeltaAutoExitNode(t *testing.T) {
-	t.Skip("TODO(tailscale/tailscale#16455): suggestExitNode does not check for online status of exit nodes")
-
 	peer1 := makePeer(1, withCap(26), withSuggest(), withOnline(true), withExitRoutes())
 	peer2 := makePeer(2, withCap(26), withSuggest(), withOnline(true), withExitRoutes())
 	derpMap := &tailcfg.DERPMap{
@@ -4278,7 +4277,11 @@ func TestSuggestExitNode(t *testing.T) {
 				allowList = set.SetOf(tt.allowPolicy)
 			}
 
-			got, err := suggestExitNode(tt.lastReport, tt.netMap, tt.lastSuggestion, selectRegion, selectNode, allowList)
+			nb := newNodeBackend(t.Context(), eventbus.New())
+			defer nb.shutdown(errShutdown)
+			nb.SetNetMap(tt.netMap)
+
+			got, err := suggestExitNode(tt.lastReport, nb, tt.lastSuggestion, selectRegion, selectNode, allowList)
 			if got.Name != tt.wantName {
 				t.Errorf("name=%v, want %v", got.Name, tt.wantName)
 			}