ipn/ipnlocal: test traffic-steering when feature is not enabled (#17997)

In PR tailscale/corp#34401, the `traffic-steering` feature flag does
not automatically enable traffic steering for all nodes. Instead, an
admin must add the `traffic-steering` node attribute to each client
node that they want opted-in.

For backwards compatibility with older clients, tailscale/corp#34401
strips out the `traffic-steering` node attribute if the feature flag
is not enabled, even if it is set in the policy file. This lets us
safely disable the feature flag.

This PR adds a missing test case for suggested exit nodes that have no
priority.

Updates tailscale/corp#34399

Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/ipnlocal/local.go

@@ -7301,6 +7301,9 @@ func suggestExitNode(report *netcheck.Report, nb *nodeBackend, prevSuggestion ta
 		// The traffic-steering feature flag is enabled on this tailnet.
 		return suggestExitNodeUsingTrafficSteering(nb, allowList)
 	default:
+		// The control plane will always strip the `traffic-steering`
+		// node attribute if it isn’t enabled for this tailnet, even if
+		// it is set in the policy file: tailscale/corp#34401
 		return suggestExitNodeUsingDERP(report, nb, prevSuggestion, selectRegion, selectNode, allowList)
 	}
 }

ipn/ipnlocal/local_test.go

@@ -5173,6 +5173,26 @@ func TestSuggestExitNodeTrafficSteering(t *testing.T) {
 			wantID:   "stable3",
 			wantName: "peer3",
 		},
+		{
+			name: "exit-nodes-without-priority-for-suggestions",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(3,
+						withExitRoutes(),
+						withLocationPriority(1)),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantPri:  0,
+		},
 		{
 			name: "exit-nodes-with-and-without-priority",
 			netMap: &netmap.NetworkMap{