ipn/ipnlocal: parse priority out of suggest-exit-node capability

When the `experimental:exit-node-steering` attribute is enabled for a
node, it is likely that the priority score for an exit node is
different when considering it as a suggested exit node versus
considering it for the routers / connectors that it hosts.

In order to distinguish between these two, the network map can now
contain the priority score for suggesting the exit node in the
`suggest-exit-node` capability, in that peer’s CapMap:

	"CapMap": {
		"suggest-exit-node": [
			{
				"Priority": 42
			}
		]
	}

Updates tailscale/corp#31011

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

ipn/ipnlocal/local.go

@@ -7952,12 +7952,32 @@ func suggestExitNodeUsingTrafficSteering(nb *nodeBackend, allowed set.Set[tailcf
 		id := n.ID()
 		s, ok := scores[id]
 		if !ok {
-			s = 0 // score of zero means incomparable
+			// score of zero means incomparable
+			s = 0
+
+			// Prefer the priority in the suggest-exit-node peer cap.
+			if caps, ok := n.CapMap().GetOk(tailcfg.NodeAttrSuggestExitNode); ok {
+				for _, cap := range caps.All() {
+					var c tailcfg.SuggestExitNode
+					if err := json.Unmarshal([]byte(cap), &c); err != nil {
+						break
+					}
+					if c.Priority == 0 {
+						break
+					}
+					s = c.Priority
+					goto SetScore
+				}
+			}
+
+			// Fallback on the peer’s location priority.
 			if hi := n.Hostinfo(); hi.Valid() {
 				if loc := hi.Location(); loc.Valid() {
 					s = loc.Priority()
 				}
 			}
+
+		SetScore:
 			scores[id] = s
 		}
 		return s

ipn/ipnlocal/local_test.go

@@ -5076,6 +5076,76 @@ func TestSuggestExitNodeTrafficSteering(t *testing.T) {
 			wantID:   "stable3",
 			wantName: "peer3",
 		},
+		{
+			name: "exit-node-empty-suggestions",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(nil)),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest([]tailcfg.RawMessage{})),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest([]tailcfg.RawMessage{
+							`{}`,
+						})),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest([]tailcfg.RawMessage{
+							`{"Priority": 0}`,
+						})),
+				},
+			},
+			// Change this, if the hashing function changes.
+			wantID:   "stable3",
+			wantName: "peer3",
+		},
+		{
+			name: "exit-node-with-suggestion-and-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest([]tailcfg.RawMessage{
+							`{"Priority": 3}`,
+						}),
+						withLocationPriority(1)), // overridden
+					makePeer(2,
+						withExitRoutes(),
+						withLocationPriority(2)),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantPri:  1, // Location.Priority
+		},
+		{
+			name: "exit-node-with-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(nil)),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest(nil)),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest(nil)),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest(nil)),
+				},
+			},
+			// Change this, if the hashing function changes.
+			wantID:   "stable3",
+			wantName: "peer3",
+		},
 		{
 			name: "exit-nodes-with-and-without-priority",
 			netMap: &netmap.NetworkMap{

tailcfg/traffic.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tailcfg
+
+type SuggestExitNode struct {
+	// Priority is the relative priority of this exit node. Nodes with a
+	// higher priority are preferred over nodes with a lower priority, nodes
+	// of equal probability may be selected arbitrarily. A priority of 0
+	// means the exit node has no a priority preference and a negative
+	// priority is not allowed.
+	Priority int `json:",omitempty"`
+}