net/traffic,ipn/ipnlocal: extract traffic steering utilities (#19682)

The traffic package contains helpers for evaluating traffic steering
scores and picking appropriate nodes. These were extracted from
ipnlocal.suggestExitNodeUsingTrafficSteering so they can be reused by
the new routecheck package to probe exit nodes in priority order.

Updates #17366
Updates tailscale/corp#33033

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

ipn/ipnlocal/local.go

@@ -9,8 +9,6 @@ import (
 	"bufio"
 	"cmp"
 	"context"
-	"crypto/sha256"
-	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -64,6 +62,7 @@ import (
 	"tailscale.com/net/netns"
 	"tailscale.com/net/netutil"
 	"tailscale.com/net/packet"
+	"tailscale.com/net/traffic"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/paths"
@@ -8133,42 +8132,18 @@ func suggestExitNodeUsingTrafficSteering(nb *nodeBackend, allowed set.Set[tailcf
 		return true
 	})
 
-	scores := make(map[tailcfg.NodeID]int, len(nodes))
-	score := func(n tailcfg.NodeView) int {
-		id := n.ID()
-		s, ok := scores[id]
-		if !ok {
-			s = 0 // score of zero means incomparable
-			if hi := n.Hostinfo(); hi.Valid() {
-				if loc := hi.Location(); loc.Valid() {
-					s = loc.Priority()
-				}
-			}
-			scores[id] = s
-		}
-		return s
-	}
-	rdvHash := makeRendezvousHasher(self.ID())
+	ss := traffic.ScoresFor(self.ID(), nodes)
 
 	var pick tailcfg.NodeView
 	if len(nodes) == 1 {
 		pick = nodes[0]
 	}
 	if len(nodes) > 1 {
-		// Find the highest scoring exit nodes.
-		slices.SortFunc(nodes, func(a, b tailcfg.NodeView) int {
-			c := cmp.Compare(score(b), score(a)) // Highest score first.
-			if c == 0 {
-				// Rendezvous hashing for reliably picking the
-				// same node from a list: tailscale/tailscale#16551.
-				return cmp.Compare(rdvHash(b.ID()), rdvHash(a.ID()))
-			}
-			return c
-		})
+		ss.SortNodes(nodes)
 
 		// TODO(sfllaw): add a temperature knob so that this client has
 		// a chance of picking the next best option.
-		pick = nodes[0]
+		pick = nodes[0] // Pick the highest score.
 	}
 
 	nb.logf("netmap: traffic steering: exit node scores: %v", logger.ArgWriter(func(bw *bufio.Writer) {
@@ -8182,7 +8157,7 @@ func suggestExitNodeUsingTrafficSteering(nb *nodeBackend, allowed set.Set[tailcf
 				bw.WriteString(", ")
 			}
 			name, _, _ := strings.Cut(n.Name(), ".")
-			fmt.Fprintf(bw, "%d:%s", score(n), name)
+			fmt.Fprintf(bw, "%d:%s", ss.Score(n), name)
 		}
 	}))
 
@@ -8284,19 +8259,6 @@ func longLatDistance(fromLat, fromLong, toLat, toLong float64) float64 {
 	return earthRadiusMeters * c
 }
 
-// makeRendezvousHasher returns a function that hashes a node ID to a uint64.
-// https://en.wikipedia.org/wiki/Rendezvous_hashing
-func makeRendezvousHasher(seed tailcfg.NodeID) func(tailcfg.NodeID) uint64 {
-	en := binary.BigEndian
-	return func(n tailcfg.NodeID) uint64 {
-		var b [16]byte
-		en.PutUint64(b[:], uint64(seed))
-		en.PutUint64(b[8:], uint64(n))
-		v := sha256.Sum256(b[:])
-		return en.Uint64(v[:])
-	}
-}
-
 const (
 	// unresolvedExitNodeID is a special [tailcfg.StableNodeID] value
 	// used as an exit node ID to install a blackhole route, preventing