state: make any-user machine-key lookup deterministic

The lookup returned the first map match, so re-auth branch choice varied
with map order once a machine key had more than one node. Prefer the tagged
node, else the lowest node ID.

Updates #3312

hscontrol/state/node_store.go

@@ -804,12 +804,15 @@ func (s *NodeStore) GetNodeByMachineKey(machineKey key.MachinePublic, userID typ
 	return types.NodeView{}, false
 }
 
-// GetNodeByMachineKeyAnyUser returns the first node with the given machine key,
+// GetNodeByMachineKeyAnyUser returns a node with the given machine key,
 // regardless of which user it belongs to. This is useful for scenarios like
 // transferring a node to a different user when re-authenticating with a
 // different user's auth key.
-// If multiple nodes exist with the same machine key (different users), the
-// first one found is returned (order is not guaranteed).
+//
+// When more than one node shares the machine key (e.g. a tagged node and a
+// user-owned node), the choice is deterministic: the tagged node is preferred,
+// otherwise the node with the lowest ID. A stable pick keeps re-auth branch
+// selection (convert vs create) from depending on map iteration order.
 func (s *NodeStore) GetNodeByMachineKeyAnyUser(machineKey key.MachinePublic) (types.NodeView, bool) {
 	timer := prometheus.NewTimer(nodeStoreOperationDuration.WithLabelValues("get_by_machine_key_any_user"))
 	defer timer.ObserveDuration()
@@ -817,14 +820,20 @@ func (s *NodeStore) GetNodeByMachineKeyAnyUser(machineKey key.MachinePublic) (ty
 	nodeStoreOperations.WithLabelValues("get_by_machine_key_any_user").Inc()
 
 	snapshot := s.data.Load()
+
+	var best types.NodeView
+
 	if userMap, exists := snapshot.nodesByMachineKey[machineKey]; exists {
-		// Return the first node found (order not guaranteed due to map iteration)
 		for _, node := range userMap {
-			return node, true
+			if !best.Valid() ||
+				(node.IsTagged() && !best.IsTagged()) ||
+				(node.IsTagged() == best.IsTagged() && node.ID() < best.ID()) {
+				best = node
+			}
 		}
 	}
 
-	return types.NodeView{}, false
+	return best, best.Valid()
 }
 
 // DebugString returns debug information about the [NodeStore].

hscontrol/state/node_store_test.go

@@ -1321,3 +1321,59 @@ func TestRebuildPeerMapsWithChangedPeersFunc(t *testing.T) {
 	assert.Equal(t, 1, peers1.Len(), "ListPeers for node1 should return 1")
 	assert.Equal(t, 1, peers2.Len(), "ListPeers for node2 should return 1")
 }
+
+// TestGetNodeByMachineKeyAnyUserDeterministic ensures the any-user machine-key
+// lookup returns a stable, well-defined node when more than one node shares a
+// machine key: the tagged node if present, otherwise the lowest node ID. A
+// nondeterministic pick makes re-auth branch choice (convert vs create) depend
+// on map iteration order.
+func TestGetNodeByMachineKeyAnyUserDeterministic(t *testing.T) {
+	mk := key.NewMachine().Public()
+
+	assertStablePick := func(t *testing.T, store *NodeStore, want types.NodeID) {
+		t.Helper()
+
+		for range 50 {
+			got, ok := store.GetNodeByMachineKeyAnyUser(mk)
+			require.True(t, ok)
+			require.Equal(t, want, got.ID(), "pick must be stable and well-defined")
+		}
+	}
+
+	t.Run("prefers lowest node ID", func(t *testing.T) {
+		store := NewNodeStore(nil, allowAllPeersFunc, TestBatchSize, TestBatchTimeout)
+
+		store.Start()
+		defer store.Stop()
+
+		n2 := createTestNode(2, 2, "user2", "node2")
+		n2.MachineKey = mk
+		n1 := createTestNode(1, 1, "user1", "node1")
+		n1.MachineKey = mk
+
+		store.PutNode(n2)
+		store.PutNode(n1)
+
+		assertStablePick(t, store, 1)
+	})
+
+	t.Run("prefers tagged node", func(t *testing.T) {
+		store := NewNodeStore(nil, allowAllPeersFunc, TestBatchSize, TestBatchTimeout)
+
+		store.Start()
+		defer store.Stop()
+
+		owned := createTestNode(1, 1, "user1", "node1")
+		owned.MachineKey = mk
+		tagged := createTestNode(3, 3, "user3", "node3")
+		tagged.MachineKey = mk
+		tagged.UserID = nil
+		tagged.User = nil
+		tagged.Tags = []string{"tag:foo"}
+
+		store.PutNode(owned)
+		store.PutNode(tagged)
+
+		assertStablePick(t, store, 3)
+	})
+}