policy: more accurate node change

This commit changes so that node changes to the policy is
calculated if any of the nodes has changed in a way that might
affect the policy.

Previously we just checked if the number of nodes had changed,
which meant that if a node was added and removed, we would be
in a bad state.

Signed-off-by: Kristoffer Dalby <kristoffer@dalby.cc>

hscontrol/types/node.go

@@ -973,6 +973,23 @@ func (nv NodeView) HasNetworkChanges(other NodeView) bool {
 	return false
 }
 
+// HasPolicyChange reports whether the node has changes that affect policy evaluation.
+func (nv NodeView) HasPolicyChange(other NodeView) bool {
+	if nv.UserID() != other.UserID() {
+		return true
+	}
+
+	if !views.SliceEqual(nv.Tags(), other.Tags()) {
+		return true
+	}
+
+	if !slices.Equal(nv.IPs(), other.IPs()) {
+		return true
+	}
+
+	return false
+}
+
 // TailNodes converts a slice of NodeViews into Tailscale tailcfg.Nodes.
 func TailNodes(
 	nodes views.Slice[NodeView],