bart: do not alias memory in Union

Union was inadvertently reusing a `*node` from the
other table which meant that when either table was
later mutated the change would appear in both tables.

Add a test and fix the condition.

Updates #16

node.go

@@ -464,7 +464,7 @@ func (n *node[V]) unionRec(o *node[V]) {
 		nNode := n.children.get(oAddr)
 		if nNode == nil {
 			// union child from oNode into nNode
-			n.children.insert(oAddr, oNode)
+			n.children.insert(oAddr, oNode.cloneRec())
 		} else {
 			// both nodes have child with addr, call union rec-descent
 			nNode.unionRec(oNode)

node_test.go

@@ -13,6 +13,7 @@ import (
 	"bytes"
 	"fmt"
 	"math/rand"
+	"net/netip"
 	"runtime"
 	"sort"
 	"testing"
@@ -405,3 +406,38 @@ func getsEqual[V comparable](a V, aOK bool, b V, bOK bool) bool {
 	}
 	return a == b
 }
+
+// TestUnionMemoryAliasing tests that the Union method does not alias memory
+// between the two tables.
+func TestUnionMemoryAliasing(t *testing.T) {
+	newTable := func(pfx ...string) *Table[struct{}] {
+		var t Table[struct{}]
+		for _, s := range pfx {
+			t.Insert(netip.MustParsePrefix(s), struct{}{})
+		}
+		return &t
+	}
+	// First create two tables with disjoint prefixes.
+	stable := newTable("0.0.0.0/24")
+	temp := newTable("100.69.1.0/24")
+
+	// Verify that the tables are disjoint.
+	if stable.Overlaps(temp) {
+		t.Error("stable should not overlap temp")
+	}
+
+	// Now union them.
+	temp.Union(stable)
+
+	// Add a new prefix to temp.
+	temp.Insert(netip.MustParsePrefix("0.0.1.0/24"), struct{}{})
+
+	// Ensure that stable is unchanged.
+	_, ok := stable.Get(netip.MustParseAddr("0.0.1.1"))
+	if ok {
+		t.Error("stable should not contain 0.0.1.1")
+	}
+	if stable.OverlapsPrefix(netip.MustParsePrefix("0.0.1.1/32")) {
+		t.Error("stable should not overlap 0.0.1.1/32")
+	}
+}