X xbtree: Verify that ForgetPast clones vδT on trim

Else the memory usage will grow indefinitely.

wcfs/internal/xbtree/δbtail_test.go

@@ -1087,15 +1087,32 @@ func TestΔBtailForget(t_ *testing.T) {
 
 	_, err  = δbtail.Update(t3.ΔZ);  X(err)
 
+	// forget calls ForgetPast(revCut) and returns state of vδT right after that.
+	forget := func(revCut zodb.Tid) []ΔTree {
+		δbtail.ForgetPast(revCut)
+		return δbtail.byRoot[t.Root()].vδT
+	}
+
+	vδT_init := δbtail.byRoot[t.Root()].vδT
 	assertΔTtail(t.T, "init",         δbtail, t3, t.Root(), t1.Δxkv, t2.Δxkv, t3.Δxkv)
-	δbtail.ForgetPast(t0.At)
+	vδT_at0 := forget(t0.At)
 	assertΔTtail(t.T, "forget ≤ at0", δbtail, t3, t.Root(), t1.Δxkv, t2.Δxkv, t3.Δxkv)
-	δbtail.ForgetPast(t1.At)
+	vδT_at1 := forget(t1.At)
 	assertΔTtail(t.T, "forget ≤ at1", δbtail, t3, t.Root(),          t2.Δxkv, t3.Δxkv)
-	δbtail.ForgetPast(t3.At)
+	_ = forget(t3.At)
 	assertΔTtail(t.T, "forget ≤ at3", δbtail, t3, t.Root(),                          )
 
-	// XXX verify no aliasing
+	// verify aliasing: init/at0 should be aliased, because there is no change @at0
+	// at1 must be unaliased from at0; at3 must be unualiased from at1.
+
+	vδT_at0[2].Rev = 0
+	if vδT_init[2].Rev != 0 {
+		t.Errorf("ForgetPast(at0) cloned vδT but should not")
+	}
+	if vδT_at1[1].Rev == 0 {
+		t.Errorf("ForgetPast(at1) remained vδT aliased but should not")
+	}
+
 }
 
 func TestΔBtailGetAt(t_ *testing.T) {