go/zodb/btree: Verify that BTree children are always all of the same kind

They are all either BTree or all Buckets.

See https://github.com/zopefoundation/ZODB/blob/3.10.7-4-gb8d7a8567/src/BTrees/Development.txt#L231
for details

go/zodb/btree/btree.go.in

@@ -331,6 +331,7 @@ func (bt *btreeState) PySetState(pystate interface{}) (err error) {
 	n := (len(t) + 1) / 2
 	bt.data = make([]_BTreeItem, 0, n)
 	var kprev int64
+	var childrenKind int // 1 - BTree, 2 - Bucket
 	for i, idx := 0, 0; i < n; i++ {
 		key := int64(math.Min<Key>) // KEY(-∞)   (qualifies for ≤)
 		if i > 0 {
@@ -354,12 +355,23 @@ func (bt *btreeState) PySetState(pystate interface{}) (err error) {
 		}
 		kprev = key
 
+		// check all children are of the same type
+		var kind int // see childrenKind ^^^
 		switch child.(type) {
 		default:
 			return fmt.Errorf("data: [%d]: child must be BTree|Bucket; got %T", i, child)
 
-		case *BTree:  // ok
-		case *Bucket: // ok
+		case *BTree:
+			kind = 1
+		case *Bucket:
+			kind = 2
+		}
+
+		if i == 0 {
+			childrenKind = kind
+		}
+		if kind != childrenKind {
+			fmt.Errorf("data: [%d]: children must be of the same type", i)
 		}
 
 		bt.data = append(bt.data, _BTreeItem{key: kkey, child: child})

go/zodb/btree/ziobtree.go

@@ -333,6 +333,7 @@ func (bt *iobtreeState) PySetState(pystate interface{}) (err error) {
 	n := (len(t) + 1) / 2
 	bt.data = make([]_IOBTreeItem, 0, n)
 	var kprev int64
+	var childrenKind int // 1 - IOBTree, 2 - IOBucket
 	for i, idx := 0, 0; i < n; i++ {
 		key := int64(math.MinInt32) // int32(-∞)   (qualifies for ≤)
 		if i > 0 {
@@ -356,12 +357,23 @@ func (bt *iobtreeState) PySetState(pystate interface{}) (err error) {
 		}
 		kprev = key
 
+		// check all children are of the same type
+		var kind int // see childrenKind ^^^
 		switch child.(type) {
 		default:
 			return fmt.Errorf("data: [%d]: child must be IOBTree|IOBucket; got %T", i, child)
 
-		case *IOBTree:  // ok
-		case *IOBucket: // ok
+		case *IOBTree:
+			kind = 1
+		case *IOBucket:
+			kind = 2
+		}
+
+		if i == 0 {
+			childrenKind = kind
+		}
+		if kind != childrenKind {
+			fmt.Errorf("data: [%d]: children must be of the same type", i)
 		}
 
 		bt.data = append(bt.data, _IOBTreeItem{key: kkey, child: child})

go/zodb/btree/zlobtree.go

@@ -333,6 +333,7 @@ func (bt *lobtreeState) PySetState(pystate interface{}) (err error) {
 	n := (len(t) + 1) / 2
 	bt.data = make([]_LOBTreeItem, 0, n)
 	var kprev int64
+	var childrenKind int // 1 - LOBTree, 2 - LOBucket
 	for i, idx := 0, 0; i < n; i++ {
 		key := int64(math.MinInt64) // int64(-∞)   (qualifies for ≤)
 		if i > 0 {
@@ -356,12 +357,23 @@ func (bt *lobtreeState) PySetState(pystate interface{}) (err error) {
 		}
 		kprev = key
 
+		// check all children are of the same type
+		var kind int // see childrenKind ^^^
 		switch child.(type) {
 		default:
 			return fmt.Errorf("data: [%d]: child must be LOBTree|LOBucket; got %T", i, child)
 
-		case *LOBTree:  // ok
-		case *LOBucket: // ok
+		case *LOBTree:
+			kind = 1
+		case *LOBucket:
+			kind = 2
+		}
+
+		if i == 0 {
+			childrenKind = kind
+		}
+		if kind != childrenKind {
+			fmt.Errorf("data: [%d]: children must be of the same type", i)
 		}
 
 		bt.data = append(bt.data, _LOBTreeItem{key: kkey, child: child})