go/zodb/btree: Expose iterating through leaf tree nodes as public API

Done for consistency with 313d2d78 (go/zodb/btree: Expose access to
BTree/Bucket entries as public API).

go/zodb/btree/btree.go

@@ -25,6 +25,8 @@
 //
 // node.Entryv() returns [] of (key, child/value).
 //
+// BTree.FirstBucket() and Bucket.Next() allow to iterate through leaf B⁺ tree nodes.
+//
 // --------
 //
 // (*) https://github.com/zopefoundation/ZODB/blob/3.10.7-4-gb8d7a8567/src/BTrees/Development.txt#L211

go/zodb/btree/btree.go.in

@@ -133,6 +133,18 @@ func (b *Bucket) Entryv() []BucketEntry {
 	return ev
 }
 
+// ---- leaf nodes iteration ----
+
+// FirstBucket returns bucket containing the smallest key in the tree.
+func (t *BTree) FirstBucket() *Bucket {
+	return t.firstbucket
+}
+
+// Next returns tree bucket with next larger keys relative to current bucket.
+func (b *Bucket) Next() *Bucket {
+	return b.next
+}
+
 // ---- point query ----
 
 // Get searches BTree by key.

go/zodb/btree/btree_test.go

@@ -253,6 +253,7 @@ func TestBTree(t *testing.T) {
 	}
 
 	// XXX verify Entryv ?
+	// XXX verify FirstBucket / Next ?
 
 	verifyFirstBucket(B3)
 }

go/zodb/btree/ziobtree.go

@@ -135,6 +135,18 @@ func (b *IOBucket) Entryv() []IOBucketEntry {
 	return ev
 }
 
+// ---- leaf nodes iteration ----
+
+// FirstBucket returns bucket containing the smallest key in the tree.
+func (t *IOBTree) FirstBucket() *IOBucket {
+	return t.firstbucket
+}
+
+// Next returns tree bucket with next larger keys relative to current bucket.
+func (b *IOBucket) Next() *IOBucket {
+	return b.next
+}
+
 // ---- point query ----
 
 // Get searches IOBTree by key.

go/zodb/btree/zlobtree.go

@@ -135,6 +135,18 @@ func (b *LOBucket) Entryv() []LOBucketEntry {
 	return ev
 }
 
+// ---- leaf nodes iteration ----
+
+// FirstBucket returns bucket containing the smallest key in the tree.
+func (t *LOBTree) FirstBucket() *LOBucket {
+	return t.firstbucket
+}
+
+// Next returns tree bucket with next larger keys relative to current bucket.
+func (b *LOBucket) Next() *LOBucket {
+	return b.next
+}
+
 // ---- point query ----
 
 // Get searches LOBTree by key.