add MoveBucket to support moving a sub-bucket from one bucket to another bucket

Signed-off-by: Benjamin Wang <benjamin.ahrtr@gmail.com>

bucket.go

@@ -322,6 +322,59 @@ func (b *Bucket) DeleteBucket(key []byte) (err error) {
 	return nil
 }
 
+// MoveBucket moves a sub-bucket from the source bucket to the destination bucket.
+// Returns an error if
+//  1. the sub-bucket cannot be found in the source bucket;
+//  2. or the key already exists in the destination bucket;
+//  3. the key represents a non-bucket value.
+func (b *Bucket) MoveBucket(key []byte, dstBucket *Bucket) error {
+	if b.tx.db == nil || dstBucket.tx.db == nil {
+		return errors.ErrTxClosed
+	} else if !dstBucket.Writable() {
+		return errors.ErrTxNotWritable
+	}
+
+	// Move cursor to correct position.
+	c := b.Cursor()
+	k, v, flags := c.seek(key)
+
+	// Return an error if bucket doesn't exist or is not a bucket.
+	if !bytes.Equal(key, k) {
+		return errors.ErrBucketNotFound
+	} else if (flags & common.BucketLeafFlag) == 0 {
+		return fmt.Errorf("key %q isn't a bucket in the source bucket: %w", key, errors.ErrIncompatibleValue)
+	}
+
+	// Do nothing (return true directly) if the source bucket and the
+	// destination bucket are actually the same bucket.
+	if b == dstBucket || (b.RootPage() == dstBucket.RootPage() && b.RootPage() != 0) {
+		return nil
+	}
+
+	// check whether the key already exists in the destination bucket
+	curDst := dstBucket.Cursor()
+	k, _, flags = curDst.seek(key)
+
+	// Return an error if there is an existing key in the destination bucket.
+	if bytes.Equal(key, k) {
+		if (flags & common.BucketLeafFlag) != 0 {
+			return errors.ErrBucketExists
+		}
+		return fmt.Errorf("key %q already exists in the target bucket: %w", key, errors.ErrIncompatibleValue)
+	}
+
+	// remove the sub-bucket from the source bucket
+	delete(b.buckets, string(key))
+	c.node().del(key)
+
+	// add te sub-bucket to the destination bucket
+	newKey := cloneBytes(key)
+	newValue := cloneBytes(v)
+	curDst.node().put(newKey, newKey, newValue, 0, common.BucketLeafFlag)
+
+	return nil
+}
+
 // Get retrieves the value for a key in the bucket.
 // Returns a nil value if the key does not exist or if the key is a nested bucket.
 // The returned value is only valid for the life of the transaction.

tx.go

@@ -127,6 +127,24 @@ func (tx *Tx) DeleteBucket(name []byte) error {
 	return tx.root.DeleteBucket(name)
 }
 
+// MoveBucket moves a sub-bucket from the source bucket to the destination bucket.
+// Returns an error if
+//  1. the sub-bucket cannot be found in the source bucket;
+//  2. or the key already exists in the destination bucket;
+//  3. the key represents a non-bucket value.
+//
+// If src is nil, it means moving a top level bucket into the target bucket.
+// If dst is nil, it means converting the child bucket into a top level bucket.
+func (tx *Tx) MoveBucket(child []byte, src *Bucket, dst *Bucket) error {
+	if src == nil {
+		src = &tx.root
+	}
+	if dst == nil {
+		dst = &tx.root
+	}
+	return src.MoveBucket(child, dst)
+}
+
 // ForEach executes a function for each bucket in the root.
 // If the provided function returns an error then the iteration is stopped and
 // the error is returned to the caller.