added KVWriter.NewBatchEx() method

index/store/boltdb/writer.go

@@ -23,6 +23,10 @@ func (w *Writer) NewBatch() store.KVBatch {
 	return store.NewEmulatedBatch(w.store.mo)
 }
 
+func (w *Writer) NewBatchEx(options store.KVBatchOptions) ([]byte, store.KVBatch, error) {
+	return make([]byte, options.TotalBytes), w.NewBatch(), nil
+}
+
 func (w *Writer) ExecuteBatch(batch store.KVBatch) error {
 
 	emulatedBatch, ok := batch.(*store.EmulatedBatch)

index/store/goleveldb/writer.go

@@ -29,6 +29,10 @@ func (w *Writer) NewBatch() store.KVBatch {
 	return &rv
 }
 
+func (w *Writer) NewBatchEx(options store.KVBatchOptions) ([]byte, store.KVBatch, error) {
+	return make([]byte, options.TotalBytes), w.NewBatch(), nil
+}
+
 func (w *Writer) ExecuteBatch(b store.KVBatch) error {
 	batch, ok := b.(*Batch)
 	if !ok {

index/store/gtreap/writer.go

@@ -29,6 +29,10 @@ func (w *Writer) NewBatch() store.KVBatch {
 	return store.NewEmulatedBatch(w.s.mo)
 }
 
+func (w *Writer) NewBatchEx(options store.KVBatchOptions) ([]byte, store.KVBatch, error) {
+	return make([]byte, options.TotalBytes), w.NewBatch(), nil
+}
+
 func (w *Writer) ExecuteBatch(batch store.KVBatch) error {
 
 	emulatedBatch, ok := batch.(*store.EmulatedBatch)

index/store/kvstore.go

@@ -94,6 +94,14 @@ type KVWriter interface {
 	// NewBatch returns a KVBatch for performaing batch operations on this kvstore
 	NewBatch() KVBatch
 
+	// NewBatchEx returns a KVBatch and an associated byte array
+	// that's pre-sized based on the KVBatchOptions.  The caller can
+	// use the returned byte array for keys and values associated with
+	// the batch.  Once the batch is either executed or closed, the
+	// associated byte array should no longer be accessed by the
+	// caller.
+	NewBatchEx(KVBatchOptions) ([]byte, KVBatch, error)
+
 	// ExecuteBatch will execute the KVBatch, the provided KVBatch **MUST** have
 	// been created by the same KVStore (though not necessarily the same KVWriter)
 	// Batch execution is atomic, either all the operations or none will be performed
@@ -103,6 +111,27 @@ type KVWriter interface {
 	Close() error
 }
 
+// KVBatchOptions provides the KVWriter.NewBatchEx() method with batch
+// preparation and preallocation information.
+type KVBatchOptions struct {
+	// TotalBytes is the sum of key and value bytes needed by the
+	// caller for the entire batch.  It affects the size of the
+	// returned byte array of KVWrite.NewBatchEx().
+	TotalBytes int
+
+	// NumSets is the number of Set() calls the caller will invoke on
+	// the KVBatch.
+	NumSets int
+
+	// NumMerges is the number of Merge() calls the caller will invoke
+	// on the KVBatch.
+	NumDeletes int
+
+	// NumMerges is the number of Merge() calls the caller will invoke
+	// on the KVBatch.
+	NumMerges int
+}
+
 // KVBatch is an abstraction for making multiple KV mutations at once
 type KVBatch interface {
 

index/store/metrics/writer.go

@@ -23,6 +23,10 @@ func (w *Writer) NewBatch() store.KVBatch {
 	return &Batch{s: w.s, o: w.o.NewBatch()}
 }
 
+func (w *Writer) NewBatchEx(options store.KVBatchOptions) ([]byte, store.KVBatch, error) {
+	return make([]byte, options.TotalBytes), w.NewBatch(), nil
+}
+
 func (w *Writer) ExecuteBatch(b store.KVBatch) (err error) {
 	batch, ok := b.(*Batch)
 	if !ok {

index/store/null/null.go

@@ -96,6 +96,10 @@ func (w *writer) NewBatch() store.KVBatch {
 	return &batch{}
 }
 
+func (w *writer) NewBatchEx(options store.KVBatchOptions) ([]byte, store.KVBatch, error) {
+	return make([]byte, options.TotalBytes), w.NewBatch(), nil
+}
+
 func (w *writer) ExecuteBatch(store.KVBatch) error {
 	return nil
 }