Expose WriteBatch::Append().

WriteBatchInternal has a method for efficiently concatenating two
WriteBatches. This commit exposes the method to the public API.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208724311

db/write_batch.cc

@@ -112,6 +112,10 @@ void WriteBatch::Delete(const Slice& key) {
   PutLengthPrefixedSlice(&rep_, key);
 }
 
+void WriteBatch::Append(const WriteBatch &source) {
+  WriteBatchInternal::Append(this, &source);
+}
+
 namespace {
 class MemTableInserter : public WriteBatch::Handler {
  public:

db/write_batch_test.cc

@@ -91,21 +91,21 @@ TEST(WriteBatchTest, Append) {
   WriteBatch b1, b2;
   WriteBatchInternal::SetSequence(&b1, 200);
   WriteBatchInternal::SetSequence(&b2, 300);
-  WriteBatchInternal::Append(&b1, &b2);
+  b1.Append(b2);
   ASSERT_EQ("",
             PrintContents(&b1));
   b2.Put("a", "va");
-  WriteBatchInternal::Append(&b1, &b2);
+  b1.Append(b2);
   ASSERT_EQ("Put(a, va)@200",
             PrintContents(&b1));
   b2.Clear();
   b2.Put("b", "vb");
-  WriteBatchInternal::Append(&b1, &b2);
+  b1.Append(b2);
   ASSERT_EQ("Put(a, va)@200"
             "Put(b, vb)@201",
             PrintContents(&b1));
   b2.Delete("foo");
-  WriteBatchInternal::Append(&b1, &b2);
+  b1.Append(b2);
   ASSERT_EQ("Put(a, va)@200"
             "Put(b, vb)@202"
             "Put(b, vb)@201"

include/leveldb/write_batch.h

@@ -54,6 +54,13 @@ class LEVELDB_EXPORT WriteBatch {
   // releases. It is intended for LevelDB usage metrics.
   size_t ApproximateSize();
 
+  // Copies the operations in "source" to this batch.
+  //
+  // This runs in O(source size) time. However, the constant factor is better
+  // than calling Iterate() over the source batch with a Handler that replicates
+  // the operations into this batch.
+  void Append(const WriteBatch& source);
+
   // Support for iterating over the contents of a batch.
   class Handler {
    public: