LevelDB: Add WriteBatch::ApproximateSize().

This can be used to report metrics on LevelDB usage.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=156934930

db/write_batch.cc

@@ -39,6 +39,10 @@ void WriteBatch::Clear() {
   rep_.resize(kHeader);
 }
 
+size_t WriteBatch::ApproximateSize() {
+  return rep_.size();
+}
+
 Status WriteBatch::Iterate(Handler* handler) const {
   Slice input(rep_);
   if (input.size() < kHeader) {

db/write_batch_test.cc

@@ -113,6 +113,23 @@ TEST(WriteBatchTest, Append) {
             PrintContents(&b1));
 }
 
+TEST(WriteBatchTest, ApproximateSize) {
+  WriteBatch batch;
+  size_t empty_size = batch.ApproximateSize();
+
+  batch.Put(Slice("foo"), Slice("bar"));
+  size_t one_key_size = batch.ApproximateSize();
+  ASSERT_LT(empty_size, one_key_size);
+
+  batch.Put(Slice("baz"), Slice("boo"));
+  size_t two_keys_size = batch.ApproximateSize();
+  ASSERT_LT(one_key_size, two_keys_size);
+
+  batch.Delete(Slice("box"));
+  size_t post_delete_size = batch.ApproximateSize();
+  ASSERT_LT(two_keys_size, post_delete_size);
+}
+
 }  // namespace leveldb
 
 int main(int argc, char** argv) {

include/leveldb/write_batch.h

@@ -42,6 +42,12 @@ class WriteBatch {
   // Clear all updates buffered in this batch.
   void Clear();
 
+  // The size of the database changes caused by this batch.
+  //
+  // This number is tied to implementation details, and may change across
+  // releases. It is intended for LevelDB usage metrics.
+  size_t ApproximateSize();
+
   // Support for iterating over the contents of a batch.
   class Handler {
    public: