An public api to fetch the latest transaction id.

Summary:
Implement a interface to retrieve the most current transaction
id from the database.

Test Plan: Added unit test.

Reviewers: sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7269

db/db_impl.cc

@@ -877,6 +877,10 @@ Status DBImpl::Flush(const FlushOptions& options) {
   return status;
 }
 
+SequenceNumber DBImpl::GetLatestSequenceNumber() {
+  return versions_->LastSequence();
+}
+
 Status DBImpl::GetUpdatesSince(SequenceNumber seq,
                                TransactionLogIterator** iter) {
 

db/db_impl.h

@@ -55,6 +55,7 @@ class DBImpl : public DB {
   virtual Status EnableFileDeletions();
   virtual Status GetLiveFiles(std::vector<std::string>&,
                               uint64_t* manifest_file_size);
+  virtual SequenceNumber GetLatestSequenceNumber();
   virtual Status GetUpdatesSince(SequenceNumber seq_number,
                                  TransactionLogIterator ** iter);
 

db/db_test.cc

@@ -2273,6 +2273,7 @@ TEST(DBTest, TransactionLogIterator) {
   Put("key1", value);
   Put("key2", value);
   Put("key2", value);
+  ASSERT_EQ(dbfull()->GetLatestSequenceNumber(), 3U);
   {
     TransactionLogIterator* iter;
     Status status = dbfull()->GetUpdatesSince(0, &iter);
@@ -2583,6 +2584,9 @@ class ModelDB: public DB {
     return Status::OK();
   }
 
+  virtual SequenceNumber GetLatestSequenceNumber() {
+    return 0;
+  }
   virtual Status GetUpdatesSince(leveldb::SequenceNumber,
                                  leveldb::TransactionLogIterator**) {
     return Status::NotSupported("Not supported in Model DB");

include/leveldb/db.h

@@ -182,6 +182,9 @@ class DB {
   virtual Status GetLiveFiles(std::vector<std::string>&,
                               uint64_t* manifest_file_size) = 0;
 
+  // The sequence number of the most recent transaction. 
+  virtual SequenceNumber GetLatestSequenceNumber() = 0;
+
   // Return's an iterator for all writes since the sequence number
   // Status::ok if iterator is valid.
   // The iterator internally holds references to the available log files.