[Rocksdb] Recover last updated sequence number from manifest also.

Summary:
During recovery, last_updated_manifest number was not set if there were no records in the Write-ahead log.
Now check for the recovered manifest also and set last_updated_manifest file to the max value.

Test Plan: unit test

Reviewers: heyongqiang

Reviewed By: heyongqiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9891

db/db_impl.cc

@@ -549,13 +549,13 @@ Status DBImpl::Recover(VersionEdit* edit, MemTable* external_table,
       // update the file number allocation counter in VersionSet.
       versions_->MarkFileNumberUsed(logs[i]);
     }
-    // This could be the last_flushed_sequence as the next sequences will be
-    // greater than this.
-    last_flushed_sequence_ = max_sequence;
 
     if (s.ok()) {
       if (versions_->LastSequence() < max_sequence) {
         versions_->SetLastSequence(max_sequence);
+        last_flushed_sequence_ = max_sequence;
+      } else {
+        last_flushed_sequence_ = versions_->LastSequence();
       }
     }
   }

db/db_test.cc

@@ -2688,6 +2688,18 @@ TEST(DBTest, TransactionLogIteratorJustEmptyFile) {
   Status status = dbfull()->GetUpdatesSince(0, &iter);
   ASSERT_TRUE(!status.ok());
 }
+
+TEST(DBTest, TransactionLogIteratorCheckAfterRestart) {
+  Options options = OptionsForLogIterTest();
+  DestroyAndReopen(&options);
+  Put("key1", DummyString(1024));
+  Put("key2", DummyString(1023));
+  dbfull()->Flush(FlushOptions());
+  Reopen(&options);
+  auto iter = OpenTransactionLogIter(0);
+  ExpectRecords(2, iter);
+}
+
 TEST(DBTest, ReadCompaction) {
   std::string value(4096, '4'); // a string of size 4K
   {