Add ReadOptions to TransactionLogIterator.

Summary:
Add an optional input parameter ReadOptions to DB::GetUpdateSince(),
which allows the verification of checksums to be disabled by setting
ReadOptions::verify_checksums to false.

Test Plan: Tests are done off-line and will not be included in the regular unit test.

Reviewers: igor

Reviewed By: igor

CC: leveldb, xjin, dhruba

Differential Revision: https://reviews.facebook.net/D16305

include/rocksdb/db.h

@@ -300,8 +300,10 @@ class DB {
   // use this api, else the WAL files will get
   // cleared aggressively and the iterator might keep getting invalid before
   // an update is read.
-  virtual Status GetUpdatesSince(SequenceNumber seq_number,
-                                 unique_ptr<TransactionLogIterator>* iter) = 0;
+  virtual Status GetUpdatesSince(
+      SequenceNumber seq_number, unique_ptr<TransactionLogIterator>* iter,
+      const TransactionLogIterator::ReadOptions&
+          read_options = TransactionLogIterator::ReadOptions()) = 0;
 
   // Delete the file name from the db directory and update the internal state to
   // reflect that. Supports deletion of sst and log files only. 'name' must be

include/rocksdb/transaction_log.h

@@ -85,6 +85,19 @@ class TransactionLogIterator {
   // earliest transaction contained in the batch.
   // ONLY use if Valid() is true and status() is OK.
   virtual BatchResult GetBatch() = 0;
+
+  // The read options for TransactionLogIterator.
+  struct ReadOptions {
+    // If true, all data read from underlying storage will be
+    // verified against corresponding checksums.
+    // Default: true
+    bool verify_checksums_;
+
+    ReadOptions() : verify_checksums_(true) {}
+
+    explicit ReadOptions(bool verify_checksums)
+        : verify_checksums_(verify_checksums) {}
+  };
 };
 } //  namespace rocksdb