Features in Transaction log iterator

Summary:
* Logstore requests a valid change of reutrning an empty iterator and not an error in case of no log files.
* Changed the code to return the writebatch containing the sequence number requested from GetupdatesSince even if it lies in the middle. Earlier we used to return the next writebatch,. This also allows me oto guarantee that no files played upon by the iterator are redundant. I mean the starting log file has at least a sequence number >= the sequence number requested form GetupdatesSince.
* Cleaned up redundant logic in Iterator::Next and made a new function SeekToStartSequence for greater readability and maintainibilty.
* Modified a test in db_test accordingly
Please check the logic carefully and suggest improvements. I have a separate patch out for more improvements like restricting reader to read till written sequences.

Test Plan:
* transaction log iterator tests in db_test,
* db_repl_stress.
* rocks_log_iterator_test in fbcode/wormhole/rocksdb/test - 2 tests thriving on hacks till now can get simplified
* testing on the shadow setup for sigma with replication

Reviewers: dhruba, haobo, kailiu, sdong

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13437

db/db_impl.cc

@@ -1117,11 +1117,6 @@ Status DBImpl::GetUpdatesSince(SequenceNumber seq,
     return s;
   }
 
-  if (wal_files->empty()) {
-    return Status::IOError(" NO WAL Files present in the db");
-  }
-  //  std::shared_ptr would have been useful here.
-
   s = RetainProbableWalFiles(*wal_files, seq);
   if (!s.ok()) {
     return s;
@@ -1133,8 +1128,7 @@ Status DBImpl::GetUpdatesSince(SequenceNumber seq,
                                    seq,
                                    std::move(wal_files),
                                    &last_flushed_sequence_));
-  iter->get()->Next();
-  return iter->get()->status();
+  return (*iter)->status();
 }
 
 Status DBImpl::RetainProbableWalFiles(VectorLogPtr& all_logs,

db/db_test.cc

@@ -3563,7 +3563,8 @@ TEST(DBTest, TransactionLogIteratorJustEmptyFile) {
     DestroyAndReopen(&options);
     unique_ptr<TransactionLogIterator> iter;
     Status status = dbfull()->GetUpdatesSince(0, &iter);
-    ASSERT_TRUE(!status.ok());
+    // Check that an empty iterator is returned
+    ASSERT_TRUE(!iter->Valid());
   } while (ChangeCompactOptions());
 }
 
@@ -3594,7 +3595,7 @@ TEST(DBTest, TransactionLogIteratorBatchOperations) {
     Reopen(&options);
     Put("key4", DummyString(1024));
     auto iter = OpenTransactionLogIter(3);
-    ExpectRecords(1, iter);
+    ExpectRecords(2, iter);
   } while (ChangeCompactOptions());
 }
 

db/transaction_log_impl.cc

@@ -20,10 +20,11 @@ TransactionLogIteratorImpl::TransactionLogIteratorImpl(
     currentFileIndex_(0),
     lastFlushedSequence_(lastFlushedSequence) {
   assert(startingSequenceNumber_ <= *lastFlushedSequence_);
-  assert(files_.get() != nullptr);
+  assert(files_ != nullptr);
 
   reporter_.env = options_->env;
   reporter_.info_log = options_->info_log.get();
+  SeekToStartSequence(); // Seek till starting sequence
 }
 
 Status TransactionLogIteratorImpl::OpenLogFile(
@@ -52,7 +53,7 @@ Status TransactionLogIteratorImpl::OpenLogFile(
 BatchResult TransactionLogIteratorImpl::GetBatch()  {
   assert(isValid_);  //  cannot call in a non valid state.
   BatchResult result;
-  result.sequence = currentSequence_;
+  result.sequence = currentBatchSeq_;
   result.writeBatchPtr = std::move(currentBatch_);
   return result;
 }
@@ -65,24 +66,21 @@ bool TransactionLogIteratorImpl::Valid() {
   return started_ && isValid_;
 }
 
-void TransactionLogIteratorImpl::Next() {
-  LogFile* currentLogFile = files_.get()->at(currentFileIndex_).get();
-
-//  First seek to the given seqNo. in the current file.
+void TransactionLogIteratorImpl::SeekToStartSequence() {
     std::string scratch;
     Slice record;
-  if (!started_) {
-    started_ = true;  // this piece only runs onced.
     isValid_ = false;
     if (startingSequenceNumber_ > *lastFlushedSequence_) {
       currentStatus_ = Status::IOError("Looking for a sequence, "
                                        "which is not flushed yet.");
       return;
     }
-    Status s = OpenLogReader(currentLogFile);
+    if (files_->size() == 0) {
+      return;
+    }
+    Status s = OpenLogReader(files_->at(0).get());
     if (!s.ok()) {
       currentStatus_ = s;
-      isValid_ = false;
       return;
     }
     while (currentLogReader_->ReadRecord(&record, &scratch)) {
@@ -92,23 +90,39 @@ void TransactionLogIteratorImpl::Next() {
         continue;
       }
       UpdateCurrentWriteBatch(record);
-      if (currentSequence_ >= startingSequenceNumber_) {
-        assert(currentSequence_ <= *lastFlushedSequence_);
+      if (currentBatchSeq_ + currentBatchCount_ - 1 >=
+          startingSequenceNumber_) {
+        assert(currentBatchSeq_ <= *lastFlushedSequence_);
         isValid_ = true;
-        break;
+        started_ = true; // set started_ as we could seek till starting sequence
+        return;
       } else {
         isValid_ = false;
       }
     }
-    if (isValid_) {
-      // Done for this iteration
-      return;
+    // Could not find start sequence in first file. Normally this must be the
+    // only file. Otherwise log the error and let the iterator return next entry
+    if (files_->size() != 1) {
+      currentStatus_ = Status::Corruption("Start sequence was not found, "
+                                          "skipping to the next available");
+      reporter_.Corruption(0, currentStatus_);
+      started_ = true; // Let Next find next available entry
+      Next();
     }
+}
+
+void TransactionLogIteratorImpl::Next() {
+  // TODO:Next() says that it requires Valid to be true but this is not true
+  // assert(Valid());
+  std::string scratch;
+  Slice record;
+  isValid_ = false;
+  if (!started_) {  // Runs every time until we can seek to the start sequence
+    return SeekToStartSequence();
   }
-  bool openNextFile = true;
-  while(openNextFile) {
+  while(true) {
     assert(currentLogReader_);
-    if (currentSequence_ < *lastFlushedSequence_) {
+    if (currentBatchSeq_ < *lastFlushedSequence_) {
       if (currentLogReader_->IsEOF()) {
         currentLogReader_->UnmarkEOF();
       }
@@ -119,16 +133,15 @@ void TransactionLogIteratorImpl::Next() {
           continue;
         } else {
           UpdateCurrentWriteBatch(record);
-          openNextFile = false;
-          break;
+          return;
         }
       }
     }
 
-    if (openNextFile) {
-      if (currentFileIndex_ < files_.get()->size() - 1) {
+    // Open the next file
+    if (currentFileIndex_ < files_->size() - 1) {
       ++currentFileIndex_;
-        Status status =OpenLogReader(files_.get()->at(currentFileIndex_).get());
+      Status status =OpenLogReader(files_->at(currentFileIndex_).get());
       if (!status.ok()) {
         isValid_ = false;
         currentStatus_ = status;
@@ -136,13 +149,12 @@ void TransactionLogIteratorImpl::Next() {
       }
     } else {
       isValid_ = false;
-        openNextFile = false;
-        if (currentSequence_ == *lastFlushedSequence_) {
+      if (currentBatchSeq_ == *lastFlushedSequence_) {
         currentStatus_ = Status::OK();
       } else {
         currentStatus_ = Status::IOError(" NO MORE DATA LEFT");
       }
-      }
+      return;
     }
   }
 }
@@ -150,7 +162,8 @@ void TransactionLogIteratorImpl::Next() {
 void TransactionLogIteratorImpl::UpdateCurrentWriteBatch(const Slice& record) {
   WriteBatch* batch = new WriteBatch();
   WriteBatchInternal::SetContents(batch, record);
-  currentSequence_ = WriteBatchInternal::Sequence(batch);
+  currentBatchSeq_ = WriteBatchInternal::Sequence(batch);
+  currentBatchCount_ = WriteBatchInternal::Count(batch);
   currentBatch_.reset(batch);
   isValid_ = true;
   currentStatus_ = Status::OK();

db/transaction_log_impl.h

@@ -88,9 +88,11 @@ class TransactionLogIteratorImpl : public TransactionLogIterator {
   Status OpenLogFile(const LogFile* logFile, unique_ptr<SequentialFile>* file);
   LogReporter reporter_;
   SequenceNumber const * const lastFlushedSequence_;
-  // represents the sequence number being read currently.
-  SequenceNumber currentSequence_;
 
+  SequenceNumber currentBatchSeq_; // sequence number at start of current batch
+  uint64_t currentBatchCount_; // count in current batch
+
+  void SeekToStartSequence();
   void UpdateCurrentWriteBatch(const Slice& record);
   Status OpenLogReader(const LogFile* file);
 };

include/rocksdb/transaction_log.h

@@ -67,8 +67,8 @@ class TransactionLogIterator {
   // REQUIRES: Valid() to be true.
   virtual void Next() = 0;
 
-  // Return's ok if the iterator is valid.
-  // Return the Error when something has gone wrong.
+  // Returns ok if the iterator is valid.
+  // Returns the Error when something has gone wrong.
   virtual Status status() = 0;
 
   // If valid return's the current write_batch and the sequence number of the