src/kudu/consensus/log_reader.cc - kudu - Git at Google

 // Licensed to the Apache Software Foundation (ASF) under one
 // or more contributor license agreements.  See the NOTICE file
 // distributed with this work for additional information
 // regarding copyright ownership.  The ASF licenses this file
 // to you under the Apache License, Version 2.0 (the
 // "License"); you may not use this file except in compliance
 // with the License.  You may obtain a copy of the License at
 //
 //   http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing,
 // software distributed under the License is distributed on an
 // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 // KIND, either express or implied.  See the License for the
 // specific language governing permissions and limitations
 // under the License.

 #include "kudu/consensus/log_reader.h"

 #include <algorithm>
 #include <cstddef>
 #include <memory>
 #include <mutex>
 #include <ostream>
 #include <type_traits>

 #include <glog/logging.h>

 #include "kudu/consensus/consensus.pb.h"
 #include "kudu/consensus/log.pb.h"
 #include "kudu/consensus/log_index.h"
 #include "kudu/consensus/opid.pb.h"
 #include "kudu/fs/fs_manager.h"
 #include "kudu/gutil/port.h"
 #include "kudu/gutil/stl_util.h"
 #include "kudu/gutil/strings/substitute.h"
 #include "kudu/gutil/strings/util.h"
 #include "kudu/util/env.h"
 #include "kudu/util/faststring.h"
 #include "kudu/util/metrics.h"
 #include "kudu/util/path_util.h"
 #include "kudu/util/pb_util.h"

 METRIC_DEFINE_counter(tablet, log_reader_bytes_read, "Bytes Read From Log",
                       kudu::MetricUnit::kBytes,
                       "Data read from the WAL since tablet start",
                       kudu::MetricLevel::kDebug);

 METRIC_DEFINE_counter(tablet, log_reader_entries_read, "Entries Read From Log",
                       kudu::MetricUnit::kEntries,
                       "Number of entries read from the WAL since tablet start",
                       kudu::MetricLevel::kDebug);

 METRIC_DEFINE_histogram(tablet, log_reader_read_batch_latency, "Log Read Latency",
                         kudu::MetricUnit::kMicroseconds,
                         "Microseconds spent reading log entry batches",
                         kudu::MetricLevel::kInfo,
                         60000000LU, 2);

 using kudu::consensus::OpId;
 using kudu::consensus::ReplicateMsg;
 using kudu::pb_util::SecureDebugString;
 using kudu::pb_util::SecureShortDebugString;
 using std::shared_ptr;
 using std::string;
 using std::unique_ptr;
 using std::vector;
 using strings::Substitute;

 namespace kudu {
 namespace log {

 namespace {
 struct LogSegmentSeqnoComparator {
   bool operator() (const scoped_refptr<ReadableLogSegment>& a,
                    const scoped_refptr<ReadableLogSegment>& b) {
     return a->header().sequence_number() < b->header().sequence_number();
   }
 };
 }

 const int64_t LogReader::kNoSizeLimit = -1;

 Status LogReader::Open(Env* env,
                        const string& tablet_wal_dir,
                        const scoped_refptr<LogIndex>& index,
                        const string& tablet_id,
                        const scoped_refptr<MetricEntity>& metric_entity,
                        FileCache* file_cache,
                        shared_ptr<LogReader>* reader) {
   auto log_reader = LogReader::make_shared(env, index, tablet_id,
                                            metric_entity, file_cache);

   RETURN_NOT_OK_PREPEND(log_reader->Init(tablet_wal_dir),
                         "Unable to initialize log reader");
   *reader = log_reader;
   return Status::OK();
 }

 Status LogReader::Open(FsManager* fs_manager,
                        const scoped_refptr<LogIndex>& index,
                        const std::string& tablet_id,
                        const scoped_refptr<MetricEntity>& metric_entity,
                        FileCache* file_cache,
                        std::shared_ptr<LogReader>* reader) {
   return LogReader::Open(fs_manager->env(), fs_manager->GetTabletWalDir(tablet_id),
                          index, tablet_id, metric_entity, file_cache, reader);
 }

 LogReader::LogReader(Env* env,
                      scoped_refptr<LogIndex> index,
                      string tablet_id,
                      const scoped_refptr<MetricEntity>& metric_entity,
                      FileCache* file_cache)
     : env_(env),
       file_cache_(file_cache),
       log_index_(std::move(index)),
       tablet_id_(std::move(tablet_id)),
       state_(kLogReaderInitialized) {
   if (metric_entity) {
     bytes_read_ = METRIC_log_reader_bytes_read.Instantiate(metric_entity);
     entries_read_ = METRIC_log_reader_entries_read.Instantiate(metric_entity);
     read_batch_latency_ = METRIC_log_reader_read_batch_latency.Instantiate(metric_entity);
   }
 }

 LogReader::~LogReader() {
 }

 Status LogReader::Init(const string& tablet_wal_path) {
   {
     std::lock_guard<simple_spinlock> lock(lock_);
     CHECK_EQ(state_, kLogReaderInitialized) << "bad state for Init(): " << state_;
   }
   VLOG(1) << "Reading wal from path:" << tablet_wal_path;

   if (!env_->FileExists(tablet_wal_path)) {
     return Status::IllegalState("Cannot find wal location at", tablet_wal_path);
   }

   VLOG(1) << "Parsing segments from path: " << tablet_wal_path;
   // list existing segment files
   vector<string> log_files;

   RETURN_NOT_OK_PREPEND(env_->GetChildren(tablet_wal_path, &log_files),
                         "Unable to read children from path");

   SegmentSequence read_segments;
   read_segments.reserve(log_files.size()); // Overestimate; will shrink_to_fit later.

   // build a log segment from each file
   for (const string &log_file : log_files) {
     if (HasPrefixString(log_file, FsManager::kWalFileNamePrefix)) {
       string fqp = JoinPathSegments(tablet_wal_path, log_file);
       scoped_refptr<ReadableLogSegment> segment;
       Status s = ReadableLogSegment::Open(env_, file_cache_, fqp, &segment);
       if (s.IsUninitialized()) {
         // This indicates that the segment was created but the writer
         // crashed before the header was successfully written. In this
         // case, we should skip it.
         LOG(WARNING) << "Ignoring log segment " << log_file << " since it was uninitialized "
                      << "(probably left after a prior tablet server crash)";
         continue;
       }

       RETURN_NOT_OK_PREPEND(s, "Unable to open readable log segment");
       DCHECK(segment);
       CHECK(segment->IsInitialized()) << "Uninitialized segment at: " << segment->path();

       if (!segment->HasFooter()) {
         VLOG(1) << "Log segment " << fqp << " was likely left in-progress "
                 << "after a previous crash. Will try to rebuild footer by scanning data.";
         RETURN_NOT_OK(segment->RebuildFooterByScanning());
       }

       read_segments.emplace_back(std::move(segment));
     }
   }
   read_segments.shrink_to_fit();

   // Sort the segments by sequence number.
   std::sort(read_segments.begin(), read_segments.end(), LogSegmentSeqnoComparator());

   {
     std::lock_guard<simple_spinlock> lock(lock_);

     string previous_seg_path;
     int64_t previous_seg_seqno = -1;
     for (auto& entry : read_segments) {
       VLOG(1) << " Log Reader Indexed: " << SecureShortDebugString(entry->footer());
       // Check that the log segments are in sequence.
       if (previous_seg_seqno != -1 && entry->header().sequence_number() != previous_seg_seqno + 1) {
         return Status::Corruption(Substitute("Segment sequence numbers are not consecutive. "
             "Previous segment: seqno $0, path $1; Current segment: seqno $2, path $3",
             previous_seg_seqno, previous_seg_path,
             entry->header().sequence_number(), entry->path()));
       }
       previous_seg_seqno = entry->header().sequence_number();
       previous_seg_path = entry->path();
       DCHECK(entry->HasFooter());
       AppendSegmentUnlocked(std::move(entry));
     }

     state_ = kLogReaderReading;
   }
   return Status::OK();
 }

 void LogReader::InitEmptyReaderForTests() {
   std::lock_guard<simple_spinlock> lock(lock_);
   state_ = kLogReaderReading;
 }

 int64_t LogReader::GetMinReplicateIndex() const {
   std::lock_guard<simple_spinlock> lock(lock_);
   int64_t min_remaining_op_idx = -1;

   for (const scoped_refptr<ReadableLogSegment>& segment : segments_) {
     if (!segment->HasFooter()) continue;
     if (!segment->footer().has_min_replicate_index()) continue;
     if (min_remaining_op_idx == -1 ||
         segment->footer().min_replicate_index() < min_remaining_op_idx) {
       min_remaining_op_idx = segment->footer().min_replicate_index();
     }
   }
   return min_remaining_op_idx;
 }


 scoped_refptr<ReadableLogSegment> LogReader::GetSegmentBySequenceNumber(int64_t seq) const {
   std::lock_guard<simple_spinlock> lock(lock_);
   if (segments_.empty()) {
     return nullptr;
   }

   // We always have a contiguous set of log segments, so we can find the requested
   // segment in our vector by calculating its offset vs the first element.
   int64_t first_seqno = segments_[0]->header().sequence_number();
   int64_t relative = seq - first_seqno;
   if (relative < 0 || relative >= segments_.size()) {
     return nullptr;
   }

   DCHECK_EQ(segments_[relative]->header().sequence_number(), seq);
   return segments_[relative];
 }

 Status LogReader::ReadBatchUsingIndexEntry(const LogIndexEntry& index_entry,
                                            faststring* tmp_buf,
                                            LogEntryBatchPB* batch) const {
   const int64_t index = index_entry.op_id.index();

   scoped_refptr<ReadableLogSegment> segment = GetSegmentBySequenceNumber(
     index_entry.segment_sequence_number);
   if (PREDICT_FALSE(!segment)) {
     return Status::NotFound(Substitute("Segment $0 which contained index $1 has been GCed",
                                        index_entry.segment_sequence_number,
                                        index));
   }

   CHECK_GT(index_entry.offset_in_segment, 0);
   int64_t offset = index_entry.offset_in_segment;
   ScopedLatencyMetric scoped(read_batch_latency_.get());
   EntryHeaderStatus unused_status_detail;
   RETURN_NOT_OK_PREPEND(segment->ReadEntryHeaderAndBatch(&offset, tmp_buf, batch,
                                                          &unused_status_detail),
                         Substitute("Failed to read LogEntry for index $0 from log segment "
                                    "$1 offset $2",
                                    index,
                                    index_entry.segment_sequence_number,
                                    index_entry.offset_in_segment));

   if (bytes_read_) {
     bytes_read_->IncrementBy(segment->entry_header_size() + tmp_buf->length());
     entries_read_->IncrementBy(batch->entry_size());
   }

   return Status::OK();
 }

 Status LogReader::ReadReplicatesInRange(int64_t starting_at,
                                         int64_t up_to,
                                         int64_t max_bytes_to_read,
                                         vector<ReplicateMsg*>* replicates) const {
   DCHECK_GT(starting_at, 0);
   DCHECK_GE(up_to, starting_at);
   DCHECK(log_index_) << "Require an index to random-read logs";

   vector<ReplicateMsg*> replicates_tmp;
   ElementDeleter d(&replicates_tmp);
   LogIndexEntry prev_index_entry;

   size_t total_size = 0;
   bool limit_exceeded = false;
   faststring tmp_buf;
   LogEntryBatchPB batch;
   for (int64_t index = starting_at; index <= up_to && !limit_exceeded; index++) {
     LogIndexEntry index_entry;
     RETURN_NOT_OK_PREPEND(log_index_->GetEntry(index, &index_entry),
                           Substitute("Failed to read log index for op $0", index));

     // Since a given LogEntryBatchPB may contain multiple REPLICATE messages,
     // it's likely that this index entry points to the same batch as the previous
     // one. If that's the case, we've already read this REPLICATE and we can
     // skip reading the batch again.
     if (index == starting_at ||
         index_entry.segment_sequence_number != prev_index_entry.segment_sequence_number ||
         index_entry.offset_in_segment != prev_index_entry.offset_in_segment) {
       RETURN_NOT_OK(ReadBatchUsingIndexEntry(index_entry, &tmp_buf, &batch));

       // Sanity-check the property that a batch should only have increasing indexes.
       int64_t prev_index = 0;
       for (int i = 0; i < batch.entry_size(); ++i) {
         LogEntryPB* entry = batch.mutable_entry(i);
         if (!entry->has_replicate()) continue;
         int64_t this_index = entry->replicate().id().index();
         CHECK_GT(this_index, prev_index)
           << "Expected that an entry batch should only include increasing log indexes: "
           << index_entry.ToString()
           << "\nBatch: " << SecureDebugString(batch);
         prev_index = this_index;
       }
     }

     bool found = false;
     for (int i = 0; i < batch.entry_size(); ++i) {
       LogEntryPB* entry = batch.mutable_entry(i);
       if (!entry->has_replicate()) {
         continue;
       }

       if (entry->replicate().id().index() != index) {
         continue;
       }

       size_t space_required = entry->replicate().SpaceUsedLong();
       if (replicates_tmp.empty() ||
           max_bytes_to_read <= 0 ||
           total_size + space_required < max_bytes_to_read) {
         total_size += space_required;
         replicates_tmp.push_back(entry->release_replicate());
       } else {
         limit_exceeded = true;
       }
       found = true;
       break;
     }
     CHECK(found) << "Incorrect index entry didn't yield expected log entry: "
                  << index_entry.ToString();

     prev_index_entry = index_entry;
   }

   replicates->swap(replicates_tmp);
   return Status::OK();
 }

 Status LogReader::LookupOpId(int64_t op_index, OpId* op_id) const {
   LogIndexEntry index_entry;
   RETURN_NOT_OK_PREPEND(log_index_->GetEntry(op_index, &index_entry),
                         strings::Substitute("Failed to read log index for op $0", op_index));
   *op_id = index_entry.op_id;
   return Status::OK();
 }

 void LogReader::GetSegmentsSnapshot(SegmentSequence* segments) const {
   std::lock_guard<simple_spinlock> lock(lock_);
   CHECK_EQ(state_, kLogReaderReading);
   segments->assign(segments_.begin(), segments_.end());
 }

 void LogReader::TrimSegmentsUpToAndIncluding(int64_t segment_sequence_number) {
   std::lock_guard<simple_spinlock> lock(lock_);
   CHECK_EQ(state_, kLogReaderReading);
   auto iter = segments_.begin();
   int num_deleted_segments = 0;

   while (iter != segments_.end()) {
     if ((*iter)->header().sequence_number() <= segment_sequence_number) {
       iter = segments_.erase(iter);
       num_deleted_segments++;
       continue;
     }
     break;
   }
   LOG(INFO) << "T " << tablet_id_ << ": removed " << num_deleted_segments
             << " log segments from log reader";
 }

 void LogReader::UpdateLastSegmentOffset(int64_t readable_to_offset) {
   std::lock_guard<simple_spinlock> lock(lock_);
   CHECK_EQ(state_, kLogReaderReading);
   DCHECK(!segments_.empty());
   // Get the last segment
   auto* segment = segments_.back().get();
   DCHECK(!segment->HasFooter());
   segment->UpdateReadableToOffset(readable_to_offset);
 }

 void LogReader::ReplaceLastSegment(scoped_refptr<ReadableLogSegment> segment) {
   // This is used to replace the last segment once we close it properly so it must
   // have a footer.
   DCHECK(segment->HasFooter());

   std::lock_guard<simple_spinlock> lock(lock_);
   CHECK_EQ(state_, kLogReaderReading);
   // Make sure the segment we're replacing has the same sequence number
   DCHECK(!segments_.empty());
   CHECK_EQ(segment->header().sequence_number(), segments_.back()->header().sequence_number());
   segments_[segments_.size() - 1] = std::move(segment);
 }

 Status LogReader::AppendSegment(scoped_refptr<ReadableLogSegment> segment) {
   DCHECK(segment->IsInitialized());
   if (PREDICT_FALSE(!segment->HasFooter())) {
     RETURN_NOT_OK(segment->RebuildFooterByScanning());
   }
   std::lock_guard<simple_spinlock> lock(lock_);
   AppendSegmentUnlocked(std::move(segment));
   return Status::OK();
 }

 void LogReader::AppendSegmentUnlocked(scoped_refptr<ReadableLogSegment> segment) {
   DCHECK(segment->IsInitialized());
   DCHECK(lock_.is_locked());

 #ifndef NDEBUG
   if (!segments_.empty()) {
     DCHECK_EQ(segments_.back()->header().sequence_number() + 1,
               segment->header().sequence_number());
   }
 #endif
   segments_.emplace_back(std::move(segment));
 }

 void LogReader::AppendEmptySegment(scoped_refptr<ReadableLogSegment> segment) {
   DCHECK(segment->IsInitialized());
   std::lock_guard<simple_spinlock> lock(lock_);
   CHECK_EQ(state_, kLogReaderReading);
   AppendSegmentUnlocked(std::move(segment));
 }

 int LogReader::num_segments() const {
   std::lock_guard<simple_spinlock> lock(lock_);
   return segments_.size();
 }

 string LogReader::ToString() const {
   std::lock_guard<simple_spinlock> lock(lock_);
   string ret = "Reader's SegmentSequence: \n";
   for (const SegmentSequence::value_type& entry : segments_) {
     ret.append(Substitute("Segment: $0 Footer: $1\n",
                           entry->header().sequence_number(),
                           !entry->HasFooter() ? "NONE" : SecureShortDebugString(entry->footer())));
   }
   return ret;
 }

 }  // namespace log
 }  // namespace kudu
	// Licensed to the Apache Software Foundation (ASF) under one
	// or more contributor license agreements. See the NOTICE file
	// distributed with this work for additional information
	// regarding copyright ownership. The ASF licenses this file
	// to you under the Apache License, Version 2.0 (the
	// "License"); you may not use this file except in compliance
	// with the License. You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing,
	// software distributed under the License is distributed on an
	// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	// KIND, either express or implied. See the License for the
	// specific language governing permissions and limitations
	// under the License.

	#include "kudu/consensus/log_reader.h"

	#include <algorithm>
	#include <cstddef>
	#include <memory>
	#include <mutex>
	#include <ostream>
	#include <type_traits>

	#include <glog/logging.h>

	#include "kudu/consensus/consensus.pb.h"
	#include "kudu/consensus/log.pb.h"
	#include "kudu/consensus/log_index.h"
	#include "kudu/consensus/opid.pb.h"
	#include "kudu/fs/fs_manager.h"
	#include "kudu/gutil/port.h"
	#include "kudu/gutil/stl_util.h"
	#include "kudu/gutil/strings/substitute.h"
	#include "kudu/gutil/strings/util.h"
	#include "kudu/util/env.h"
	#include "kudu/util/faststring.h"
	#include "kudu/util/metrics.h"
	#include "kudu/util/path_util.h"
	#include "kudu/util/pb_util.h"

	METRIC_DEFINE_counter(tablet, log_reader_bytes_read, "Bytes Read From Log",
	kudu::MetricUnit::kBytes,
	"Data read from the WAL since tablet start",
	kudu::MetricLevel::kDebug);

	METRIC_DEFINE_counter(tablet, log_reader_entries_read, "Entries Read From Log",
	kudu::MetricUnit::kEntries,
	"Number of entries read from the WAL since tablet start",
	kudu::MetricLevel::kDebug);

	METRIC_DEFINE_histogram(tablet, log_reader_read_batch_latency, "Log Read Latency",
	kudu::MetricUnit::kMicroseconds,
	"Microseconds spent reading log entry batches",
	kudu::MetricLevel::kInfo,
	60000000LU, 2);

	using kudu::consensus::OpId;
	using kudu::consensus::ReplicateMsg;
	using kudu::pb_util::SecureDebugString;
	using kudu::pb_util::SecureShortDebugString;
	using std::shared_ptr;
	using std::string;
	using std::unique_ptr;
	using std::vector;
	using strings::Substitute;

	namespace kudu {
	namespace log {

	namespace {
	struct LogSegmentSeqnoComparator {
	bool operator() (const scoped_refptr<ReadableLogSegment>& a,
	const scoped_refptr<ReadableLogSegment>& b) {
	return a->header().sequence_number() < b->header().sequence_number();
	}
	};
	}

	const int64_t LogReader::kNoSizeLimit = -1;

	Status LogReader::Open(Env* env,
	const string& tablet_wal_dir,
	const scoped_refptr<LogIndex>& index,
	const string& tablet_id,
	const scoped_refptr<MetricEntity>& metric_entity,
	FileCache* file_cache,
	shared_ptr<LogReader>* reader) {
	auto log_reader = LogReader::make_shared(env, index, tablet_id,
	metric_entity, file_cache);

	RETURN_NOT_OK_PREPEND(log_reader->Init(tablet_wal_dir),
	"Unable to initialize log reader");
	*reader = log_reader;
	return Status::OK();
	}

	Status LogReader::Open(FsManager* fs_manager,
	const scoped_refptr<LogIndex>& index,
	const std::string& tablet_id,
	const scoped_refptr<MetricEntity>& metric_entity,
	FileCache* file_cache,
	std::shared_ptr<LogReader>* reader) {
	return LogReader::Open(fs_manager->env(), fs_manager->GetTabletWalDir(tablet_id),
	index, tablet_id, metric_entity, file_cache, reader);
	}

	LogReader::LogReader(Env* env,
	scoped_refptr<LogIndex> index,
	string tablet_id,
	const scoped_refptr<MetricEntity>& metric_entity,
	FileCache* file_cache)
	: env_(env),
	file_cache_(file_cache),
	log_index_(std::move(index)),
	tablet_id_(std::move(tablet_id)),
	state_(kLogReaderInitialized) {
	if (metric_entity) {
	bytes_read_ = METRIC_log_reader_bytes_read.Instantiate(metric_entity);
	entries_read_ = METRIC_log_reader_entries_read.Instantiate(metric_entity);
	read_batch_latency_ = METRIC_log_reader_read_batch_latency.Instantiate(metric_entity);
	}
	}

	LogReader::~LogReader() {
	}

	Status LogReader::Init(const string& tablet_wal_path) {
	{
	std::lock_guard<simple_spinlock> lock(lock_);
	CHECK_EQ(state_, kLogReaderInitialized) << "bad state for Init(): " << state_;
	}
	VLOG(1) << "Reading wal from path:" << tablet_wal_path;

	if (!env_->FileExists(tablet_wal_path)) {
	return Status::IllegalState("Cannot find wal location at", tablet_wal_path);
	}

	VLOG(1) << "Parsing segments from path: " << tablet_wal_path;
	// list existing segment files
	vector<string> log_files;

	RETURN_NOT_OK_PREPEND(env_->GetChildren(tablet_wal_path, &log_files),
	"Unable to read children from path");

	SegmentSequence read_segments;
	read_segments.reserve(log_files.size()); // Overestimate; will shrink_to_fit later.

	// build a log segment from each file
	for (const string &log_file : log_files) {
	if (HasPrefixString(log_file, FsManager::kWalFileNamePrefix)) {
	string fqp = JoinPathSegments(tablet_wal_path, log_file);
	scoped_refptr<ReadableLogSegment> segment;
	Status s = ReadableLogSegment::Open(env_, file_cache_, fqp, &segment);
	if (s.IsUninitialized()) {
	// This indicates that the segment was created but the writer
	// crashed before the header was successfully written. In this
	// case, we should skip it.
	LOG(WARNING) << "Ignoring log segment " << log_file << " since it was uninitialized "
	<< "(probably left after a prior tablet server crash)";
	continue;
	}

	RETURN_NOT_OK_PREPEND(s, "Unable to open readable log segment");
	DCHECK(segment);
	CHECK(segment->IsInitialized()) << "Uninitialized segment at: " << segment->path();

	if (!segment->HasFooter()) {
	VLOG(1) << "Log segment " << fqp << " was likely left in-progress "
	<< "after a previous crash. Will try to rebuild footer by scanning data.";
	RETURN_NOT_OK(segment->RebuildFooterByScanning());
	}

	read_segments.emplace_back(std::move(segment));
	}
	}
	read_segments.shrink_to_fit();

	// Sort the segments by sequence number.
	std::sort(read_segments.begin(), read_segments.end(), LogSegmentSeqnoComparator());

	{
	std::lock_guard<simple_spinlock> lock(lock_);

	string previous_seg_path;
	int64_t previous_seg_seqno = -1;
	for (auto& entry : read_segments) {
	VLOG(1) << " Log Reader Indexed: " << SecureShortDebugString(entry->footer());
	// Check that the log segments are in sequence.
	if (previous_seg_seqno != -1 && entry->header().sequence_number() != previous_seg_seqno + 1) {
	return Status::Corruption(Substitute("Segment sequence numbers are not consecutive. "
	"Previous segment: seqno $0, path $1; Current segment: seqno $2, path $3",
	previous_seg_seqno, previous_seg_path,
	entry->header().sequence_number(), entry->path()));
	}
	previous_seg_seqno = entry->header().sequence_number();
	previous_seg_path = entry->path();
	DCHECK(entry->HasFooter());
	AppendSegmentUnlocked(std::move(entry));
	}

	state_ = kLogReaderReading;
	}
	return Status::OK();
	}

	void LogReader::InitEmptyReaderForTests() {
	std::lock_guard<simple_spinlock> lock(lock_);
	state_ = kLogReaderReading;
	}

	int64_t LogReader::GetMinReplicateIndex() const {
	std::lock_guard<simple_spinlock> lock(lock_);
	int64_t min_remaining_op_idx = -1;

	for (const scoped_refptr<ReadableLogSegment>& segment : segments_) {
	if (!segment->HasFooter()) continue;
	if (!segment->footer().has_min_replicate_index()) continue;
	if (min_remaining_op_idx == -1 \|\|
	segment->footer().min_replicate_index() < min_remaining_op_idx) {
	min_remaining_op_idx = segment->footer().min_replicate_index();
	}
	}
	return min_remaining_op_idx;
	}


	scoped_refptr<ReadableLogSegment> LogReader::GetSegmentBySequenceNumber(int64_t seq) const {
	std::lock_guard<simple_spinlock> lock(lock_);
	if (segments_.empty()) {
	return nullptr;
	}

	// We always have a contiguous set of log segments, so we can find the requested
	// segment in our vector by calculating its offset vs the first element.
	int64_t first_seqno = segments_[0]->header().sequence_number();
	int64_t relative = seq - first_seqno;
	if (relative < 0 \|\| relative >= segments_.size()) {
	return nullptr;
	}

	DCHECK_EQ(segments_[relative]->header().sequence_number(), seq);
	return segments_[relative];
	}

	Status LogReader::ReadBatchUsingIndexEntry(const LogIndexEntry& index_entry,
	faststring* tmp_buf,
	LogEntryBatchPB* batch) const {
	const int64_t index = index_entry.op_id.index();

	scoped_refptr<ReadableLogSegment> segment = GetSegmentBySequenceNumber(
	index_entry.segment_sequence_number);
	if (PREDICT_FALSE(!segment)) {
	return Status::NotFound(Substitute("Segment $0 which contained index $1 has been GCed",
	index_entry.segment_sequence_number,
	index));
	}

	CHECK_GT(index_entry.offset_in_segment, 0);
	int64_t offset = index_entry.offset_in_segment;
	ScopedLatencyMetric scoped(read_batch_latency_.get());
	EntryHeaderStatus unused_status_detail;
	RETURN_NOT_OK_PREPEND(segment->ReadEntryHeaderAndBatch(&offset, tmp_buf, batch,
	&unused_status_detail),
	Substitute("Failed to read LogEntry for index $0 from log segment "
	"$1 offset $2",
	index,
	index_entry.segment_sequence_number,
	index_entry.offset_in_segment));

	if (bytes_read_) {
	bytes_read_->IncrementBy(segment->entry_header_size() + tmp_buf->length());
	entries_read_->IncrementBy(batch->entry_size());
	}

	return Status::OK();
	}

	Status LogReader::ReadReplicatesInRange(int64_t starting_at,
	int64_t up_to,
	int64_t max_bytes_to_read,
	vector<ReplicateMsg> replicates) const {
	DCHECK_GT(starting_at, 0);
	DCHECK_GE(up_to, starting_at);
	DCHECK(log_index_) << "Require an index to random-read logs";

	vector<ReplicateMsg*> replicates_tmp;
	ElementDeleter d(&replicates_tmp);
	LogIndexEntry prev_index_entry;

	size_t total_size = 0;
	bool limit_exceeded = false;
	faststring tmp_buf;
	LogEntryBatchPB batch;
	for (int64_t index = starting_at; index <= up_to && !limit_exceeded; index++) {
	LogIndexEntry index_entry;
	RETURN_NOT_OK_PREPEND(log_index_->GetEntry(index, &index_entry),
	Substitute("Failed to read log index for op $0", index));

	// Since a given LogEntryBatchPB may contain multiple REPLICATE messages,
	// it's likely that this index entry points to the same batch as the previous
	// one. If that's the case, we've already read this REPLICATE and we can
	// skip reading the batch again.
	if (index == starting_at \|\|
	index_entry.segment_sequence_number != prev_index_entry.segment_sequence_number \|\|
	index_entry.offset_in_segment != prev_index_entry.offset_in_segment) {
	RETURN_NOT_OK(ReadBatchUsingIndexEntry(index_entry, &tmp_buf, &batch));

	// Sanity-check the property that a batch should only have increasing indexes.
	int64_t prev_index = 0;
	for (int i = 0; i < batch.entry_size(); ++i) {
	LogEntryPB* entry = batch.mutable_entry(i);
	if (!entry->has_replicate()) continue;
	int64_t this_index = entry->replicate().id().index();
	CHECK_GT(this_index, prev_index)
	<< "Expected that an entry batch should only include increasing log indexes: "
	<< index_entry.ToString()
	<< "\nBatch: " << SecureDebugString(batch);
	prev_index = this_index;
	}
	}

	bool found = false;
	for (int i = 0; i < batch.entry_size(); ++i) {
	LogEntryPB* entry = batch.mutable_entry(i);
	if (!entry->has_replicate()) {
	continue;
	}

	if (entry->replicate().id().index() != index) {
	continue;
	}

	size_t space_required = entry->replicate().SpaceUsedLong();
	if (replicates_tmp.empty() \|\|
	max_bytes_to_read <= 0 \|\|
	total_size + space_required < max_bytes_to_read) {
	total_size += space_required;
	replicates_tmp.push_back(entry->release_replicate());
	} else {
	limit_exceeded = true;
	}
	found = true;
	break;
	}
	CHECK(found) << "Incorrect index entry didn't yield expected log entry: "
	<< index_entry.ToString();

	prev_index_entry = index_entry;
	}

	replicates->swap(replicates_tmp);
	return Status::OK();
	}

	Status LogReader::LookupOpId(int64_t op_index, OpId* op_id) const {
	LogIndexEntry index_entry;
	RETURN_NOT_OK_PREPEND(log_index_->GetEntry(op_index, &index_entry),
	strings::Substitute("Failed to read log index for op $0", op_index));
	*op_id = index_entry.op_id;
	return Status::OK();
	}

	void LogReader::GetSegmentsSnapshot(SegmentSequence* segments) const {
	std::lock_guard<simple_spinlock> lock(lock_);
	CHECK_EQ(state_, kLogReaderReading);
	segments->assign(segments_.begin(), segments_.end());
	}

	void LogReader::TrimSegmentsUpToAndIncluding(int64_t segment_sequence_number) {
	std::lock_guard<simple_spinlock> lock(lock_);
	CHECK_EQ(state_, kLogReaderReading);
	auto iter = segments_.begin();
	int num_deleted_segments = 0;

	while (iter != segments_.end()) {
	if ((*iter)->header().sequence_number() <= segment_sequence_number) {
	iter = segments_.erase(iter);
	num_deleted_segments++;
	continue;
	}
	break;
	}
	LOG(INFO) << "T " << tablet_id_ << ": removed " << num_deleted_segments
	<< " log segments from log reader";
	}

	void LogReader::UpdateLastSegmentOffset(int64_t readable_to_offset) {
	std::lock_guard<simple_spinlock> lock(lock_);
	CHECK_EQ(state_, kLogReaderReading);
	DCHECK(!segments_.empty());
	// Get the last segment
	auto* segment = segments_.back().get();
	DCHECK(!segment->HasFooter());
	segment->UpdateReadableToOffset(readable_to_offset);
	}

	void LogReader::ReplaceLastSegment(scoped_refptr<ReadableLogSegment> segment) {
	// This is used to replace the last segment once we close it properly so it must
	// have a footer.
	DCHECK(segment->HasFooter());

	std::lock_guard<simple_spinlock> lock(lock_);
	CHECK_EQ(state_, kLogReaderReading);
	// Make sure the segment we're replacing has the same sequence number
	DCHECK(!segments_.empty());
	CHECK_EQ(segment->header().sequence_number(), segments_.back()->header().sequence_number());
	segments_[segments_.size() - 1] = std::move(segment);
	}

	Status LogReader::AppendSegment(scoped_refptr<ReadableLogSegment> segment) {
	DCHECK(segment->IsInitialized());
	if (PREDICT_FALSE(!segment->HasFooter())) {
	RETURN_NOT_OK(segment->RebuildFooterByScanning());
	}
	std::lock_guard<simple_spinlock> lock(lock_);
	AppendSegmentUnlocked(std::move(segment));
	return Status::OK();
	}

	void LogReader::AppendSegmentUnlocked(scoped_refptr<ReadableLogSegment> segment) {
	DCHECK(segment->IsInitialized());
	DCHECK(lock_.is_locked());

	#ifndef NDEBUG
	if (!segments_.empty()) {
	DCHECK_EQ(segments_.back()->header().sequence_number() + 1,
	segment->header().sequence_number());
	}
	#endif
	segments_.emplace_back(std::move(segment));
	}

	void LogReader::AppendEmptySegment(scoped_refptr<ReadableLogSegment> segment) {
	DCHECK(segment->IsInitialized());
	std::lock_guard<simple_spinlock> lock(lock_);
	CHECK_EQ(state_, kLogReaderReading);
	AppendSegmentUnlocked(std::move(segment));
	}

	int LogReader::num_segments() const {
	std::lock_guard<simple_spinlock> lock(lock_);
	return segments_.size();
	}

	string LogReader::ToString() const {
	std::lock_guard<simple_spinlock> lock(lock_);
	string ret = "Reader's SegmentSequence: \n";
	for (const SegmentSequence::value_type& entry : segments_) {
	ret.append(Substitute("Segment: $0 Footer: $1\n",
	entry->header().sequence_number(),
	!entry->HasFooter() ? "NONE" : SecureShortDebugString(entry->footer())));
	}
	return ret;
	}

	} // namespace log
	} // namespace kudu