ENG-3612: Fix to execute GetSegmentsSnapshot using lock

Summary:
There still could be a case when LogReader becomes null during remote bootstrap and shutdown process.
So instead of calling GetSegmentsSnapshot directly from the remote bootstrap session, we introduce intermediate
GetSegmentsSnapshot in Log, that will make sure the reader is alive while GetSegmentsSnapshot is executed.

Test Plan: ybd --cxx-test redisserver_redisserver-test --gtest_filter TestRedisService.AbortQueueOnShutdown -n 1000 -- -p 12

Reviewers: mikhail, bogdan, bharat, hector

Reviewed By: hector

Subscribers: ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D5322

src/yb/consensus/log.cc

@@ -862,6 +862,15 @@ LogReader* Log::GetLogReader() const {
   return reader_.get();
 }
 
+Status Log::GetSegmentsSnapshot(SegmentSequence* segments) const {
+  boost::shared_lock<rw_spinlock> read_lock(state_lock_.get_lock());
+  if (!reader_) {
+    return STATUS(IllegalState, "Log already closed");
+  }
+
+  return reader_->GetSegmentsSnapshot(segments);
+}
+
 uint64_t Log::OnDiskSize() {
   SegmentSequence segments;
   {

src/yb/consensus/log.h

@@ -161,6 +161,8 @@ class Log : public RefCountedThreadSafe<Log> {
   // guaranteed to be live as long as the log itself is initialized and live.
   LogReader* GetLogReader() const;
 
+  CHECKED_STATUS GetSegmentsSnapshot(SegmentSequence* segments) const;
+
   void SetMaxSegmentSizeForTests(uint64_t max_segment_size) {
     max_segment_size_ = max_segment_size;
   }

src/yb/tserver/remote_bootstrap_session.cc

@@ -259,7 +259,7 @@ Status RemoteBootstrapSession::Init() {
   // Get the current segments from the log, including the active segment.
   // The Log doesn't add the active segment to the log reader's list until
   // a header has been written to it (but it will not have a footer).
-  RETURN_NOT_OK(tablet_peer_->log()->GetLogReader()->GetSegmentsSnapshot(&log_segments_));
+  RETURN_NOT_OK(tablet_peer_->log()->GetSegmentsSnapshot(&log_segments_));
   for (const scoped_refptr<ReadableLogSegment>& segment : log_segments_) {
     RETURN_NOT_OK(OpenLogSegmentUnlocked(segment->header().sequence_number()));
   }