MB-25156 scan-coordinator: fix stats update panic with drop index

scan_coordinator has race condition in stats update with drop index.
drop index can cleanup the stats while the scan has acquired the snapshot
and will execute the request. In all such cases, the stat update needs
to be skipped.

Change-Id: If03ec2ebce841255074eb7efc578bbb3a0432cf2

secondary/indexer/scan_coordinator.go

@@ -1481,16 +1481,19 @@ func (s *scanCoordinator) serverCallback(protoReq interface{}, conn net.Conn,
 		return
 	}
 
-	req.Stats.scanReqAllocDuration.Add(time.Now().Sub(atime).Nanoseconds())
+	if req.Stats != nil {
+		req.Stats.scanReqAllocDuration.Add(time.Now().Sub(atime).Nanoseconds())
+	}
 
 	if err := s.isScanAllowed(*req.Consistency, req); err != nil {
 		s.tryRespondWithError(w, req, err)
 		return
 	}
 
-	req.Stats.numRequests.Add(1)
-
-	req.Stats.scanReqInitDuration.Add(time.Now().Sub(ttime).Nanoseconds())
+	if req.Stats != nil {
+		req.Stats.numRequests.Add(1)
+		req.Stats.scanReqInitDuration.Add(time.Now().Sub(ttime).Nanoseconds())
+	}
 
 	t0 := time.Now()
 	is, err := s.getRequestedIndexSnapshot(req)
@@ -1506,7 +1509,9 @@ func (s *scanCoordinator) serverCallback(protoReq interface{}, conn net.Conn,
 	})
 
 	defer func() {
-		req.Stats.scanReqDuration.Add(time.Now().Sub(ttime).Nanoseconds())
+		if req.Stats != nil {
+			req.Stats.scanReqDuration.Add(time.Now().Sub(ttime).Nanoseconds())
+		}
 	}()
 
 	s.processRequest(req, w, is, t0)
@@ -1546,10 +1551,12 @@ func (s *scanCoordinator) handleScanRequest(req *ScanRequest, w ScanResponseWrit
 	err := scanPipeline.Execute()
 	scanTime := time.Now().Sub(t0)
 
-	req.Stats.numRowsReturned.Add(int64(scanPipeline.RowsReturned()))
-	req.Stats.scanBytesRead.Add(int64(scanPipeline.BytesRead()))
-	req.Stats.scanDuration.Add(scanTime.Nanoseconds())
-	req.Stats.scanWaitDuration.Add(waitTime.Nanoseconds())
+	if req.Stats != nil {
+		req.Stats.numRowsReturned.Add(int64(scanPipeline.RowsReturned()))
+		req.Stats.scanBytesRead.Add(int64(scanPipeline.BytesRead()))
+		req.Stats.scanDuration.Add(scanTime.Nanoseconds())
+		req.Stats.scanWaitDuration.Add(waitTime.Nanoseconds())
+	}
 
 	if err != nil {
 		status := fmt.Sprintf("(error = %s)", err)