Fix bugs reading and checking for certain files (siglens#1667)

* Fix checking for a file

* Avoid panic reading empty sst file

pkg/blob/local/localstorage.go

@@ -164,9 +164,18 @@ func SetBlobAsInUse(fName string) error {
 }
 
 /*
-Returns if the file exists in the local SegSetKeys struct
+Returns true if the file is on disk or in the local SegSetKeys struct
 */
 func IsFilePresentOnLocal(fName string) bool {
+	_, err := os.Stat(fName)
+	if err == nil {
+		return true
+	}
+	if !os.IsNotExist(err) {
+		// We're not sure if we have the file, so don't early exit.
+		log.Errorf("IsFilePresentOnLocal: Error checking file %s: %v", fName, err)
+	}
+
 	segSetKeysLock.Lock()
 	defer segSetKeysLock.Unlock()
 	if _, exists := segSetKeys[fName]; exists {

pkg/segment/reader/segread/segstatsreader.go

@@ -26,7 +26,6 @@ import (
 	"github.com/siglens/siglens/pkg/blob"
 	"github.com/siglens/siglens/pkg/segment/structs"
 	"github.com/siglens/siglens/pkg/segment/utils"
-
 	toputils "github.com/siglens/siglens/pkg/utils"
 	log "github.com/sirupsen/logrus"
 )
@@ -54,6 +53,10 @@ func ReadSegStats(segkey string, qid uint64) (map[string]*structs.SegStats, erro
 		}
 	}()
 
+	if len(fdata) == 0 {
+		return nil, toputils.TeeErrorf("qid=%d, ReadSegStats: empty sst file: %v", qid, fName)
+	}
+
 	rIdx := uint32(0)
 
 	// version