Fix(cleanup): Avoid truncating in value.Open on error (#1465)

* Fix(cleanup): Avoid truncating in value.Open on error

The vlog.Open() function can return an error denoting that the open was
unsuccessful but we have `db.cleanup` which will be called to stop all
the running go routines in case badger couldn't start. The db.cleanup
function calls vlog.Close() which will truncate the maxFid vlog file
based on the vlog.writableLogOffset. The vlog.writableLogOffset was not
being updated in case open failed. As a result of this, we will end up
truncating the vlog file to lenght 0 if open fails.

This PR fixes this by using vlog.stopDiscardStatFlush() instead of vlog.close.

Author: Ibrahim Jarif

db.go

@@ -448,7 +448,10 @@ func (db *DB) cleanup() {
 	}
 
 	db.orc.Stop()
-	db.vlog.Close()
+
+	// Do not use vlog.Close() here. vlog.Close truncates the files. We don't
+	// want to truncate files unless the user has specified the truncate flag.
+	db.vlog.stopFlushDiscardStats()
 }
 
 // BlockCacheMetrics returns the metrics for the underlying block cache.

value.go

@@ -1047,6 +1047,8 @@ func (vlog *valueLog) replayLog(lf *logFile, offset uint32, replayFn logEntry) e
 	// End offset is different from file size. So, we should truncate the file
 	// to that size.
 	if !vlog.opt.Truncate {
+		vlog.db.opt.Warningf("Truncate Needed. File %s size: %d Endoffset: %d",
+			lf.fd.Name(), fi.Size(), endOffset)
 		return ErrTruncateNeeded
 	}
 
@@ -1063,6 +1065,7 @@ func (vlog *valueLog) replayLog(lf *logFile, offset uint32, replayFn logEntry) e
 		return lf.bootstrap()
 	}
 
+	vlog.db.opt.Infof("Truncating vlog file %s to offset: %d", lf.fd.Name(), endOffset)
 	if err := lf.fd.Truncate(int64(endOffset)); err != nil {
 		return errFile(err, lf.path, fmt.Sprintf(
 			"Truncation needed at offset %d. Can be done manually as well.", endOffset))
@@ -1228,6 +1231,12 @@ func (lf *logFile) init() error {
 	return nil
 }
 
+func (vlog *valueLog) stopFlushDiscardStats() {
+	if vlog.lfDiscardStats != nil {
+		vlog.lfDiscardStats.closer.Signal()
+	}
+}
+
 func (vlog *valueLog) Close() error {
 	if vlog == nil || vlog.db == nil || vlog.db.opt.InMemory {
 		return nil
@@ -1245,6 +1254,9 @@ func (vlog *valueLog) Close() error {
 		}
 
 		maxFid := vlog.maxFid
+		// TODO(ibrahim) - Do we need the following truncations on non-windows
+		// platforms? We expand the file only on windows and the vlog.woffset()
+		// should point to end of file on all other platforms.
 		if !vlog.opt.ReadOnly && id == maxFid {
 			// truncate writable log file to correct offset.
 			if truncErr := f.fd.Truncate(