wal: Export SegmentSizeBytes as a variable

In test situations, it's useful to create smaller than usual WAL files
to test rotation and to avoid the overhead of preallocation on old-style
filesystems that don't handle it efficiently. This commit changes
segmentSizeBytes to an exported variable so that tests can override it
from an init() function.

wal/wal.go

@@ -41,16 +41,18 @@ const (
 	crcType
 	snapshotType
 
-	// the expected size of each wal segment file.
-	// the actual size might be bigger than it.
-	segmentSizeBytes = 64 * 1000 * 1000 // 64MB
-
 	// warnSyncDuration is the amount of time allotted to an fsync before
 	// logging a warning
 	warnSyncDuration = time.Second
 )
 
 var (
+	// SegmentSizeBytes is the preallocated size of each wal segment file.
+	// The actual size might be larger than this. In general, the default
+	// value should be used, but this is defined as an exported variable
+	// so that tests can set a different segment size.
+	SegmentSizeBytes int64 = 64 * 1000 * 1000 // 64MB
+
 	plog = capnslog.NewPackageLogger("github.com/coreos/etcd", "wal")
 
 	ErrMetadataConflict = errors.New("wal: conflicting metadata found")
@@ -109,7 +111,7 @@ func Create(dirpath string, metadata []byte) (*WAL, error) {
 	if _, err := f.Seek(0, os.SEEK_END); err != nil {
 		return nil, err
 	}
-	if err := fileutil.Preallocate(f.File, segmentSizeBytes, true); err != nil {
+	if err := fileutil.Preallocate(f.File, SegmentSizeBytes, true); err != nil {
 		return nil, err
 	}
 
@@ -219,7 +221,7 @@ func openAtIndex(dirpath string, snap walpb.Snapshot, write bool) (*WAL, error)
 			closer()
 			return nil, err
 		}
-		w.fp = newFilePipeline(w.dir, segmentSizeBytes)
+		w.fp = newFilePipeline(w.dir, SegmentSizeBytes)
 	}
 
 	return w, nil
@@ -526,7 +528,7 @@ func (w *WAL) Save(st raftpb.HardState, ents []raftpb.Entry) error {
 	if err != nil {
 		return err
 	}
-	if curOff < segmentSizeBytes {
+	if curOff < SegmentSizeBytes {
 		if mustSync {
 			return w.sync()
 		}