Initialize vlog before starting compactions in db.Open (#1226)

If compaction is started before opening vlog, badger might crash with
nil pointer error when trying to push discard stats at the end of
compactions because discard stats are initialized in vlog open.

This commit fixes it.

db.go

@@ -331,6 +331,9 @@ func Open(opt Options) (db *DB, err error) {
 		return nil, err
 	}
 
+	// Initialize vlog struct.
+	db.vlog.init(db)
+
 	if !opt.ReadOnly {
 		db.closers.compactors = y.NewCloser(1)
 		db.lc.startCompact(db.closers.compactors)

value.go

@@ -1061,13 +1061,15 @@ func (vlog *valueLog) replayLog(lf *logFile, offset uint32, replayFn logEntry) e
 	return nil
 }
 
-func (vlog *valueLog) open(db *DB, ptr valuePointer, replayFn logEntry) error {
+// init initializes the value log struct. This initialization needs to happen
+// before compactions start.
+func (vlog *valueLog) init(db *DB) {
 	vlog.opt = db.opt
 	vlog.db = db
 	// We don't need to open any vlog files or collect stats for GC if DB is opened
 	// in InMemory mode. InMemory mode doesn't create any files/directories on disk.
 	if vlog.opt.InMemory {
-		return nil
+		return
 	}
 	vlog.dirPath = vlog.opt.ValueDir
 	vlog.elog = y.NoEventLog
@@ -1080,6 +1082,15 @@ func (vlog *valueLog) open(db *DB, ptr valuePointer, replayFn logEntry) error {
 		closer:    y.NewCloser(1),
 		flushChan: make(chan map[uint32]int64, 16),
 	}
+}
+
+func (vlog *valueLog) open(db *DB, ptr valuePointer, replayFn logEntry) error {
+	// We don't need to open any vlog files or collect stats for GC if DB is opened
+	// in InMemory mode. InMemory mode doesn't create any files/directories on disk.
+	if db.opt.InMemory {
+		return nil
+	}
+
 	go vlog.flushDiscardStats()
 	if err := vlog.populateFilesMap(); err != nil {
 		return err

value_test.go

@@ -412,6 +412,7 @@ func TestValueGC4(t *testing.T) {
 	err = kv.vlog.Close()
 	require.NoError(t, err)
 
+	kv.vlog.init(kv)
 	err = kv.vlog.open(kv, valuePointer{Fid: 2}, kv.replayFunction())
 	require.NoError(t, err)
 
@@ -642,8 +643,11 @@ func TestPartialAppendToValueLog(t *testing.T) {
 	checkKeys(t, kv, [][]byte{k3})
 	// Replay value log from beginning, badger head is past k2.
 	require.NoError(t, kv.vlog.Close())
-	require.NoError(t,
-		kv.vlog.open(kv, valuePointer{Fid: 0}, kv.replayFunction()))
+
+	kv.vlog.init(kv)
+	require.NoError(
+		t, kv.vlog.open(kv, valuePointer{Fid: 0}, kv.replayFunction()),
+	)
 	require.NoError(t, kv.Close())
 }