Update head while replaying value log (#1372)

Fixes #1363

The head pointer is not updated when we perform replays. The head
pointer would be updated only when the replay completes. If badger crashes
between the point when replay started and replay finished, we would end up
replaying all the value log files. This PR fixes this issue.

db.go

@@ -136,6 +136,10 @@ func (db *DB) replayFunction() func(Entry, valuePointer) error {
 		} else {
 			nv = vp.Encode()
 			meta = meta | bitValuePointer
+			// Update vhead. If the crash happens while replay was in progess
+			// and the head is not updated, we will end up replaying all the
+			// files again.
+			db.updateHead([]valuePointer{vp})
 		}
 
 		v := y.ValueStruct{
@@ -671,6 +675,8 @@ func (db *DB) get(key []byte) (y.ValueStruct, error) {
 	return db.lc.get(key, maxVs, 0)
 }
 
+// updateHead should not be called without the db.Lock() since db.vhead is used
+// by the writer go routines and memtable flushing goroutine.
 func (db *DB) updateHead(ptrs []valuePointer) {
 	var ptr valuePointer
 	for i := len(ptrs) - 1; i >= 0; i-- {
@@ -684,8 +690,6 @@ func (db *DB) updateHead(ptrs []valuePointer) {
 		return
 	}
 
-	db.Lock()
-	defer db.Unlock()
 	y.AssertTrue(!ptr.Less(db.vhead))
 	db.vhead = ptr
 }
@@ -784,7 +788,9 @@ func (db *DB) writeRequests(reqs []*request) error {
 			done(err)
 			return errors.Wrap(err, "writeRequests")
 		}
+		db.Lock()
 		db.updateHead(b.Ptrs)
+		db.Unlock()
 	}
 	done(nil)
 	db.opt.Debugf("%d entries written", count)

value_test.go

@@ -370,7 +370,6 @@ func TestValueGC4(t *testing.T) {
 
 	kv, err := Open(opt)
 	require.NoError(t, err)
-	defer kv.Close()
 
 	sz := 128 << 10 // 5 entries per value log file.
 	txn := kv.NewTransaction(true)
@@ -409,11 +408,9 @@ func TestValueGC4(t *testing.T) {
 	kv.vlog.rewrite(lf0, tr)
 	kv.vlog.rewrite(lf1, tr)
 
-	err = kv.vlog.Close()
-	require.NoError(t, err)
+	require.NoError(t, kv.Close())
 
-	kv.vlog.init(kv)
-	err = kv.vlog.open(kv, valuePointer{Fid: 2}, kv.replayFunction())
+	kv, err = Open(opt)
 	require.NoError(t, err)
 
 	for i := 0; i < 8; i++ {
@@ -435,6 +432,7 @@ func TestValueGC4(t *testing.T) {
 			return nil
 		}))
 	}
+	require.NoError(t, kv.Close())
 }
 
 func TestPersistLFDiscardStats(t *testing.T) {
@@ -646,6 +644,11 @@ func TestPartialAppendToValueLog(t *testing.T) {
 	// Replay value log from beginning, badger head is past k2.
 	require.NoError(t, kv.vlog.Close())
 
+	// clean up the current db.vhead so that we can replay from the beginning.
+	// If we don't clear the current vhead, badger will error out since new
+	// head passed while opening vlog is zero in the following lines.
+	kv.vhead = valuePointer{}
+
 	kv.vlog.init(kv)
 	require.NoError(
 		t, kv.vlog.open(kv, valuePointer{Fid: 0}, kv.replayFunction()),