Improve ChangePack creation logic for p.Clear() when document detach

server/backend/database/database.go

@@ -219,6 +219,13 @@ type Database interface {
 		docRefKey types.DocRefKey,
 	) error
 
+	// FindChangeInfoByServerSeq returns the change by the given server sequence.
+	FindChangeInfoByServerSeq(
+		ctx context.Context,
+		docRefKey types.DocRefKey,
+		serverSeq int64,
+	) (*ChangeInfo, error)
+
 	// FindChangesBetweenServerSeqs returns the changes between two server sequences.
 	FindChangesBetweenServerSeqs(
 		ctx context.Context,

server/backend/database/memory/database.go

@@ -1012,6 +1012,28 @@ func (d *DB) PurgeStaleChanges(
 	return nil
 }
 
+// FindChangeInfoByServerSeq returns the change by the given server sequence.
+func (d *DB) FindChangeInfoByServerSeq(
+	_ context.Context,
+	docRefKey types.DocRefKey,
+	serverSeq int64,
+) (*database.ChangeInfo, error) {
+	txn := d.db.Txn(false)
+	defer txn.Abort()
+	raw, err := txn.First(tblSnapshots, "doc_id_server_seq",
+		docRefKey.DocID.String(),
+		serverSeq,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("find snapshot by serverSeq: %w", err)
+	}
+	if raw == nil {
+		return nil, fmt.Errorf("%s: %w", docRefKey, database.ErrChangeNotFound)
+	}
+
+	return raw.(*database.ChangeInfo).DeepCopy(), nil
+}
+
 // FindChangesBetweenServerSeqs returns the changes between two server sequences.
 func (d *DB) FindChangesBetweenServerSeqs(
 	ctx context.Context,

server/backend/database/mongo/client.go

@@ -977,6 +977,33 @@ func (c *Client) PurgeStaleChanges(
 	return nil
 }
 
+// FindChangeInfoByServerSeq returns the change by the given server sequence.
+func (c *Client) FindChangeInfoByServerSeq(
+	ctx context.Context,
+	docRefKey types.DocRefKey,
+	serverSeq int64,
+) (*database.ChangeInfo, error) {
+	result := c.collection(ColChanges).FindOne(ctx, bson.M{
+		"project_id": docRefKey.ProjectID,
+		"doc_id":     docRefKey.DocID,
+		"server_seq": serverSeq,
+	})
+
+	changeInfo := &database.ChangeInfo{}
+	if result.Err() == mongo.ErrNoDocuments {
+		return changeInfo, nil
+	}
+	if result.Err() != nil {
+		return nil, fmt.Errorf("find change: %w", result.Err())
+	}
+
+	if err := result.Decode(changeInfo); err != nil {
+		return nil, fmt.Errorf("decode change: %w", err)
+	}
+
+	return changeInfo, nil
+}
+
 // FindChangesBetweenServerSeqs returns the changes between two server sequences.
 func (c *Client) FindChangesBetweenServerSeqs(
 	ctx context.Context,

server/rpc/cluster_server.go

@@ -25,7 +25,8 @@ import (
 	"github.com/yorkie-team/yorkie/api/types"
 	api "github.com/yorkie-team/yorkie/api/yorkie/v1"
 	"github.com/yorkie-team/yorkie/pkg/document"
-	"github.com/yorkie-team/yorkie/pkg/document/json"
+	"github.com/yorkie-team/yorkie/pkg/document/change"
+	"github.com/yorkie-team/yorkie/pkg/document/innerpresence"
 	"github.com/yorkie-team/yorkie/pkg/document/presence"
 	"github.com/yorkie-team/yorkie/pkg/document/time"
 	"github.com/yorkie-team/yorkie/server/backend"
@@ -93,29 +94,32 @@ func (s *clusterServer) DetachDocument(
 		return nil, err
 	}
 
-	// TODO(hackerwins): BuildDocForCheckpoint is expensive because it reads the entire document.
-	// We need to optimize this by creating a ChangePack directly.
-	// 01. Create ChangePack with clear presence.
-	doc, err := packs.BuildDocForCheckpoint(ctx, s.backend, docInfo, clientInfo.Checkpoint(summary.ID), actorID)
+	// 02. Create changePack with presence clear change
+	cp := clientInfo.Checkpoint(summary.ID)
+	latestChange, err := s.backend.DB.FindChangeInfoByServerSeq(ctx, docRefKey, cp.ServerSeq)
 	if err != nil {
 		return nil, err
 	}
-
-	if err := doc.Update(func(root *json.Object, p *presence.Presence) error {
-		p.Clear()
-		return nil
-	}); err != nil {
-		return nil, err
-	}
-
-	// 02. PushPull with the created ChangePack.
+	changeCtx := change.NewContext(
+		change.NewID(cp.ClientSeq, cp.ServerSeq, latestChange.Lamport, actorID, latestChange.VersionVector).Next(),
+		"",
+		nil,
+	)
+	p := presence.New(changeCtx, innerpresence.NewPresence())
+	p.Clear()
+
+	changes := []*change.Change{changeCtx.ToChange()}
+	pack := change.NewPack(docInfo.Key, cp, changes, nil, nil)
+
+	// 03. PushPull with the created ChangePack.
 	if _, err := packs.PushPull(
 		ctx,
 		s.backend,
 		project,
 		clientInfo,
 		docInfo,
-		doc.CreateChangePack(),
+		//doc.CreateChangePack(),
+		pack,
 		packs.PushPullOptions{
 			Mode:   types.SyncModePushPull,
 			Status: document.StatusDetached,