diff --git a/pkg/cron/cron_test.go b/pkg/cron/cron_test.go
deleted file mode 100644
index f949a58..0000000
--- a/pkg/cron/cron_test.go
+++ /dev/null
@@ -1,25 +0,0 @@
-package cron
-
-import (
-	"sync"
-	"testing"
-	"time"
-
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
-)
-
-func TestCron(t *testing.T) {
-	var wg sync.WaitGroup
-	now := time.Now()
-	next := now.Add(time.Second * 4)
-	wg.Add(1)
-	cron := New()
-	err := cron.AddJob("test", "@every 3s", func() {
-		wg.Done()
-	})
-	require.NoError(t, err)
-	wg.Wait()
-	now2 := time.Now()
-	assert.True(t, now2.After(now) && now2.Before(next))
-}
diff --git a/pkg/docker/fake/cli.go b/pkg/docker/fake/cli.go
index 00e6559..8ac8785 100644
--- a/pkg/docker/fake/cli.go
+++ b/pkg/docker/fake/cli.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/docker/docker/api/types"
@@ -15,10 +16,13 @@ import (
 )
 
 type Client struct {
+	mu         sync.Mutex
 	containers []types.Container
 }
 
 func (f *Client) RunContainer(ctx context.Context, config *container.Config, hostConfig *container.HostConfig, networkingConfig *network.NetworkingConfig, containerName string) (id string, err error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	for _, ct := range f.containers {
 		if ct.Names[0] == containerName {
 			return "", errdefs.Conflict(errors.New("container already exists"))
@@ -40,6 +44,8 @@ func (f *Client) PullImage(ctx context.Context, image string) error {
 }
 
 func (f *Client) WaitContainer(ctx context.Context, id string) (int, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	for i, ct := range f.containers {
 		if ct.ID == id {
 			time.Sleep(5 * time.Second)
@@ -51,6 +57,8 @@ func (f *Client) WaitContainer(ctx context.Context, id string) (int, error) {
 }
 
 func (f *Client) RemoveContainerWithTimeout(id string, timeout time.Duration) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	cts := make([]types.Container, 0, len(f.containers))
 	for _, ct := range f.containers {
 		if ct.ID != id {
@@ -62,6 +70,8 @@ func (f *Client) RemoveContainerWithTimeout(id string, timeout time.Duration) er
 }
 
 func (f *Client) ListContainersWithTimeout(running bool, timeout time.Duration) ([]types.Container, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
 	return f.containers, nil
 }
 
diff --git a/pkg/server/main.go b/pkg/server/main.go
index 10a92bc..556faa3 100644
--- a/pkg/server/main.go
+++ b/pkg/server/main.go
@@ -55,8 +55,10 @@ func New(configPath string) (*Server, error) {
 }
 
 func NewWithConfig(cfg Config) (*Server, error) {
+	// TODO: enforce shared cache mode?
 	db, err := gorm.Open(sqlite.Open(cfg.DbURL), &gorm.Config{
-		QueryFields: true,
+		QueryFields:            true,
+		SkipDefaultTransaction: true,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("open db: %w", err)
diff --git a/pkg/server/main_test.go b/pkg/server/main_test.go
index 33e0c08..3a38494 100644
--- a/pkg/server/main_test.go
+++ b/pkg/server/main_test.go
@@ -56,9 +56,15 @@ func NewTestEnv(t *testing.T) *TestEnv {
 		_ = os.Remove(dbFile.Name())
 	})
 	db, err := gorm.Open(sqlite.Open(dbFile.Name()), &gorm.Config{
-		QueryFields: true,
+		QueryFields:            true,
+		SkipDefaultTransaction: true,
 	})
 	require.NoError(t, err)
+	sqlDB, err := db.DB()
+	require.NoError(t, err)
+	// To resolve the "database is locked" error.
+	// See also https://github.com/mattn/go-sqlite3/issues/209
+	sqlDB.SetMaxOpenConns(1)
 	require.NoError(t, model.AutoMigrate(db))
 
 	s := &Server{
diff --git a/pkg/server/utils.go b/pkg/server/utils.go
index a6337a4..b6d067c 100644
--- a/pkg/server/utils.go
+++ b/pkg/server/utils.go
@@ -97,7 +97,6 @@ func (s *Server) waitForSync(name, ctID, storageDir string) {
 			code = -2
 		}
 	}
-	s.syncingContainers.Delete(name)
 	err = s.dockerCli.RemoveContainerWithTimeout(ctID, time.Second*20)
 	if err != nil {
 		l.Error("Fail to remove container", slogErrAttr(err))
@@ -119,6 +118,10 @@ func (s *Server) waitForSync(name, ctID, storageDir string) {
 		l.Error("Fail to update RepoMeta", slogErrAttr(err))
 	}
 
+	// NOTE: Only change status after RepoMeta is updated, b/c we need to determine
+	// if synchronization is completed in unit test, and then verify RepoMeta.
+	s.syncingContainers.Delete(name)
+
 	if len(s.config.PostSync) == 0 {
 		return
 	}