fix: serialize concurrent demo seeding

backend/internal/seed/demo.go

@@ -1,14 +1,18 @@
 package seed
 
 import (
+	"errors"
 	"strings"
 	"time"
 
 	"golang.org/x/crypto/bcrypt"
 	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
 	"senlinai-agent/backend/internal/models"
 )
 
+const demoTransactionAttempts = 3
+
 type DemoOptions struct {
 	Email       string
 	DisplayName string
@@ -35,43 +39,68 @@ func Demo(database *gorm.DB, options DemoOptions) (DemoResult, error) {
 	}
 
 	var result DemoResult
-	err := database.Transaction(func(tx *gorm.DB) error {
-		user, err := upsertUser(tx, email, displayName, password)
-		if err != nil {
-			return err
-		}
-		result.User = user
+	var err error
+	for attempt := 0; attempt < demoTransactionAttempts; attempt++ {
+		result = DemoResult{}
+		err = database.Transaction(func(tx *gorm.DB) error {
+			user, err := upsertAndLockUser(tx, email, displayName, password)
+			if err != nil {
+				return err
+			}
+			result.User = user
 
-		projects, err := seedProjects(tx, user.ID)
-		if err != nil {
-			return err
+			projects, err := seedProjects(tx, user.ID)
+			if err != nil {
+				return err
+			}
+			result.Projects = projects
+			return nil
+		})
+		if err == nil || !isRetryableDemoTransactionError(err) || attempt == demoTransactionAttempts-1 {
+			return result, err
 		}
-		result.Projects = projects
-		return nil
-	})
+		// 重试发生在事务完全回滚之后，避免持锁等待自身的新事务。
+		time.Sleep(time.Duration(attempt+1) * 10 * time.Millisecond)
+	}
 	return result, err
 }
 
-func upsertUser(tx *gorm.DB, email string, displayName string, password string) (models.SenlinAgentUser, error) {
-	var user models.SenlinAgentUser
-	err := tx.Where("email = ?", email).First(&user).Error
-	if err == nil {
-		return user, nil
-	}
-	if err != gorm.ErrRecordNotFound {
-		return user, err
-	}
+func upsertAndLockUser(tx *gorm.DB, email string, displayName string, password string) (models.SenlinAgentUser, error) {
 	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
 	if err != nil {
-		return user, err
+		return models.SenlinAgentUser{}, err
 	}
-	user = models.SenlinAgentUser{
+	candidate := models.SenlinAgentUser{
 		Email:        email,
 		DisplayName:  displayName,
 		PasswordHash: string(hash),
 		Role:         "user",
 	}
-	return user, tx.Create(&user).Error
+	if err := tx.Clauses(clause.OnConflict{
+		Columns:   []clause.Column{{Name: "email"}},
+		DoNothing: true,
+	}).Create(&candidate).Error; err != nil {
+		return models.SenlinAgentUser{}, err
+	}
+
+	// PostgreSQL 会先等待并发的同 email INSERT 提交，再在 READ COMMITTED 的下一条语句中看见该用户。
+	// 锁定 owner 后，项目、标签和子对象的 find-or-create 在不同进程间也会按同一用户串行执行。
+	var user models.SenlinAgentUser
+	err = tx.Clauses(clause.Locking{Strength: "UPDATE"}).Where("email = ?", email).First(&user).Error
+	return user, err
+}
+
+func isRetryableDemoTransactionError(err error) bool {
+	var sqlState interface{ SQLState() string }
+	if !errors.As(err, &sqlState) {
+		return false
+	}
+	switch sqlState.SQLState() {
+	case "40001", "40P01":
+		return true
+	default:
+		return false
+	}
 }
 
 func seedProjects(tx *gorm.DB, ownerID uint) ([]models.SenlinAgentProject, error) {