修复会话安全与实时消息

server/internal/model/message.go

@@ -0,0 +1,34 @@
+package model
+
+import (
+	"errors"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+)
+
+var ErrSessionNotFound = errors.New("会话不存在")
+
+// CreateMessage 在会话行锁保护下分配消息序号，避免并发写入出现重复序号。
+func CreateMessage(message *Message) error {
+	return DB.Transaction(func(tx *gorm.DB) error {
+		var session Session
+		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).First(&session, message.SessionID).Error; err != nil {
+			if errors.Is(err, gorm.ErrRecordNotFound) {
+				return ErrSessionNotFound
+			}
+			return err
+		}
+
+		var maxSeq int
+		if err := tx.Model(&Message{}).
+			Where("session_id = ?", message.SessionID).
+			Select("COALESCE(MAX(seq), 0)").
+			Scan(&maxSeq).Error; err != nil {
+			return err
+		}
+
+		message.Seq = maxSeq + 1
+		return tx.Create(message).Error
+	})
+}