feat(canvas): 画布同步 - 新增操作事务、预演保存与并发恢复能力

backend/internal/repository/canvas_mutation_receipts.go

@@ -0,0 +1,47 @@
+package repository
+
+import (
+	"errors"
+	"time"
+
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+	"yingce/backend/internal/model"
+)
+
+// LockCanvasMutation serializes all operations for an existing canvas, including
+// deletion. On SQLite this first write also avoids a deferred read transaction
+// trying to upgrade its stale snapshot to a write lock.
+func (r *Repository) LockCanvasMutation(userID, canvasID string) error {
+	return r.db.Model(&model.CanvasProject{}).Where("id = ? AND user_id = ?", canvasID, userID).
+		UpdateColumn("revision", gorm.Expr("revision")).Error
+}
+
+func (r *Repository) CanvasMutationReceiptForUser(userID, canvasID, mutationID string) (*model.CanvasMutationReceipt, error) {
+	var receipt model.CanvasMutationReceipt
+	err := r.db.First(&receipt, "user_id = ? AND canvas_id = ? AND mutation_id = ?", userID, canvasID, mutationID).Error
+	return &receipt, err
+}
+
+func (r *Repository) TryCreateCanvasMutationReceipt(receipt *model.CanvasMutationReceipt) (bool, error) {
+	if receipt.CreatedAt.IsZero() {
+		receipt.CreatedAt = time.Now().UTC()
+	}
+	result := r.db.Clauses(clause.OnConflict{Columns: []clause.Column{{Name: "user_id"}, {Name: "canvas_id"}, {Name: "mutation_id"}}, DoNothing: true}).Create(receipt)
+	return result.RowsAffected == 1, result.Error
+}
+
+func (r *Repository) FinalizeCanvasMutationReceipt(receipt *model.CanvasMutationReceipt) error {
+	if receipt.ResultJSON == "" {
+		return errors.New("canvas mutation receipt requires a committed result")
+	}
+	result := r.db.Model(&model.CanvasMutationReceipt{}).Where("id = ? AND payload_digest = ?", receipt.ID, receipt.PayloadDigest).
+		Updates(map[string]any{"result_json": receipt.ResultJSON, "revision": receipt.Revision})
+	if result.Error != nil {
+		return result.Error
+	}
+	if result.RowsAffected != 1 {
+		return errors.New("canvas mutation receipt was not finalized")
+	}
+	return nil
+}