fix(subscriptions): reject concurrent duplicate rules

Fixes #66

internal/service/subscription_identity.go

@@ -0,0 +1,89 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+
+	"github.com/ShukeBta/MediaStationGo/internal/model"
+)
+
+var ErrSubscriptionAlreadyExists = errors.New("subscription already exists")
+
+type SubscriptionAlreadyExistsError struct {
+	ExistingID string
+}
+
+func (e *SubscriptionAlreadyExistsError) Error() string {
+	if e == nil || e.ExistingID == "" {
+		return ErrSubscriptionAlreadyExists.Error()
+	}
+	return fmt.Sprintf("%s: %s", ErrSubscriptionAlreadyExists, e.ExistingID)
+}
+
+func (e *SubscriptionAlreadyExistsError) Unwrap() error {
+	return ErrSubscriptionAlreadyExists
+}
+
+func newSubscriptionAlreadyExistsError(existingID string) error {
+	return &SubscriptionAlreadyExistsError{ExistingID: existingID}
+}
+
+func SubscriptionAlreadyExistsID(err error) string {
+	var conflict *SubscriptionAlreadyExistsError
+	if errors.As(err, &conflict) && conflict != nil {
+		return conflict.ExistingID
+	}
+	return ""
+}
+
+func (s *SubscriptionService) subscriptionDuplicate(ctx context.Context, sub *model.Subscription, excludeID string) (*model.Subscription, error) {
+	if s == nil || s.repo == nil || s.repo.Subscription == nil || sub == nil {
+		return nil, nil
+	}
+	return s.repo.Subscription.FindActiveByIdentity(ctx, sub.UserID, sub.IdentityKey, excludeID)
+}
+
+// Update applies API patch fields while recomputing the functional identity.
+// The database partial unique index remains the final concurrency guard.
+func (s *SubscriptionService) Update(ctx context.Context, id string, updates map[string]any) error {
+	if s == nil || s.repo == nil || s.repo.DB == nil {
+		return errors.New("subscription service unavailable")
+	}
+	var sub model.Subscription
+	if err := s.repo.DB.WithContext(ctx).Where("id = ?", id).First(&sub).Error; err != nil {
+		return err
+	}
+	raw, err := json.Marshal(updates)
+	if err != nil {
+		return err
+	}
+	if err := json.Unmarshal(raw, &sub); err != nil {
+		return err
+	}
+	if sub.Name == "" || sub.FeedURL == "" {
+		return errors.New("name and feed_url required")
+	}
+	normalizeSubscriptionDefaults(&sub)
+	model.RefreshSubscriptionIdentity(&sub)
+	if duplicate, err := s.subscriptionDuplicate(ctx, &sub, sub.ID); err != nil {
+		return err
+	} else if duplicate != nil {
+		return newSubscriptionAlreadyExistsError(duplicate.ID)
+	}
+
+	updates["search_mode"] = sub.SearchMode
+	updates["resolution"] = sub.Resolution
+	updates["wash_priority"] = sub.WashPriority
+	updates["priority"] = sub.Priority
+	updates["identity_key"] = sub.IdentityKey
+	if err := s.repo.DB.WithContext(ctx).Model(&model.Subscription{}).
+		Where("id = ?", sub.ID).Updates(updates).Error; err != nil {
+		if duplicate, lookupErr := s.subscriptionDuplicate(ctx, &sub, sub.ID); lookupErr == nil && duplicate != nil {
+			return newSubscriptionAlreadyExistsError(duplicate.ID)
+		}
+		return err
+	}
+	return nil
+}