fix nil panic in AddToList()

internal/processing/list/updateentries.go

@@ -52,8 +52,8 @@ func (p *Processor) AddToList(ctx context.Context, account *gtsmodel.Account, li
 		return gtserror.NewErrorInternalError(err)
 	}
 
-	// Convert the follows to a map keyed by the target account ID.
-	followsMap := util.KeyBy(follows, func(follow *gtsmodel.Follow) string {
+	// Convert the follows to a hash set containing the target account IDs.
+	inFollows := util.ToSetFunc(follows, func(follow *gtsmodel.Follow) string {
 		return follow.TargetAccountID
 	})
 
@@ -65,14 +65,30 @@ func (p *Processor) AddToList(ctx context.Context, account *gtsmodel.Account, li
 	// Iterate all the account IDs in given target list.
 	for _, targetAccountID := range targetAccountIDs {
 
-		// Look for existing follow targetting ID.
-		follow, ok := followsMap[targetAccountID]
-
-		if ok {
+		// Look for follow to target account.
+		if inFollows.Has(targetAccountID) {
 			text := fmt.Sprintf("account %s is already in list %s", targetAccountID, listID)
 			return gtserror.NewErrorUnprocessableEntity(errors.New(text), text)
 		}
 
+		// Get the actual follow to target.
+		follow, err := p.state.DB.GetFollow(
+
+			// We don't need any sub-models.
+			gtscontext.SetBarebones(ctx),
+			account.ID,
+			targetAccountID,
+		)
+		if err != nil && !errors.Is(err, db.ErrNoEntries) {
+			err := gtserror.Newf("db error getting follow: %w", err)
+			return gtserror.NewErrorInternalError(err)
+		}
+
+		if follow == nil {
+			text := fmt.Sprintf("account %s not currently followed", targetAccountID)
+			return gtserror.NewErrorUnprocessableEntity(errors.New(text), text)
+		}
+
 		// Generate new entry for this follow in list.
 		entries = append(entries, &gtsmodel.ListEntry{
 			ID:       id.NewULID(),

internal/util/unique.go

@@ -42,6 +42,18 @@ func ToSet[T comparable](in []T) Set[T] {
 	return set
 }
 
+// ToSetFunc creates a Set[T] from input slice, keys provided by func.
+func ToSetFunc[S any, T comparable](in []S, key func(S) T) Set[T] {
+	if key == nil {
+		panic("nil func")
+	}
+	set := make(Set[T], len(in))
+	for _, v := range in {
+		set[key(v)] = struct{}{}
+	}
+	return set
+}
+
 // FromSet extracts the values from set to slice.
 func FromSet[T comparable](in Set[T]) []T {
 	out := make([]T, len(in))