Fix gob encoding the URL

encoding_gob.go

@@ -68,6 +68,19 @@ func gobEncodeItems(col ItemCollection) ([]byte, error) {
 	return b.Bytes(), err
 }
 
+func gobEncodeItemOrLink(it LinkOrIRI) ([]byte, error) {
+	if ob, ok := it.(Item); ok {
+		return gobEncodeItem(ob)
+	}
+	b := bytes.Buffer{}
+	err := OnLink(it, func(l *Link) error {
+		bytes, err := l.GobEncode()
+		b.Write(bytes)
+		return err
+	})
+	return b.Bytes(), err
+}
+
 func gobEncodeItem(it Item) ([]byte, error) {
 	if IsIRI(it) {
 		if i, ok := it.(IRI); ok {
@@ -97,6 +110,7 @@ func gobEncodeItem(it Item) ([]byte, error) {
 				return err
 			})
 		case LinkType, MentionType:
+			// TODO(marius): this shouldn't work, as Link does not implement Item? (or rather, should not)
 			err = OnLink(it, func(l *Link) error {
 				bytes, err := l.GobEncode()
 				b.Write(bytes)
@@ -333,7 +347,7 @@ func mapObjectProperties(mm map[string][]byte, o *Object) (hasData bool, err err
 		hasData = true
 	}
 	if o.URL != nil {
-		if mm["url"], err = gobEncodeItem(o.URL); err != nil {
+		if mm["url"], err = gobEncodeItemOrLink(o.URL); err != nil {
 			return hasData, err
 		}
 		hasData = true

helpers.go

@@ -41,7 +41,7 @@ type WithOrderedCollectionPageFn func(*OrderedCollectionPage) error
 type WithItemCollectionFn func(*ItemCollection) error
 
 // OnLink calls function fn on it Item if it can be asserted to type Link
-func OnLink(it Item, fn WithLinkFn) error {
+func OnLink(it LinkOrIRI, fn WithLinkFn) error {
 	if it == nil {
 		return nil
 	}

object.go

@@ -135,10 +135,10 @@ func (a ActivityVocabularyType) MarshalBinary() ([]byte, error) {
 
 type Objects interface {
 	Object | Tombstone | Place | Profile | Relationship |
-	Activities |
-	IntransitiveActivities |
-	Collections |
-	IRI
+		Activities |
+		IntransitiveActivities |
+		Collections |
+		IRI
 }
 
 // Object describes an ActivityPub object of any kind.
@@ -427,14 +427,14 @@ func (m MimeType) MarshalBinary() ([]byte, error) {
 }
 
 // ToLink returns a Link pointer to the data in the current Item
-func ToLink(it Item) (*Link, error) {
+func ToLink(it LinkOrIRI) (*Link, error) {
 	switch i := it.(type) {
 	case *Link:
 		return i, nil
 	case Link:
 		return &i, nil
 	}
-	return nil, fmt.Errorf("unable to convert %q", it.GetType())
+	return nil, fmt.Errorf("unable to convert %T", it)
 }
 
 // ToObject returns an Object pointer to the data in the current Item