package pub import ( "context" "encoding/json" "fmt" "github.com/go-fed/activity/streams" "github.com/go-fed/activity/streams/vocab" "net/url" ) // OnFollowBehavior enumerates the different default actions that the go-fed // library can provide when receiving a Follow Activity from a peer. type OnFollowBehavior int const ( // OnFollowDoNothing does not take any action when a Follow Activity // is received. OnFollowDoNothing OnFollowBehavior = iota // OnFollowAutomaticallyAccept triggers the side effect of sending an // Accept of this Follow request in response. OnFollowAutomaticallyAccept // OnFollowAutomaticallyAccept triggers the side effect of sending a // Reject of this Follow request in response. OnFollowAutomaticallyReject ) // FederatingWrappedCallbacks lists the callback functions that already have // some side effect behavior provided by the pub library. // // These functions are wrapped for the Federating Protocol. type FederatingWrappedCallbacks struct { // Create handles additional side effects for the Create ActivityStreams // type, specific to the application using go-fed. // // The wrapping callback for the Federating Protocol ensures the // 'object' property is created in the database. // // Create calls Create for each object in the federated Activity. Create func(context.Context, vocab.ActivityStreamsCreate) error // Update handles additional side effects for the Update ActivityStreams // type, specific to the application using go-fed. // // The wrapping callback for the Federating Protocol ensures the // 'object' property is updated in the database. // // Update calls Update on the federated entry from the database, with a // new value. Update func(context.Context, vocab.ActivityStreamsUpdate) error // Delete handles additional side effects for the Delete ActivityStreams // type, specific to the application using go-fed. // // Delete removes the federated entry from the database. Delete func(context.Context, vocab.ActivityStreamsDelete) error // Follow handles additional side effects for the Follow ActivityStreams // type, specific to the application using go-fed. // // The wrapping function can have one of several default behaviors, // depending on the value of the OnFollow setting. Follow func(context.Context, vocab.ActivityStreamsFollow) error // OnFollow determines what action to take for this particular callback // if a Follow Activity is handled. OnFollow OnFollowBehavior // Accept handles additional side effects for the Accept ActivityStreams // type, specific to the application using go-fed. // // The wrapping function determines if this 'Accept' is in response to a // 'Follow'. If so, then the 'actor' is added to the original 'actor's // 'following' collection. // // Otherwise, no side effects are done by go-fed. Accept func(context.Context, vocab.ActivityStreamsAccept) error // Reject handles additional side effects for the Reject ActivityStreams // type, specific to the application using go-fed. // // The wrapping function has no default side effects. However, if this // 'Reject' is in response to a 'Follow' then the client MUST NOT go // forward with adding the 'actor' to the original 'actor's 'following' // collection by the client application. Reject func(context.Context, vocab.ActivityStreamsReject) error // Add handles additional side effects for the Add ActivityStreams // type, specific to the application using go-fed. // // The wrapping function will add the 'object' IRIs to a specific // 'target' collection if the 'target' collection(s) live on this // server. Add func(context.Context, vocab.ActivityStreamsAdd) error // Remove handles additional side effects for the Remove ActivityStreams // type, specific to the application using go-fed. // // The wrapping function will remove all 'object' IRIs from a specific // 'target' collection if the 'target' collection(s) live on this // server. Remove func(context.Context, vocab.ActivityStreamsRemove) error // Like handles additional side effects for the Like ActivityStreams // type, specific to the application using go-fed. // // The wrapping function will add the activity to the "likes" collection // on all 'object' targets owned by this server. Like func(context.Context, vocab.ActivityStreamsLike) error // Announce handles additional side effects for the Announce // ActivityStreams type, specific to the application using go-fed. // // The wrapping function will add the activity to the "shares" // collection on all 'object' targets owned by this server. Announce func(context.Context, vocab.ActivityStreamsAnnounce) error // Undo handles additional side effects for the Undo ActivityStreams // type, specific to the application using go-fed. // // The wrapping function ensures the 'actor' on the 'Undo' // is be the same as the 'actor' on all Activities being undone. // It enforces that the actors on the Undo must correspond to all of the // 'object' actors in some manner. // // It is expected that the application will implement the proper // reversal of activities that are being undone. Undo func(context.Context, vocab.ActivityStreamsUndo) error // Block handles additional side effects for the Block ActivityStreams // type, specific to the application using go-fed. // // The wrapping function provides no default side effects. It simply // calls the wrapped function. However, note that Blocks should not be // received from a federated peer, as delivering Blocks explicitly // deviates from the original ActivityPub specification. Block func(context.Context, vocab.ActivityStreamsBlock) error // Sidechannel data -- this is set at request handling time. These must // be set before the callbacks are used. // db is the Database the FederatingWrappedCallbacks should use. db Database // inboxIRI is the inboxIRI that is handling this callback. inboxIRI *url.URL // addNewIds creates new 'id' entries on an activity and its objects if // it is a Create activity. addNewIds func(c context.Context, activity Activity) error // deliver delivers an outgoing message. deliver func(c context.Context, outboxIRI *url.URL, activity Activity) error // newTransport creates a new Transport. newTransport func(c context.Context, actorBoxIRI *url.URL, gofedAgent string) (t Transport, err error) } // callbacks returns the WrappedCallbacks members into a single interface slice // for use in streams.Resolver callbacks. // // If the given functions have a type that collides with the default behavior, // then disable our default behavior func (w FederatingWrappedCallbacks) callbacks(fns []interface{}) []interface{} { enableCreate := true enableUpdate := true enableDelete := true enableFollow := true enableAccept := true enableReject := true enableAdd := true enableRemove := true enableLike := true enableAnnounce := true enableUndo := true enableBlock := true for _, fn := range fns { switch fn.(type) { default: continue case func(context.Context, vocab.ActivityStreamsCreate) error: enableCreate = false case func(context.Context, vocab.ActivityStreamsUpdate) error: enableUpdate = false case func(context.Context, vocab.ActivityStreamsDelete) error: enableDelete = false case func(context.Context, vocab.ActivityStreamsFollow) error: enableFollow = false case func(context.Context, vocab.ActivityStreamsAccept) error: enableAccept = false case func(context.Context, vocab.ActivityStreamsReject) error: enableReject = false case func(context.Context, vocab.ActivityStreamsAdd) error: enableAdd = false case func(context.Context, vocab.ActivityStreamsRemove) error: enableRemove = false case func(context.Context, vocab.ActivityStreamsLike) error: enableLike = false case func(context.Context, vocab.ActivityStreamsAnnounce) error: enableAnnounce = false case func(context.Context, vocab.ActivityStreamsUndo) error: enableUndo = false case func(context.Context, vocab.ActivityStreamsBlock) error: enableBlock = false } } if enableCreate { fns = append(fns, w.create) } if enableUpdate { fns = append(fns, w.update) } if enableDelete { fns = append(fns, w.deleteFn) } if enableFollow { fns = append(fns, w.follow) } if enableAccept { fns = append(fns, w.accept) } if enableReject { fns = append(fns, w.reject) } if enableAdd { fns = append(fns, w.add) } if enableRemove { fns = append(fns, w.remove) } if enableLike { fns = append(fns, w.like) } if enableAnnounce { fns = append(fns, w.announce) } if enableUndo { fns = append(fns, w.undo) } if enableBlock { fns = append(fns, w.block) } return fns } // create implements the federating Create activity side effects. func (w FederatingWrappedCallbacks) create(c context.Context, a vocab.ActivityStreamsCreate) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } // Create anonymous loop function to be able to properly scope the defer // for the database lock at each iteration. loopFn := func(iter vocab.ActivityStreamsObjectPropertyIterator) error { t := iter.GetType() if t == nil && iter.IsIRI() { // Attempt to dereference the IRI instead tport, err := w.newTransport(c, w.inboxIRI, goFedUserAgent()) if err != nil { return err } b, err := tport.Dereference(c, iter.GetIRI()) if err != nil { return err } var m map[string]interface{} if err = json.Unmarshal(b, &m); err != nil { return err } t, err = streams.ToType(c, m) if err != nil { return err } } else if t == nil { return fmt.Errorf("cannot handle federated create: object is neither a value nor IRI") } id, err := GetId(t) if err != nil { return err } err = w.db.Lock(c, id) if err != nil { return err } defer w.db.Unlock(c, id) if err := w.db.Create(c, t); err != nil { return err } return nil } for iter := op.Begin(); iter != op.End(); iter = iter.Next() { if err := loopFn(iter); err != nil { return err } } if w.Create != nil { return w.Create(c, a) } return nil } // update implements the federating Update activity side effects. func (w FederatingWrappedCallbacks) update(c context.Context, a vocab.ActivityStreamsUpdate) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } if err := mustHaveActivityOriginMatchObjects(a); err != nil { return err } // Create anonymous loop function to be able to properly scope the defer // for the database lock at each iteration. loopFn := func(iter vocab.ActivityStreamsObjectPropertyIterator) error { t := iter.GetType() if t == nil { return fmt.Errorf("update requires an object to be wholly provided") } id, err := GetId(t) if err != nil { return err } err = w.db.Lock(c, id) if err != nil { return err } defer w.db.Unlock(c, id) if err := w.db.Update(c, t); err != nil { return err } return nil } for iter := op.Begin(); iter != op.End(); iter = iter.Next() { if err := loopFn(iter); err != nil { return err } } if w.Update != nil { return w.Update(c, a) } return nil } // deleteFn implements the federating Delete activity side effects. func (w FederatingWrappedCallbacks) deleteFn(c context.Context, a vocab.ActivityStreamsDelete) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } if err := mustHaveActivityOriginMatchObjects(a); err != nil { return err } // Create anonymous loop function to be able to properly scope the defer // for the database lock at each iteration. loopFn := func(iter vocab.ActivityStreamsObjectPropertyIterator) error { id, err := ToId(iter) if err != nil { return err } err = w.db.Lock(c, id) if err != nil { return err } defer w.db.Unlock(c, id) if err := w.db.Delete(c, id); err != nil { return err } return nil } for iter := op.Begin(); iter != op.End(); iter = iter.Next() { if err := loopFn(iter); err != nil { return err } } if w.Delete != nil { return w.Delete(c, a) } return nil } // follow implements the federating Follow activity side effects. func (w FederatingWrappedCallbacks) follow(c context.Context, a vocab.ActivityStreamsFollow) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } // Check that we own at least one of the 'object' properties, and ensure // it is to the actor that owns this inbox. // // If not then don't send a response. It was federated to us as an FYI, // by mistake, or some other reason. if err := w.db.Lock(c, w.inboxIRI); err != nil { return err } // WARNING: Unlock not deferred. actorIRI, err := w.db.ActorForInbox(c, w.inboxIRI) if err != nil { w.db.Unlock(c, w.inboxIRI) return err } w.db.Unlock(c, w.inboxIRI) // Unlock must be called by now and every branch above. isMe := false if w.OnFollow != OnFollowDoNothing { for iter := op.Begin(); iter != op.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { return err } if id.String() == actorIRI.String() { isMe = true break } } } if isMe { // Prepare the response. var response Activity if w.OnFollow == OnFollowAutomaticallyAccept { response = streams.NewActivityStreamsAccept() } else if w.OnFollow == OnFollowAutomaticallyReject { response = streams.NewActivityStreamsReject() } else { return fmt.Errorf("unknown OnFollowBehavior: %d", w.OnFollow) } // Set us as the 'actor'. me := streams.NewActivityStreamsActorProperty() response.SetActivityStreamsActor(me) me.AppendIRI(actorIRI) // Set the Follow as the 'object' property. op := streams.NewActivityStreamsObjectProperty() response.SetActivityStreamsObject(op) op.AppendActivityStreamsFollow(a) // Add all actors on the original Follow to the 'to' property. recipients := make([]*url.URL, 0) to := streams.NewActivityStreamsToProperty() response.SetActivityStreamsTo(to) followActors := a.GetActivityStreamsActor() for iter := followActors.Begin(); iter != followActors.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { return err } to.AppendIRI(id) recipients = append(recipients, id) } if w.OnFollow == OnFollowAutomaticallyAccept { // If automatically accepting, then also update our // followers collection with the new actors. // // If automatically rejecting, do not update the // followers collection. if err := w.db.Lock(c, actorIRI); err != nil { return err } // WARNING: Unlock not deferred. followers, err := w.db.Followers(c, actorIRI) if err != nil { w.db.Unlock(c, actorIRI) return err } items := followers.GetActivityStreamsItems() if items == nil { items = streams.NewActivityStreamsItemsProperty() followers.SetActivityStreamsItems(items) } for _, elem := range recipients { items.PrependIRI(elem) } if err = w.db.Update(c, followers); err != nil { w.db.Unlock(c, actorIRI) return err } w.db.Unlock(c, actorIRI) // Unlock must be called by now and every branch above. } // Lock without defer! w.db.Lock(c, w.inboxIRI) outboxIRI, err := w.db.OutboxForInbox(c, w.inboxIRI) if err != nil { w.db.Unlock(c, w.inboxIRI) return err } w.db.Unlock(c, w.inboxIRI) // Everything must be unlocked by now. if err := w.addNewIds(c, response); err != nil { return err } else if err := w.deliver(c, outboxIRI, response); err != nil { return err } } if w.Follow != nil { return w.Follow(c, a) } return nil } // accept implements the federating Accept activity side effects. func (w FederatingWrappedCallbacks) accept(c context.Context, a vocab.ActivityStreamsAccept) error { op := a.GetActivityStreamsObject() if op != nil && op.Len() > 0 { // Get this actor's id. if err := w.db.Lock(c, w.inboxIRI); err != nil { return err } // WARNING: Unlock not deferred. actorIRI, err := w.db.ActorForInbox(c, w.inboxIRI) if err != nil { w.db.Unlock(c, w.inboxIRI) return err } w.db.Unlock(c, w.inboxIRI) // Unlock must be called by now and every branch above. // // Determine if we are in a follow on the 'object' property. // // TODO: Handle Accept multiple Follow. var maybeMyFollowIRI *url.URL for iter := op.Begin(); iter != op.End(); iter = iter.Next() { t := iter.GetType() if t == nil && iter.IsIRI() { // Attempt to dereference the IRI instead tport, err := w.newTransport(c, w.inboxIRI, goFedUserAgent()) if err != nil { return err } b, err := tport.Dereference(c, iter.GetIRI()) if err != nil { return err } var m map[string]interface{} if err = json.Unmarshal(b, &m); err != nil { return err } t, err = streams.ToType(c, m) if err != nil { return err } } else if t == nil { return fmt.Errorf("cannot handle federated create: object is neither a value nor IRI") } // Ensure it is a Follow. if !streams.IsOrExtendsActivityStreamsFollow(t) { continue } follow, ok := t.(Activity) if !ok { return fmt.Errorf("a Follow in an Accept does not satisfy the Activity interface") } followId, err := GetId(follow) if err != nil { return err } // Ensure that we are one of the actors on the Follow. actors := follow.GetActivityStreamsActor() for iter := actors.Begin(); iter != actors.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { return err } if id.String() == actorIRI.String() { maybeMyFollowIRI = followId break } } // Continue breaking if we found ourselves if maybeMyFollowIRI != nil { break } } // If we received an Accept whose 'object' is a Follow with an // Accept that we sent, add to the following collection. if maybeMyFollowIRI != nil { // Verify our Follow request exists and the peer didn't // fabricate it. activityActors := a.GetActivityStreamsActor() if activityActors == nil || activityActors.Len() == 0 { return fmt.Errorf("an Accept with a Follow has no actors") } // This may be a duplicate check if we dereferenced the // Follow above. TODO: Separate this logic to avoid // redundancy. // // Use an anonymous function to properly scope the // database lock, immediately call it. err = func() error { if err := w.db.Lock(c, maybeMyFollowIRI); err != nil { return err } defer w.db.Unlock(c, maybeMyFollowIRI) t, err := w.db.Get(c, maybeMyFollowIRI) if err != nil { return err } if !streams.IsOrExtendsActivityStreamsFollow(t) { return fmt.Errorf("peer gave an Accept wrapping a Follow but provided a non-Follow id") } follow, ok := t.(Activity) if !ok { return fmt.Errorf("a Follow in an Accept does not satisfy the Activity interface") } // Ensure that we are one of the actors on the Follow. ok = false actors := follow.GetActivityStreamsActor() for iter := actors.Begin(); iter != actors.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { return err } if id.String() == actorIRI.String() { ok = true break } } if !ok { return fmt.Errorf("peer gave an Accept wrapping a Follow but we are not the actor on that Follow") } // Build map of original Accept actors acceptActors := make(map[string]bool) for iter := activityActors.Begin(); iter != activityActors.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { return err } acceptActors[id.String()] = false } // Verify all actor(s) were on the original Follow. followObj := follow.GetActivityStreamsObject() for iter := followObj.Begin(); iter != followObj.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { return err } if _, ok := acceptActors[id.String()]; ok { acceptActors[id.String()] = true } } for _, found := range acceptActors { if !found { return fmt.Errorf("peer gave an Accept wrapping a Follow but was not an object in the original Follow") } } return nil }() if err != nil { return err } // Add the peer to our following collection. if err := w.db.Lock(c, actorIRI); err != nil { return err } // WARNING: Unlock not deferred. following, err := w.db.Following(c, actorIRI) if err != nil { w.db.Unlock(c, actorIRI) return err } items := following.GetActivityStreamsItems() if items == nil { items = streams.NewActivityStreamsItemsProperty() following.SetActivityStreamsItems(items) } for iter := activityActors.Begin(); iter != activityActors.End(); iter = iter.Next() { id, err := ToId(iter) if err != nil { w.db.Unlock(c, actorIRI) return err } items.PrependIRI(id) } if err = w.db.Update(c, following); err != nil { w.db.Unlock(c, actorIRI) return err } w.db.Unlock(c, actorIRI) // Unlock must be called by now and every branch above. } } if w.Accept != nil { return w.Accept(c, a) } return nil } // reject implements the federating Reject activity side effects. func (w FederatingWrappedCallbacks) reject(c context.Context, a vocab.ActivityStreamsReject) error { if w.Reject != nil { return w.Reject(c, a) } return nil } // add implements the federating Add activity side effects. func (w FederatingWrappedCallbacks) add(c context.Context, a vocab.ActivityStreamsAdd) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } target := a.GetActivityStreamsTarget() if target == nil || target.Len() == 0 { return ErrTargetRequired } if err := add(c, op, target, w.db); err != nil { return err } if w.Add != nil { return w.Add(c, a) } return nil } // remove implements the federating Remove activity side effects. func (w FederatingWrappedCallbacks) remove(c context.Context, a vocab.ActivityStreamsRemove) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } target := a.GetActivityStreamsTarget() if target == nil || target.Len() == 0 { return ErrTargetRequired } if err := remove(c, op, target, w.db); err != nil { return err } if w.Remove != nil { return w.Remove(c, a) } return nil } // like implements the federating Like activity side effects. func (w FederatingWrappedCallbacks) like(c context.Context, a vocab.ActivityStreamsLike) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } id, err := GetId(a) if err != nil { return err } // Create anonymous loop function to be able to properly scope the defer // for the database lock at each iteration. loopFn := func(iter vocab.ActivityStreamsObjectPropertyIterator) error { objId, err := ToId(iter) if err != nil { return err } if err := w.db.Lock(c, objId); err != nil { return err } defer w.db.Unlock(c, objId) if owns, err := w.db.Owns(c, objId); err != nil { return err } else if !owns { return nil } t, err := w.db.Get(c, objId) if err != nil { return err } l, ok := t.(likeser) if !ok { return fmt.Errorf("cannot add Like to likes collection for type %T", t) } // Get 'likes' property on the object, creating default if // necessary. likes := l.GetActivityStreamsLikes() if likes == nil { likes = streams.NewActivityStreamsLikesProperty() l.SetActivityStreamsLikes(likes) } // Get 'likes' value, defaulting to a collection. likesT := likes.GetType() if likesT == nil { col := streams.NewActivityStreamsCollection() likesT = col likes.SetActivityStreamsCollection(col) } // Prepend the activity's 'id' on the 'likes' Collection or // OrderedCollection. if col, ok := likesT.(itemser); ok { items := col.GetActivityStreamsItems() if items == nil { items = streams.NewActivityStreamsItemsProperty() col.SetActivityStreamsItems(items) } items.PrependIRI(id) } else if oCol, ok := likesT.(orderedItemser); ok { oItems := oCol.GetActivityStreamsOrderedItems() if oItems == nil { oItems = streams.NewActivityStreamsOrderedItemsProperty() oCol.SetActivityStreamsOrderedItems(oItems) } oItems.PrependIRI(id) } else { return fmt.Errorf("likes type is neither a Collection nor an OrderedCollection: %T", likesT) } err = w.db.Update(c, t) if err != nil { return err } return nil } for iter := op.Begin(); iter != op.End(); iter = iter.Next() { if err := loopFn(iter); err != nil { return err } } if w.Like != nil { return w.Like(c, a) } return nil } // announce implements the federating Announce activity side effects. func (w FederatingWrappedCallbacks) announce(c context.Context, a vocab.ActivityStreamsAnnounce) error { id, err := GetId(a) if err != nil { return err } op := a.GetActivityStreamsObject() // Create anonymous loop function to be able to properly scope the defer // for the database lock at each iteration. loopFn := func(iter vocab.ActivityStreamsObjectPropertyIterator) error { objId, err := ToId(iter) if err != nil { return err } if err := w.db.Lock(c, objId); err != nil { return err } defer w.db.Unlock(c, objId) if owns, err := w.db.Owns(c, objId); err != nil { return err } else if !owns { return nil } t, err := w.db.Get(c, objId) if err != nil { return err } s, ok := t.(shareser) if !ok { return fmt.Errorf("cannot add Announce to Shares collection for type %T", t) } // Get 'shares' property on the object, creating default if // necessary. shares := s.GetActivityStreamsShares() if shares == nil { shares = streams.NewActivityStreamsSharesProperty() s.SetActivityStreamsShares(shares) } // Get 'shares' value, defaulting to a collection. sharesT := shares.GetType() if sharesT == nil { col := streams.NewActivityStreamsCollection() sharesT = col shares.SetActivityStreamsCollection(col) } // Prepend the activity's 'id' on the 'shares' Collection or // OrderedCollection. if col, ok := sharesT.(itemser); ok { items := col.GetActivityStreamsItems() if items == nil { items = streams.NewActivityStreamsItemsProperty() col.SetActivityStreamsItems(items) } items.PrependIRI(id) } else if oCol, ok := sharesT.(orderedItemser); ok { oItems := oCol.GetActivityStreamsOrderedItems() if oItems == nil { oItems = streams.NewActivityStreamsOrderedItemsProperty() oCol.SetActivityStreamsOrderedItems(oItems) } oItems.PrependIRI(id) } else { return fmt.Errorf("shares type is neither a Collection nor an OrderedCollection: %T", sharesT) } err = w.db.Update(c, t) if err != nil { return err } return nil } if op != nil { for iter := op.Begin(); iter != op.End(); iter = iter.Next() { if err := loopFn(iter); err != nil { return err } } } if w.Announce != nil { return w.Announce(c, a) } return nil } // undo implements the federating Undo activity side effects. func (w FederatingWrappedCallbacks) undo(c context.Context, a vocab.ActivityStreamsUndo) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } actors := a.GetActivityStreamsActor() if err := mustHaveActivityActorsMatchObjectActors(c, actors, op, w.newTransport, w.inboxIRI); err != nil { return err } if w.Undo != nil { return w.Undo(c, a) } return nil } // block implements the federating Block activity side effects. func (w FederatingWrappedCallbacks) block(c context.Context, a vocab.ActivityStreamsBlock) error { op := a.GetActivityStreamsObject() if op == nil || op.Len() == 0 { return ErrObjectRequired } if w.Block != nil { return w.Block(c, a) } return nil }