Federate pinned statuses over ActivityPub (#6610)

* Federate pinned statuses over ActivityPub

* Display pinned toots in web UI

Fix #6117

* Fix migration

* Fix tests

* Update outbox_serializer.rb

* Update remove_serializer.rb

* Update add_serializer.rb

* Update fetch_featured_collection_service.rb

29 files changed, 383 insertions, 37 deletions

diff --git a/app/controllers/activitypub/collections_controller.rb b/app/controllers/activitypub/collections_controller.rb
new file mode 100644
index 000000000..081914016
--- /dev/null
+++ b/app/controllers/activitypub/collections_controller.rb
@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+class ActivityPub::CollectionsController < Api::BaseController
+  include SignatureVerification
+
+  before_action :set_account
+  before_action :set_size
+  before_action :set_statuses
+
+  def show
+    render json: collection_presenter,
+           serializer: ActivityPub::CollectionSerializer,
+           adapter: ActivityPub::Adapter,
+           content_type: 'application/activity+json',
+           skip_activities: true
+  end
+
+  private
+
+  def set_account
+    @account = Account.find_local!(params[:account_username])
+  end
+
+  def set_statuses
+    @statuses = scope_for_collection.paginate_by_max_id(20, params[:max_id], params[:since_id])
+    @statuses = cache_collection(@statuses, Status)
+  end
+
+  def set_size
+    case params[:id]
+    when 'featured'
+      @account.pinned_statuses.count
+    else
+      raise ActiveRecord::NotFound
+    end
+  end
+
+  def scope_for_collection
+    case params[:id]
+    when 'featured'
+      @account.statuses.permitted_for(@account, signed_request_account).tap do |scope|
+        scope.merge!(@account.pinned_statuses)
+      end
+    else
+      raise ActiveRecord::NotFound
+    end
+  end
+
+  def collection_presenter
+    ActivityPub::CollectionPresenter.new(
+      id: account_collection_url(@account, params[:id]),
+      type: :ordered,
+      size: @size,
+      items: @statuses
+    )
+  end
+end
diff --git a/app/controllers/activitypub/outboxes_controller.rb b/app/controllers/activitypub/outboxes_controller.rb
index a431e3557..9ed700c1e 100644
--- a/app/controllers/activitypub/outboxes_controller.rb
+++ b/app/controllers/activitypub/outboxes_controller.rb
@@ -9,7 +9,7 @@ class ActivityPub::OutboxesController < Api::BaseController
     @statuses = @account.statuses.permitted_for(@account, signed_request_account).paginate_by_max_id(20, params[:max_id], params[:since_id])
     @statuses = cache_collection(@statuses, Status)
 
-    render json: outbox_presenter, serializer: ActivityPub::CollectionSerializer, adapter: ActivityPub::Adapter, content_type: 'application/activity+json'
+    render json: outbox_presenter, serializer: ActivityPub::OutboxSerializer, adapter: ActivityPub::Adapter, content_type: 'application/activity+json'
   end
 
   private
diff --git a/app/controllers/api/v1/statuses/pins_controller.rb b/app/controllers/api/v1/statuses/pins_controller.rb
index 3de1009b8..bba6a6f48 100644
--- a/app/controllers/api/v1/statuses/pins_controller.rb
+++ b/app/controllers/api/v1/statuses/pins_controller.rb
@@ -11,12 +11,18 @@ class Api::V1::Statuses::PinsController < Api::BaseController
 
   def create
     StatusPin.create!(account: current_account, status: @status)
+    distribute_add_activity!
     render json: @status, serializer: REST::StatusSerializer
   end
 
   def destroy
     pin = StatusPin.find_by(account: current_account, status: @status)
-    pin&.destroy!
+
+    if pin
+      pin.destroy!
+      distribute_remove_activity!
+    end
+
     render json: @status, serializer: REST::StatusSerializer
   end
 
@@ -25,4 +31,24 @@ class Api::V1::Statuses::PinsController < Api::BaseController
   def set_status
     @status = Status.find(params[:status_id])
   end
+
+  def distribute_add_activity!
+    json = ActiveModelSerializers::SerializableResource.new(
+      @status,
+      serializer: ActivityPub::AddSerializer,
+      adapter: ActivityPub::Adapter
+    ).as_json
+
+    ActivityPub::RawDistributionWorker.perform_async(Oj.dump(json), current_account)
+  end
+
+  def distribute_remove_activity!
+    json = ActiveModelSerializers::SerializableResource.new(
+      @status,
+      serializer: ActivityPub::RemoveSerializer,
+      adapter: ActivityPub::Adapter
+    ).as_json
+
+    ActivityPub::RawDistributionWorker.perform_async(Oj.dump(json), current_account)
+  end
 end
diff --git a/app/javascript/mastodon/actions/timelines.js b/app/javascript/mastodon/actions/timelines.js
index 858a12b15..f0ab16a2d 100644
--- a/app/javascript/mastodon/actions/timelines.js
+++ b/app/javascript/mastodon/actions/timelines.js
@@ -117,13 +117,14 @@ export function refreshTimeline(timelineId, path, params = {}) {
   };
 };
 
-export const refreshHomeTimeline         = () => refreshTimeline('home', '/api/v1/timelines/home');
-export const refreshPublicTimeline       = () => refreshTimeline('public', '/api/v1/timelines/public');
-export const refreshCommunityTimeline    = () => refreshTimeline('community', '/api/v1/timelines/public', { local: true });
-export const refreshAccountTimeline      = (accountId, withReplies) => refreshTimeline(`account:${accountId}${withReplies ? ':with_replies' : ''}`, `/api/v1/accounts/${accountId}/statuses`, { exclude_replies: !withReplies });
-export const refreshAccountMediaTimeline = accountId => refreshTimeline(`account:${accountId}:media`, `/api/v1/accounts/${accountId}/statuses`, { only_media: true });
-export const refreshHashtagTimeline      = hashtag => refreshTimeline(`hashtag:${hashtag}`, `/api/v1/timelines/tag/${hashtag}`);
-export const refreshListTimeline         = id => refreshTimeline(`list:${id}`, `/api/v1/timelines/list/${id}`);
+export const refreshHomeTimeline            = () => refreshTimeline('home', '/api/v1/timelines/home');
+export const refreshPublicTimeline          = () => refreshTimeline('public', '/api/v1/timelines/public');
+export const refreshCommunityTimeline       = () => refreshTimeline('community', '/api/v1/timelines/public', { local: true });
+export const refreshAccountTimeline         = (accountId, withReplies) => refreshTimeline(`account:${accountId}${withReplies ? ':with_replies' : ''}`, `/api/v1/accounts/${accountId}/statuses`, { exclude_replies: !withReplies });
+export const refreshAccountFeaturedTimeline = accountId => refreshTimeline(`account:${accountId}:pinned`, `/api/v1/accounts/${accountId}/statuses`, { pinned: true });
+export const refreshAccountMediaTimeline    = accountId => refreshTimeline(`account:${accountId}:media`, `/api/v1/accounts/${accountId}/statuses`, { only_media: true });
+export const refreshHashtagTimeline         = hashtag => refreshTimeline(`hashtag:${hashtag}`, `/api/v1/timelines/tag/${hashtag}`);
+export const refreshListTimeline            = id => refreshTimeline(`list:${id}`, `/api/v1/timelines/list/${id}`);
 
 export function refreshTimelineFail(timeline, error, skipLoading) {
   return {
diff --git a/app/javascript/mastodon/components/status.js b/app/javascript/mastodon/components/status.js
index c52cd5f09..85baeffca 100644
--- a/app/javascript/mastodon/components/status.js
+++ b/app/javascript/mastodon/components/status.js
@@ -138,7 +138,7 @@ export default class Status extends ImmutablePureComponent {
     let media = null;
     let statusAvatar, prepend;
 
-    const { hidden }     = this.props;
+    const { hidden, featured } = this.props;
     const { isExpanded } = this.state;
 
     let { status, account, ...other } = this.props;
@@ -156,7 +156,14 @@ export default class Status extends ImmutablePureComponent {
       );
     }
 
-    if (status.get('reblog', null) !== null && typeof status.get('reblog') === 'object') {
+    if (featured) {
+      prepend = (
+        <div className='status__prepend'>
+          <div className='status__prepend-icon-wrapper'><i className='fa fa-fw fa-thumb-tack status__prepend-icon' /></div>
+          <FormattedMessage id='status.pinned' defaultMessage='Pinned toot' />
+        </div>
+      );
+    } else if (status.get('reblog', null) !== null && typeof status.get('reblog') === 'object') {
       const display_name_html = { __html: status.getIn(['account', 'display_name_html']) };
 
       prepend = (
diff --git a/app/javascript/mastodon/components/status_list.js b/app/javascript/mastodon/components/status_list.js
index 5acaf714e..eb65838ce 100644
--- a/app/javascript/mastodon/components/status_list.js
+++ b/app/javascript/mastodon/components/status_list.js
@@ -11,6 +11,7 @@ export default class StatusList extends ImmutablePureComponent {
   static propTypes = {
     scrollKey: PropTypes.string.isRequired,
     statusIds: ImmutablePropTypes.list.isRequired,
+    featuredStatusIds: ImmutablePropTypes.list,
     onScrollToBottom: PropTypes.func,
     onScrollToTop: PropTypes.func,
     onScroll: PropTypes.func,
@@ -50,7 +51,7 @@ export default class StatusList extends ImmutablePureComponent {
   }
 
   render () {
-    const { statusIds, ...other }  = this.props;
+    const { statusIds, featuredStatusIds, ...other }  = this.props;
     const { isLoading, isPartial } = other;
 
     if (isPartial) {
@@ -68,8 +69,8 @@ export default class StatusList extends ImmutablePureComponent {
       );
     }
 
-    const scrollableContent = (isLoading || statusIds.size > 0) ? (
-      statusIds.map((statusId) => (
+    let scrollableContent = (isLoading || statusIds.size > 0) ? (
+      statusIds.map(statusId => (
         <StatusContainer
           key={statusId}
           id={statusId}
@@ -79,6 +80,18 @@ export default class StatusList extends ImmutablePureComponent {
       ))
     ) : null;
 
+    if (scrollableContent && featuredStatusIds) {
+      scrollableContent = featuredStatusIds.map(statusId => (
+        <StatusContainer
+          key={`f-${statusId}`}
+          id={statusId}
+          featured
+          onMoveUp={this.handleMoveUp}
+          onMoveDown={this.handleMoveDown}
+        />
+      )).concat(scrollableContent);
+    }
+
     return (
       <ScrollableList {...other} ref={this.setRef}>
         {scrollableContent}
diff --git a/app/javascript/mastodon/features/account_timeline/components/header.js b/app/javascript/mastodon/features/account_timeline/components/header.js
index 5cd4af1d3..b143e1d36 100644
--- a/app/javascript/mastodon/features/account_timeline/components/header.js
+++ b/app/javascript/mastodon/features/account_timeline/components/header.js
@@ -21,6 +21,7 @@ export default class Header extends ImmutablePureComponent {
     onMute: PropTypes.func.isRequired,
     onBlockDomain: PropTypes.func.isRequired,
     onUnblockDomain: PropTypes.func.isRequired,
+    hideTabs: PropTypes.bool,
   };
 
   static contextTypes = {
@@ -68,7 +69,7 @@ export default class Header extends ImmutablePureComponent {
   }
 
   render () {
-    const { account } = this.props;
+    const { account, hideTabs } = this.props;
 
     if (account === null) {
       return <MissingIndicator />;
@@ -94,11 +95,13 @@ export default class Header extends ImmutablePureComponent {
           onUnblockDomain={this.handleUnblockDomain}
         />
 
-        <div className='account__section-headline'>
-          <NavLink exact to={`/accounts/${account.get('id')}`}><FormattedMessage id='account.posts' defaultMessage='Toots' /></NavLink>
-          <NavLink exact to={`/accounts/${account.get('id')}/with_replies`}><FormattedMessage id='account.posts_with_replies' defaultMessage='Toots with replies' /></NavLink>
-          <NavLink exact to={`/accounts/${account.get('id')}/media`}><FormattedMessage id='account.media' defaultMessage='Media' /></NavLink>
-        </div>
+        {!hideTabs && (
+          <div className='account__section-headline'>
+            <NavLink exact to={`/accounts/${account.get('id')}`}><FormattedMessage id='account.posts' defaultMessage='Toots' /></NavLink>
+            <NavLink exact to={`/accounts/${account.get('id')}/with_replies`}><FormattedMessage id='account.posts_with_replies' defaultMessage='Toots with replies' /></NavLink>
+            <NavLink exact to={`/accounts/${account.get('id')}/media`}><FormattedMessage id='account.media' defaultMessage='Media' /></NavLink>
+          </div>
+        )}
       </div>
     );
   }
diff --git a/app/javascript/mastodon/features/account_timeline/index.js b/app/javascript/mastodon/features/account_timeline/index.js
index aed009ef0..95ae5fd06 100644
--- a/app/javascript/mastodon/features/account_timeline/index.js
+++ b/app/javascript/mastodon/features/account_timeline/index.js
@@ -3,7 +3,7 @@ import { connect } from 'react-redux';
 import ImmutablePropTypes from 'react-immutable-proptypes';
 import PropTypes from 'prop-types';
 import { fetchAccount } from '../../actions/accounts';
-import { refreshAccountTimeline, expandAccountTimeline } from '../../actions/timelines';
+import { refreshAccountTimeline, refreshAccountFeaturedTimeline, expandAccountTimeline } from '../../actions/timelines';
 import StatusList from '../../components/status_list';
 import LoadingIndicator from '../../components/loading_indicator';
 import Column from '../ui/components/column';
@@ -17,6 +17,7 @@ const mapStateToProps = (state, { params: { accountId }, withReplies = false })
 
   return {
     statusIds: state.getIn(['timelines', `account:${path}`, 'items'], ImmutableList()),
+    featuredStatusIds: state.getIn(['timelines', `account:${accountId}:pinned`, 'items'], ImmutableList()),
     isLoading: state.getIn(['timelines', `account:${path}`, 'isLoading']),
     hasMore: !!state.getIn(['timelines', `account:${path}`, 'next']),
   };
@@ -29,19 +30,24 @@ export default class AccountTimeline extends ImmutablePureComponent {
     params: PropTypes.object.isRequired,
     dispatch: PropTypes.func.isRequired,
     statusIds: ImmutablePropTypes.list,
+    featuredStatusIds: ImmutablePropTypes.list,
     isLoading: PropTypes.bool,
     hasMore: PropTypes.bool,
     withReplies: PropTypes.bool,
   };
 
   componentWillMount () {
-    this.props.dispatch(fetchAccount(this.props.params.accountId));
-    this.props.dispatch(refreshAccountTimeline(this.props.params.accountId, this.props.withReplies));
+    const { params: { accountId }, withReplies } = this.props;
+
+    this.props.dispatch(fetchAccount(accountId));
+    this.props.dispatch(refreshAccountFeaturedTimeline(accountId));
+    this.props.dispatch(refreshAccountTimeline(accountId, withReplies));
   }
 
   componentWillReceiveProps (nextProps) {
     if ((nextProps.params.accountId !== this.props.params.accountId && nextProps.params.accountId) || nextProps.withReplies !== this.props.withReplies) {
       this.props.dispatch(fetchAccount(nextProps.params.accountId));
+      this.props.dispatch(refreshAccountFeaturedTimeline(nextProps.params.accountId));
       this.props.dispatch(refreshAccountTimeline(nextProps.params.accountId, nextProps.params.withReplies));
     }
   }
@@ -53,7 +59,7 @@ export default class AccountTimeline extends ImmutablePureComponent {
   }
 
   render () {
-    const { statusIds, isLoading, hasMore } = this.props;
+    const { statusIds, featuredStatusIds, isLoading, hasMore } = this.props;
 
     if (!statusIds && isLoading) {
       return (
@@ -71,6 +77,7 @@ export default class AccountTimeline extends ImmutablePureComponent {
           prepend={<HeaderContainer accountId={this.props.params.accountId} />}
           scrollKey='account_timeline'
           statusIds={statusIds}
+          featuredStatusIds={featuredStatusIds}
           isLoading={isLoading}
           hasMore={hasMore}
           onScrollToBottom={this.handleScrollToBottom}
diff --git a/app/javascript/mastodon/features/followers/index.js b/app/javascript/mastodon/features/followers/index.js
index f64ed7948..919a89332 100644
--- a/app/javascript/mastodon/features/followers/index.js
+++ b/app/javascript/mastodon/features/followers/index.js
@@ -80,7 +80,7 @@ export default class Followers extends ImmutablePureComponent {
         <ScrollContainer scrollKey='followers'>
           <div className='scrollable' onScroll={this.handleScroll}>
             <div className='followers'>
-              <HeaderContainer accountId={this.props.params.accountId} />
+              <HeaderContainer accountId={this.props.params.accountId} hideTabs />
               {accountIds.map(id => <AccountContainer key={id} id={id} withNote={false} />)}
               {loadMore}
             </div>
diff --git a/app/javascript/mastodon/features/following/index.js b/app/javascript/mastodon/features/following/index.js
index a0c0fac05..5719259d1 100644
--- a/app/javascript/mastodon/features/following/index.js
+++ b/app/javascript/mastodon/features/following/index.js
@@ -80,7 +80,7 @@ export default class Following extends ImmutablePureComponent {
         <ScrollContainer scrollKey='following'>
           <div className='scrollable' onScroll={this.handleScroll}>
             <div className='following'>
-              <HeaderContainer accountId={this.props.params.accountId} />
+              <HeaderContainer accountId={this.props.params.accountId} hideTabs />
               {accountIds.map(id => <AccountContainer key={id} id={id} withNote={false} />)}
               {loadMore}
             </div>
diff --git a/app/lib/activitypub/activity.rb b/app/lib/activitypub/activity.rb
index 6f4a3b491..9b00f0f52 100644
--- a/app/lib/activitypub/activity.rb
+++ b/app/lib/activitypub/activity.rb
@@ -46,6 +46,10 @@ class ActivityPub::Activity
         ActivityPub::Activity::Reject
       when 'Flag'
         ActivityPub::Activity::Flag
+      when 'Add'
+        ActivityPub::Activity::Add
+      when 'Remove'
+        ActivityPub::Activity::Remove
       end
     end
   end
diff --git a/app/lib/activitypub/activity/add.rb b/app/lib/activitypub/activity/add.rb
new file mode 100644
index 000000000..ea94d2f98
--- /dev/null
+++ b/app/lib/activitypub/activity/add.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class ActivityPub::Activity::Add < ActivityPub::Activity
+  def perform
+    return unless @json['target'].present? && value_or_id(@json['target']) == @account.featured_collection_url
+
+    status = status_from_uri(object_uri)
+
+    return unless status.account_id == @account.id && !@account.pinned?(status)
+
+    StatusPin.create!(account: @account, status: status)
+  end
+end
diff --git a/app/lib/activitypub/activity/remove.rb b/app/lib/activitypub/activity/remove.rb
new file mode 100644
index 000000000..97cee5116
--- /dev/null
+++ b/app/lib/activitypub/activity/remove.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class ActivityPub::Activity::Remove < ActivityPub::Activity
+  def perform
+    return unless @json['origin'].present? && value_or_id(@json['origin']) == @account.featured_collection_url
+
+    status = status_from_uri(object_uri)
+
+    return unless status.account_id == @account.id
+
+    pin = StatusPin.find_by(account: @account, status: status)
+    pin&.destroy!
+  end
+end
diff --git a/app/lib/activitypub/adapter.rb b/app/lib/activitypub/adapter.rb
index 8198ac580..f19b04ae6 100644
--- a/app/lib/activitypub/adapter.rb
+++ b/app/lib/activitypub/adapter.rb
@@ -18,6 +18,7 @@ class ActivityPub::Adapter < ActiveModelSerializers::Adapter::Base
         'toot'                      => 'http://joinmastodon.org/ns#',
         'Emoji'                     => 'toot:Emoji',
         'focalPoint'                => { '@container' => '@list', '@id' => 'toot:focalPoint' },
+        'featured'                  => 'toot:featured',
       },
     ],
   }.freeze
diff --git a/app/models/account.rb b/app/models/account.rb
index 13692d0d7..c1347fe65 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -43,6 +43,7 @@
 #  protocol                :integer          default("ostatus"), not null
 #  memorial                :boolean          default(FALSE), not null
 #  moved_to_account_id     :integer
+#  featured_collection_url :string
 #
 
 class Account < ApplicationRecord
diff --git a/app/serializers/activitypub/actor_serializer.rb b/app/serializers/activitypub/actor_serializer.rb
index 622bdde0c..afcd37771 100644
--- a/app/serializers/activitypub/actor_serializer.rb
+++ b/app/serializers/activitypub/actor_serializer.rb
@@ -4,7 +4,7 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :id, :type, :following, :followers,
-             :inbox, :outbox,
+             :inbox, :outbox, :featured,
              :preferred_username, :name, :summary,
              :url, :manually_approves_followers
 
@@ -53,6 +53,10 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
     account_outbox_url(object)
   end
 
+  def featured
+    account_collection_url(object, :featured)
+  end
+
   def endpoints
     object
   end
diff --git a/app/serializers/activitypub/add_serializer.rb b/app/serializers/activitypub/add_serializer.rb
new file mode 100644
index 000000000..a5f091e37
--- /dev/null
+++ b/app/serializers/activitypub/add_serializer.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class ActivityPub::AddSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :type, :actor, :target
+  attribute :proper_object, key: :object
+
+  def type
+    'Add'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+
+  def proper_object
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def target
+    account_collection_url(object, :featured)
+  end
+end
diff --git a/app/serializers/activitypub/collection_serializer.rb b/app/serializers/activitypub/collection_serializer.rb
index d43af3f8e..1ae492945 100644
--- a/app/serializers/activitypub/collection_serializer.rb
+++ b/app/serializers/activitypub/collection_serializer.rb
@@ -2,7 +2,7 @@
 
 class ActivityPub::CollectionSerializer < ActiveModel::Serializer
   def self.serializer_for(model, options)
-    return ActivityPub::ActivitySerializer if model.class.name == 'Status'
+    return ActivityPub::NoteSerializer if model.class.name == 'Status'
     return ActivityPub::CollectionSerializer if model.class.name == 'ActivityPub::CollectionPresenter'
     super
   end
diff --git a/app/serializers/activitypub/outbox_serializer.rb b/app/serializers/activitypub/outbox_serializer.rb
new file mode 100644
index 000000000..48fbad0fd
--- /dev/null
+++ b/app/serializers/activitypub/outbox_serializer.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class ActivityPub::OutboxSerializer < ActivityPub::CollectionSerializer
+  def self.serializer_for(model, options)
+    return ActivityPub::ActivitySerializer if model.is_a?(Status)
+    super
+  end
+end
diff --git a/app/serializers/activitypub/remove_serializer.rb b/app/serializers/activitypub/remove_serializer.rb
new file mode 100644
index 000000000..6da7e35d3
--- /dev/null
+++ b/app/serializers/activitypub/remove_serializer.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+class ActivityPub::RemoveSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :type, :actor, :origin
+  attribute :proper_object, key: :object
+
+  def type
+    'Remove'
+  end
+
+  def actor
+    ActivityPub::TagManager.instance.uri_for(object.account)
+  end
+
+  def proper_object
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def origin
+    account_collection_url(object, :featured)
+  end
+end
diff --git a/app/services/activitypub/fetch_featured_collection_service.rb b/app/services/activitypub/fetch_featured_collection_service.rb
new file mode 100644
index 000000000..40714e980
--- /dev/null
+++ b/app/services/activitypub/fetch_featured_collection_service.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchFeaturedCollectionService < BaseService
+  include JsonLdHelper
+
+  def call(account)
+    @account = account
+    @json    = fetch_resource(@account.featured_collection_url, true)
+
+    return unless supported_context?
+    return if @account.suspended? || @account.local?
+
+    case @json['type']
+    when 'Collection', 'CollectionPage'
+      process_items @json['items']
+    when 'OrderedCollection', 'OrderedCollectionPage'
+      process_items @json['orderedItems']
+    end
+  end
+
+  private
+
+  def process_items(items)
+    status_ids = items.map { |item| value_or_id(item) }
+                      .reject { |uri| ActivityPub::TagManager.instance.local_uri?(uri) }
+                      .map { |uri| ActivityPub::FetchRemoteStatusService.new.call(uri) }
+                      .compact
+                      .select { |status| status.account_id == @account.id }
+                      .map(&:id)
+
+    to_remove = []
+    to_add    = status_ids
+
+    StatusPin.where(account: @account).pluck(:status_id).each do |status_id|
+      if status_ids.include?(status_id)
+        to_add.delete(status_id)
+      else
+        to_remove << status_id
+      end
+    end
+
+    StatusPin.where(account: @account, status_id: to_remove).delete_all unless to_remove.empty?
+
+    to_add.each do |status_id|
+      StatusPin.create!(account: @account, status_id: status_id)
+    end
+  end
+
+  def supported_context?
+    super(@json)
+  end
+end
diff --git a/app/services/activitypub/process_account_service.rb b/app/services/activitypub/process_account_service.rb
index f43edafe7..68e9db766 100644
--- a/app/services/activitypub/process_account_service.rb
+++ b/app/services/activitypub/process_account_service.rb
@@ -27,6 +27,7 @@ class ActivityPub::ProcessAccountService < BaseService
 
     after_protocol_change! if protocol_changed?
     after_key_change! if key_changed?
+    check_featured_collection! if @account.featured_collection_url.present?
 
     @account
   rescue Oj::ParseError
@@ -57,14 +58,15 @@ class ActivityPub::ProcessAccountService < BaseService
   end
 
   def set_immediate_attributes!
-    @account.inbox_url        = @json['inbox'] || ''
-    @account.outbox_url       = @json['outbox'] || ''
-    @account.shared_inbox_url = (@json['endpoints'].is_a?(Hash) ? @json['endpoints']['sharedInbox'] : @json['sharedInbox']) || ''
-    @account.followers_url    = @json['followers'] || ''
-    @account.url              = url || @uri
-    @account.display_name     = @json['name'] || ''
-    @account.note             = @json['summary'] || ''
-    @account.locked           = @json['manuallyApprovesFollowers'] || false
+    @account.inbox_url               = @json['inbox'] || ''
+    @account.outbox_url              = @json['outbox'] || ''
+    @account.shared_inbox_url        = (@json['endpoints'].is_a?(Hash) ? @json['endpoints']['sharedInbox'] : @json['sharedInbox']) || ''
+    @account.followers_url           = @json['followers'] || ''
+    @account.featured_collection_url = @json['featured'] || ''
+    @account.url                     = url || @uri
+    @account.display_name            = @json['name'] || ''
+    @account.note                    = @json['summary'] || ''
+    @account.locked                  = @json['manuallyApprovesFollowers'] || false
   end
 
   def set_fetchable_attributes!
@@ -85,6 +87,10 @@ class ActivityPub::ProcessAccountService < BaseService
     RefollowWorker.perform_async(@account.id)
   end
 
+  def check_featured_collection!
+    ActivityPub::SynchronizeFeaturedCollectionWorker.perform_async(@account.id)
+  end
+
   def image_url(key)
     value = first_of_value(@json[key])
 
diff --git a/app/workers/activitypub/synchronize_featured_collection_worker.rb b/app/workers/activitypub/synchronize_featured_collection_worker.rb
new file mode 100644
index 000000000..dd676a3ee
--- /dev/null
+++ b/app/workers/activitypub/synchronize_featured_collection_worker.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class ActivityPub::SynchronizeFeaturedCollectionWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'pull'
+
+  def perform(account_id)
+    ActivityPub::FetchFeaturedCollectionService.new.call(Account.find(account_id))
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+end
diff --git a/config/routes.rb b/config/routes.rb
index 7ed2d61f1..0542cb680 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -58,8 +58,10 @@ Rails.application.routes.draw do
     resources :following, only: [:index], controller: :following_accounts
     resource :follow, only: [:create], controller: :account_follow
     resource :unfollow, only: [:create], controller: :account_unfollow
+
     resource :outbox, only: [:show], module: :activitypub
     resource :inbox, only: [:create], module: :activitypub
+    resources :collections, only: [:show], module: :activitypub
   end
 
   resource :inbox, only: [:create], module: :activitypub
diff --git a/db/migrate/20180304013859_add_featured_collection_url_to_accounts.rb b/db/migrate/20180304013859_add_featured_collection_url_to_accounts.rb
new file mode 100644
index 000000000..e0b8ed5cc
--- /dev/null
+++ b/db/migrate/20180304013859_add_featured_collection_url_to_accounts.rb
@@ -0,0 +1,5 @@
+class AddFeaturedCollectionUrlToAccounts < ActiveRecord::Migration[5.1]
+  def change
+    add_column :accounts, :featured_collection_url, :string
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index c6b81df34..c52a6f0d4 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180211015820) do
+ActiveRecord::Schema.define(version: 20180304013859) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -73,6 +73,7 @@ ActiveRecord::Schema.define(version: 20180211015820) do
     t.integer "protocol", default: 0, null: false
     t.boolean "memorial", default: false, null: false
     t.bigint "moved_to_account_id"
+    t.string "featured_collection_url"
     t.index "(((setweight(to_tsvector('simple'::regconfig, (display_name)::text), 'A'::\"char\") || setweight(to_tsvector('simple'::regconfig, (username)::text), 'B'::\"char\")) || setweight(to_tsvector('simple'::regconfig, (COALESCE(domain, ''::character varying))::text), 'C'::\"char\")))", name: "search_index", using: :gin
     t.index "lower((username)::text), lower((domain)::text)", name: "index_accounts_on_username_and_domain_lower"
     t.index ["uri"], name: "index_accounts_on_uri"
diff --git a/spec/lib/activitypub/activity/add_spec.rb b/spec/lib/activitypub/activity/add_spec.rb
new file mode 100644
index 000000000..3ebab4e37
--- /dev/null
+++ b/spec/lib/activitypub/activity/add_spec.rb
@@ -0,0 +1,29 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::Activity::Add do
+  let(:sender) { Fabricate(:account, featured_collection_url: 'https://example.com/featured') }
+  let(:status) { Fabricate(:status, account: sender) }
+
+  let(:json) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      id: 'foo',
+      type: 'Add',
+      actor: ActivityPub::TagManager.instance.uri_for(sender),
+      object: ActivityPub::TagManager.instance.uri_for(status),
+      target: sender.featured_collection_url,
+    }.with_indifferent_access
+  end
+
+  describe '#perform' do
+    subject { described_class.new(json, sender) }
+
+    before do
+      subject.perform
+    end
+
+    it 'creates a pin' do
+      expect(sender.pinned?(status)).to be true
+    end
+  end
+end
diff --git a/spec/lib/activitypub/activity/remove_spec.rb b/spec/lib/activitypub/activity/remove_spec.rb
new file mode 100644
index 000000000..c3f015053
--- /dev/null
+++ b/spec/lib/activitypub/activity/remove_spec.rb
@@ -0,0 +1,30 @@
+require 'rails_helper'
+
+RSpec.describe ActivityPub::Activity::Remove do
+  let(:sender) { Fabricate(:account, featured_collection_url: 'https://example.com/featured') }
+  let(:status) { Fabricate(:status, account: sender) }
+
+  let(:json) do
+    {
+      '@context': 'https://www.w3.org/ns/activitystreams',
+      id: 'foo',
+      type: 'Add',
+      actor: ActivityPub::TagManager.instance.uri_for(sender),
+      object: ActivityPub::TagManager.instance.uri_for(status),
+      origin: sender.featured_collection_url,
+    }.with_indifferent_access
+  end
+
+  describe '#perform' do
+    subject { described_class.new(json, sender) }
+
+    before do
+      StatusPin.create!(account: sender, status: status)
+      subject.perform
+    end
+
+    it 'removes a pin' do
+      expect(sender.pinned?(status)).to be false
+    end
+  end
+end
diff --git a/spec/lib/activitypub/activity/update_spec.rb b/spec/lib/activitypub/activity/update_spec.rb
index ea308e35c..fbfc585cf 100644
--- a/spec/lib/activitypub/activity/update_spec.rb
+++ b/spec/lib/activitypub/activity/update_spec.rb
@@ -7,6 +7,7 @@ RSpec.describe ActivityPub::Activity::Update do
     stub_request(:get, actor_json[:outbox]).to_return(status: 404)
     stub_request(:get, actor_json[:followers]).to_return(status: 404)
     stub_request(:get, actor_json[:following]).to_return(status: 404)
+    stub_request(:get, actor_json[:featured]).to_return(status: 404)
 
     sender.update!(uri: ActivityPub::TagManager.instance.uri_for(sender))
   end