synchronize remote posts on first follow

app/services/activitypub/fetch_account_statuses_service.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchAccountStatusesService < BaseService
+  include JsonLdHelper
+
+  MAX_PAGES = 50
+
+  def call(account, url = nil)
+    @account = account
+    return if account.local? || account.suspended?
+
+    page = 1
+
+    @json = fetch_collection(url || account.outbox_url)
+    @items = Rails.cache.fetch("account_sync:#{account.id}") || []
+
+    if @items.empty?
+      until page == MAX_PAGES || @json.blank?
+        items = collection_items(@json).select { |item| item['type'] == 'Create' }
+        @items.concat(items)
+        break if @json['next'].blank?
+        page += 1
+        @json = fetch_collection(@json['next'])
+      end
+    end
+
+    Rails.cache.write("account_sync:#{account.id}", @items, expires_in: 1.day)
+    process_items(@items)
+    Rails.cache.delete("account_sync:#{account.id}")
+
+    @items
+  end
+
+  private
+
+  def fetch_collection(collection_or_uri)
+    return collection_or_uri if collection_or_uri.is_a?(Hash)
+
+    collection = _fetch_collection(collection_or_uri)
+    return unless collection.is_a?(Hash)
+
+    if collection['first'].present?
+      collection = _fetch_collection(collection['first'])
+      return unless collection.is_a?(Hash)
+    end
+
+    collection
+  end
+
+  def _fetch_collection(collection_or_uri)
+    return collection_or_uri if collection_or_uri.is_a?(Hash)
+    return if invalid_origin?(collection_or_uri)
+    fetch_resource_without_id_validation(collection_or_uri, nil, true)
+  end
+
+  def collection_items(collection)
+    case collection['type']
+    when 'Collection', 'CollectionPage'
+      collection['items']
+    when 'OrderedCollection', 'OrderedCollectionPage'
+      collection['orderedItems']
+    end
+  end
+
+  def process_items(items)
+    items.reverse_each.map { |item| process_item(item) }.compact
+  end
+
+  def process_item(item)
+    return unless item.is_a?(Hash) && item['type'].present?
+    ActivityPub::Activity.factory(item, @account, override_timestamps: true, requested: true)&.perform
+  rescue => e
+    Rails.logger.error("Failed to process #{item['type']} #{item['id']} due to #{e}: #{e.message}")
+    Rails.logger.error("Stack trace: #{backtrace.map {|l| "  #{l}\n"}.join}")
+  end
+end

app/services/follow_service.rb

@@ -17,6 +17,8 @@ class FollowService < BaseService
 
     target_account.mark_known! unless !Setting.auto_mark_known || target_account.known?
 
+    SyncRemoteAccountWorker.perform_async(target_account.id) unless target_account.local? || target_account.passive_relationships.exists?
+
     if source_account.following?(target_account)
       # We're already following this account, but we'll call follow! again to
       # make sure the reblogs status is set correctly.

app/workers/sync_remote_account_worker.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class SyncRemoteAccountWorker
+  include Sidekiq::Worker
+
+  def perform(account_id)
+    account = Account.find(account_id)
+    ActivityPub::FetchAccountStatusesService.new.call(account)
+  rescue ActiveRecord::RecordNotFound, ActiveRecord::RecordInvalid
+    true
+  end
+end