Fix addressing of remote groups' followers (#16700)

Fixes #16699

app/lib/activitypub/tag_manager.rb

@@ -64,6 +64,10 @@ class ActivityPub::TagManager
     account_status_replies_url(target.account, target, page_params)
   end
 
+  def followers_uri_for(target)
+    target.local? ? account_followers_url(target) : target.followers_url.presence
+  end
+
   # Primary audience of a status
   # Public statuses go out to primarily the public collection
   # Unlisted and private statuses go out primarily to the followers collection
@@ -80,17 +84,17 @@ class ActivityPub::TagManager
         account_ids = status.active_mentions.pluck(:account_id)
         to = status.account.followers.where(id: account_ids).each_with_object([]) do |account, result|
           result << uri_for(account)
-          result << account_followers_url(account) if account.group?
+          result << followers_uri_for(account) if account.group?
         end
         to.concat(FollowRequest.where(target_account_id: status.account_id, account_id: account_ids).each_with_object([]) do |request, result|
           result << uri_for(request.account)
-          result << account_followers_url(request.account) if request.account.group?
-        end)
+          result << followers_uri_for(request.account) if request.account.group?
+        end).compact
       else
         status.active_mentions.each_with_object([]) do |mention, result|
           result << uri_for(mention.account)
-          result << account_followers_url(mention.account) if mention.account.group?
-        end
+          result << followers_uri_for(mention.account) if mention.account.group?
+        end.compact
       end
     end
   end
@@ -118,17 +122,17 @@ class ActivityPub::TagManager
         account_ids = status.active_mentions.pluck(:account_id)
         cc.concat(status.account.followers.where(id: account_ids).each_with_object([]) do |account, result|
           result << uri_for(account)
-          result << account_followers_url(account) if account.group?
-        end)
+          result << followers_uri_for(account) if account.group?
+        end.compact)
         cc.concat(FollowRequest.where(target_account_id: status.account_id, account_id: account_ids).each_with_object([]) do |request, result|
           result << uri_for(request.account)
-          result << account_followers_url(request.account) if request.account.group?
-        end)
+          result << followers_uri_for(request.account) if request.account.group?
+        end.compact)
       else
         cc.concat(status.active_mentions.each_with_object([]) do |mention, result|
           result << uri_for(mention.account)
-          result << account_followers_url(mention.account) if mention.account.group?
-        end)
+          result << followers_uri_for(mention.account) if mention.account.group?
+        end.compact)
       end
     end
 

spec/lib/activitypub/tag_manager_spec.rb

@@ -42,6 +42,14 @@ RSpec.describe ActivityPub::TagManager do
       expect(subject.to(status)).to eq [subject.uri_for(mentioned)]
     end
 
+    it "returns URIs of mentioned group's followers for direct statuses to groups" do
+      status    = Fabricate(:status, visibility: :direct)
+      mentioned = Fabricate(:account, domain: 'remote.org', uri: 'https://remote.org/group', followers_url: 'https://remote.org/group/followers', actor_type: 'Group')
+      status.mentions.create(account: mentioned)
+      expect(subject.to(status)).to include(subject.uri_for(mentioned))
+      expect(subject.to(status)).to include(subject.followers_uri_for(mentioned))
+    end
+
     it "returns URIs of mentions for direct silenced author's status only if they are followers or requesting to be" do
       bob    = Fabricate(:account, username: 'bob')
       alice  = Fabricate(:account, username: 'alice')