Fix processing of remote Delete activities (#16084)

* Add tests

* Ensure deleted statuses are marked as such

* Save some redis memory by not storing URIs in delete_upon_arrival values

* Avoid possible race condition when processing incoming Deletes

* Avoid potential duplicate Delete forwards

* Lower lock durations to reduce issues in case of hard crash of the Rails process

* Check for `lock.aquired?` and improve comment

* Refactor RedisLock usage in app/lib/activitypub

* Fix using incorrect or non-existent sender for relaying Deletes

app/lib/activitypub/activity.rb

@@ -144,7 +144,7 @@ class ActivityPub::Activity
   end
 
   def delete_later!(uri)
-    redis.setex("delete_upon_arrival:#{@account.id}:#{uri}", 6.hours.seconds, uri)
+    redis.setex("delete_upon_arrival:#{@account.id}:#{uri}", 6.hours.seconds, true)
   end
 
   def status_from_object
@@ -210,12 +210,22 @@ class ActivityPub::Activity
     end
   end
 
-  def lock_or_return(key, expire_after = 7.days.seconds)
+  def lock_or_return(key, expire_after = 2.hours.seconds)
     yield if redis.set(key, true, nx: true, ex: expire_after)
   ensure
     redis.del(key)
   end
 
+  def lock_or_fail(key)
+    RedisLock.acquire({ redis: Redis.current, key: key }) do |lock|
+      if lock.acquired?
+        yield
+      else
+        raise Mastodon::RaceConditionError
+      end
+    end
+  end
+
   def fetch?
     !@options[:delivery]
   end

app/lib/activitypub/activity/announce.rb

@@ -4,8 +4,7 @@ class ActivityPub::Activity::Announce < ActivityPub::Activity
   def perform
     return reject_payload! if delete_arrived_first?(@json['id']) || !related_to_local_activity?
 
-    RedisLock.acquire(lock_options) do |lock|
-      if lock.acquired?
+    lock_or_fail("announce:#{@object['id']}") do
       original_status = status_from_object
 
       return reject_payload! if original_status.nil? || !announceable?(original_status)
@@ -24,9 +23,6 @@ class ActivityPub::Activity::Announce < ActivityPub::Activity
       )
 
       distribute(@status)
-      else
-        raise Mastodon::RaceConditionError
-      end
     end
 
     @status
@@ -69,8 +65,4 @@ class ActivityPub::Activity::Announce < ActivityPub::Activity
   def reblog_of_local_status?
     status_from_uri(object_uri)&.account&.local?
   end
-
-  def lock_options
-    { redis: Redis.current, key: "announce:#{@object['id']}" }
-  end
 end

app/lib/activitypub/activity/create.rb

@@ -45,8 +45,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
   def create_status
     return reject_payload! if unsupported_object_type? || invalid_origin?(object_uri) || tombstone_exists? || !related_to_local_activity?
 
-    RedisLock.acquire(lock_options) do |lock|
-      if lock.acquired?
+    lock_or_fail("create:#{object_uri}") do
       return if delete_arrived_first?(object_uri) || poll_vote? # rubocop:disable Lint/NonLocalExitFromIterator
 
       @status = find_existing_status
@@ -56,9 +55,6 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
       elsif @options[:delivered_to_account_id].present?
         postprocess_audience_and_deliver
       end
-      else
-        raise Mastodon::RaceConditionError
-      end
     end
 
     @status
@@ -314,13 +310,9 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     poll = replied_to_status.preloadable_poll
     already_voted = true
 
-    RedisLock.acquire(poll_lock_options) do |lock|
-      if lock.acquired?
+    lock_or_fail("vote:#{replied_to_status.poll_id}:#{@account.id}") do
       already_voted = poll.votes.where(account: @account).exists?
       poll.votes.create!(account: @account, choice: poll.options.index(@object['name']), uri: object_uri)
-      else
-        raise Mastodon::RaceConditionError
-      end
     end
 
     increment_voters_count! unless already_voted
@@ -513,12 +505,4 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     poll.reload
     retry
   end
-
-  def lock_options
-    { redis: Redis.current, key: "create:#{object_uri}" }
-  end
-
-  def poll_lock_options
-    { redis: Redis.current, key: "vote:#{replied_to_status.poll_id}:#{@account.id}" }
-  end
 end

app/lib/activitypub/activity/delete.rb

@@ -20,8 +20,15 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
   def delete_note
     return if object_uri.nil?
 
+    lock_or_return("delete_status_in_progress:#{object_uri}", 5.minutes.seconds) do
       unless invalid_origin?(object_uri)
-      RedisLock.acquire(lock_options) { |_lock| delete_later!(object_uri) }
+        # This lock ensures a concurrent `ActivityPub::Activity::Create` either
+        # does not create a status at all, or has finished saving it to the
+        # database before we try to load it.
+        # Without the lock, `delete_later!` could be called after `delete_arrived_first?`
+        # and `Status.find` before `Status.create!`
+        lock_or_fail("create:#{object_uri}") { delete_later!(object_uri) }
+
         Tombstone.find_or_create_by(uri: object_uri, account: @account)
       end
 
@@ -30,23 +37,18 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
 
       return if @status.nil?
 
-    if @status.distributable?
-      forward_for_reply
-      forward_for_reblogs
-    end
-
+      forward! if @json['signature'].present? && @status.distributable?
       delete_now!
     end
-
-  def forward_for_reblogs
-    return if @json['signature'].blank?
-
-    rebloggers_ids = @status.reblogs.includes(:account).references(:account).merge(Account.local).pluck(:account_id)
-    inboxes        = Account.where(id: ::Follow.where(target_account_id: rebloggers_ids).select(:account_id)).inboxes - [@account.preferred_inbox_url]
-
-    ActivityPub::LowPriorityDeliveryWorker.push_bulk(inboxes) do |inbox_url|
-      [payload, rebloggers_ids.first, inbox_url]
   end
+
+  def rebloggers_ids
+    return @rebloggers_ids if defined?(@rebloggers_ids)
+    @rebloggers_ids = @status.reblogs.includes(:account).references(:account).merge(Account.local).pluck(:account_id)
+  end
+
+  def inboxes_for_reblogs
+    Account.where(id: ::Follow.where(target_account_id: rebloggers_ids).select(:account_id)).inboxes
   end
 
   def replied_to_status
@@ -58,13 +60,19 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
     !replied_to_status.nil? && replied_to_status.account.local?
   end
 
-  def forward_for_reply
-    return unless @json['signature'].present? && reply_to_local?
+  def inboxes_for_reply
+    replied_to_status.account.followers.inboxes
+  end
 
-    inboxes = replied_to_status.account.followers.inboxes - [@account.preferred_inbox_url]
+  def forward!
+    inboxes = inboxes_for_reblogs
+    inboxes += inboxes_for_reply if reply_to_local?
+    inboxes -= [@account.preferred_inbox_url]
 
-    ActivityPub::LowPriorityDeliveryWorker.push_bulk(inboxes) do |inbox_url|
-      [payload, replied_to_status.account_id, inbox_url]
+    sender_id = reply_to_local? ? replied_to_status.account_id : rebloggers_ids.first
+
+    ActivityPub::LowPriorityDeliveryWorker.push_bulk(inboxes.uniq) do |inbox_url|
+      [payload, sender_id, inbox_url]
     end
   end
 
@@ -75,8 +83,4 @@ class ActivityPub::Activity::Delete < ActivityPub::Activity
   def payload
     @payload ||= Oj.dump(@json)
   end
-
-  def lock_options
-    { redis: Redis.current, key: "create:#{object_uri}" }
-  end
 end

app/services/remove_status_service.rb

@@ -16,6 +16,8 @@ class RemoveStatusService < BaseService
     @account  = status.account
     @options  = options
 
+    @status.discard
+
     RedisLock.acquire(lock_options) do |lock|
       if lock.acquired?
         remove_from_self if @account.local?

spec/lib/activitypub/activity/delete_spec.rb

@@ -49,4 +49,24 @@ RSpec.describe ActivityPub::Activity::Delete do
       end
     end
   end
+
+  context 'when the status has been reported' do
+    describe '#perform' do
+      subject { described_class.new(json, sender) }
+      let!(:reporter) { Fabricate(:account) }
+
+      before do
+        reporter.reports.create!(target_account: status.account, status_ids: [status.id], forwarded: false)
+        subject.perform
+      end
+
+      it 'marks the status as deleted' do
+        expect(Status.find_by(id: status.id)).to be_nil
+      end
+
+      it 'actually keeps a copy for inspection' do
+        expect(Status.with_discarded.find_by(id: status.id)).to_not be_nil
+      end
+    end
+  end
 end