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