After 7 days of repeated delivery failures, give up on inbox (#5131)

- A successful delivery cancels it out
- An incoming delivery from account of the inbox cancels it out

app/lib/delivery_failure_tracker.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+class DeliveryFailureTracker
+  FAILURE_DAYS_THRESHOLD = 7
+
+  def initialize(inbox_url)
+    @inbox_url = inbox_url
+  end
+
+  def track_failure!
+    Redis.current.sadd(exhausted_deliveries_key, today)
+    Redis.current.sadd('unavailable_inboxes', @inbox_url) if reached_failure_threshold?
+  end
+
+  def track_success!
+    Redis.current.del(exhausted_deliveries_key)
+    Redis.current.srem('unavailable_inboxes', @inbox_url)
+  end
+
+  def days
+    Redis.current.scard(exhausted_deliveries_key) || 0
+  end
+
+  class << self
+    def filter(arr)
+      arr.reject(&method(:unavailable?))
+    end
+
+    def unavailable?(url)
+      Redis.current.sismember('unavailable_inboxes', url)
+    end
+
+    def available?(url)
+      !unavailable?(url)
+    end
+
+    def track_inverse_success!(from_account)
+      new(from_account.inbox_url).track_success! if from_account.inbox_url.present?
+      new(from_account.shared_inbox_url).track_success! if from_account.shared_inbox_url.present?
+    end
+  end
+
+  private
+
+  def exhausted_deliveries_key
+    "exhausted_deliveries:#{@inbox_url}"
+  end
+
+  def today
+    Time.now.utc.strftime('%Y%m%d')
+  end
+
+  def reached_failure_threshold?
+    days >= FAILURE_DAYS_THRESHOLD
+  end
+end