Add request pool to improve delivery performance (#10353)

* Add request pool to improve delivery performance

Fix #7909

* Ensure connection is closed when exception interrupts execution

* Remove Timeout#timeout from socket connection

* Fix infinite retrial loop on HTTP::ConnectionError

* Close sockets on failure, reduce idle time to 90 seconds

* Add MAX_REQUEST_POOL_SIZE option to limit concurrent connections to the same server

* Use a shared pool size, 512 by default, to stay below open file limit

* Add some tests

* Add more tests

* Reduce MAX_IDLE_TIME from 90 to 30 seconds, reap every 30 seconds

* Use a shared pool that returns preferred connection but re-purposes other ones when needed

* Fix wrong connection being returned on subsequent calls within the same thread

* Reduce mutex calls on flushes from 2 to 1 and add test for reaping

app/workers/activitypub/delivery_worker.rb

@@ -17,6 +17,7 @@ class ActivityPub::DeliveryWorker
     @json           = json
     @source_account = Account.find(source_account_id)
     @inbox_url      = inbox_url
+    @host           = Addressable::URI.parse(inbox_url).normalized_site
 
     perform_request
 
@@ -28,16 +29,18 @@ class ActivityPub::DeliveryWorker
 
   private
 
-  def build_request
-    request = Request.new(:post, @inbox_url, body: @json)
+  def build_request(http_client)
+    request = Request.new(:post, @inbox_url, body: @json, http_client: http_client)
     request.on_behalf_of(@source_account, :uri, sign_with: @options[:sign_with])
     request.add_headers(HEADERS)
   end
 
   def perform_request
     light = Stoplight(@inbox_url) do
-      build_request.perform do |response|
-        raise Mastodon::UnexpectedResponseError, response unless response_successful?(response) || response_error_unsalvageable?(response)
+      request_pool.with(@host) do |http_client|
+        build_request(http_client).perform do |response|
+          raise Mastodon::UnexpectedResponseError, response unless response_successful?(response) || response_error_unsalvageable?(response)
+        end
       end
     end
 
@@ -57,4 +60,8 @@ class ActivityPub::DeliveryWorker
   def failure_tracker
     @failure_tracker ||= DeliveryFailureTracker.new(@inbox_url)
   end
+
+  def request_pool
+    RequestPool.current
+  end
 end