Check for instance availability before attempting to fetch mentioned …

…users (mastodon#22713)

app/lib/activitypub/activity/create.rb

@@ -222,8 +222,13 @@ def process_mention(tag)
     return if tag['href'].blank?
 
     account = account_from_uri(tag['href'])
-    account = ActivityPub::FetchRemoteAccountService.new.call(tag['href'], request_id: @options[:request_id]) if account.nil?
-
+    begin
+      return unless DeliveryFailureTracker.available?(@domain) || !account.nil
+      account = ActivityPub::FetchRemoteAccountService.new.call(tag['href'], request_id: @options[:request_id]) if account.nil?
+    rescue HTTP::ConnectionError => e
+      Rails.logger.info "Fetching account #{tag['href']} failed: #{e}"
+      raise
+    end
     return if account.nil?
 
     @mentions << Mention.new(account: account, silent: false)