Fix single Redis connection being used across all threads (#18135)

* Fix single Redis connection being used across all Sidekiq threads

* Fix tests

lib/mastodon/sidekiq_middleware.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+class Mastodon::SidekiqMiddleware
+  BACKTRACE_LIMIT = 3
+
+  def call(*)
+    yield
+  rescue Mastodon::HostValidationError
+    # Do not retry
+  rescue => e
+    limit_backtrace_and_raise(e)
+  ensure
+    clean_up_sockets!
+  end
+
+  private
+
+  def limit_backtrace_and_raise(exception)
+    exception.set_backtrace(exception.backtrace.first(BACKTRACE_LIMIT))
+    raise exception
+  end
+
+  def clean_up_sockets!
+    clean_up_redis_socket!
+    clean_up_statsd_socket!
+  end
+
+  def clean_up_redis_socket!
+    Thread.current[:redis]&.close
+    Thread.current[:redis] = nil
+  end
+
+  def clean_up_statsd_socket!
+    Thread.current[:statsd_socket]&.close
+    Thread.current[:statsd_socket] = nil
+  end
+end