Permit symbols when loading mail_room.yml

The `YAML.safe_load` changes made by
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58042
broke the diagnostic rake task gitlab:incoming_email:check
due to use of symbols for keys inside the ERB template
file `config/mail_room.yml`.

This change adds an exclusion permitting loading of
Symbol classes when parsing the YAML syntax for this
specific file.

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/331683

Changelog: fixed

lib/system_check/incoming_email/imap_authentication_check.rb

@@ -52,7 +52,7 @@ module SystemCheck
       def load_config
         erb = ERB.new(File.read(mail_room_config_path))
         erb.filename = mail_room_config_path
-        config_file = YAML.safe_load(erb.result)
+        config_file = YAML.safe_load(erb.result, permitted_classes: [Symbol])
 
         config_file[:mailboxes]
       end

spec/lib/system_check/incoming_email/imap_authentication_check_spec.rb

+# frozen_string_literal: true
+
+require 'fast_spec_helper'
+
+MAIL_ROOM_CONFIG_ENABLED_SAMPLE =
+  ":mailboxes:\n"\
+  "  \n"\
+  "    -\n"\
+  "      :host: \"gitlab.example.com\"\n"\
+  "      :port: 143\n"\
+  ""
+
+RSpec.describe SystemCheck::IncomingEmail::ImapAuthenticationCheck do
+  subject(:system_check) { described_class.new }
+
+  describe '#load_config' do
+    subject { system_check.send(:load_config) }
+
+    context 'returns no mailbox configurations with mailroom default configuration' do
+      it { is_expected.to be_nil }
+    end
+
+    context 'returns an array of mailbox configurations with mailroom configured' do
+      before do
+        allow(File).to receive(:read).and_return(MAIL_ROOM_CONFIG_ENABLED_SAMPLE)
+      end
+
+      it { is_expected.to eq([{ host: "gitlab.example.com", port: 143 }]) }
+    end
+  end
+end