new pop3_polling_enabled setting validator to ensure credentials are working before enabling it

config/locales/server.en.yml

@@ -1236,6 +1236,10 @@ en:
       invalid_string_min: "Must be at least %{min} characters."
       invalid_string_max: "Must be no more than %{max} characters."
       invalid_reply_by_email_address: "Value must contain '%{reply_key}' and be different from the notification email."
+      pop3_polling_host_is_empty: "You must set a 'pop3 polling host' before enabling POP3 polling."
+      pop3_polling_username_is_empty: "You must set a 'pop3 polling username' before enabling POP3 polling."
+      pop3_polling_password_is_empty: "You must set a 'pop3 polling password' before enabling POP3 polling."
+      pop3_polling_authentication_failed: "POP3 authentication failed. Please verify your pop3 credentials."
 
   notification_types:
     group_mentioned: "%{group_name} was mentioned in %{link}"

config/site_settings.yml

@@ -506,7 +506,9 @@ email:
   reply_by_email_address:
     default: ''
     validator: "ReplyByEmailAddressValidator"
-  pop3_polling_enabled: false
+  pop3_polling_enabled:
+    default: false
+    validator: "POP3PollingEnabledSettingValidator"
   pop3_polling_ssl: true
   pop3_polling_period_mins: 5
   pop3_polling_host: ''

lib/validators/pop3_polling_enabled_setting_validator.rb

@@ -0,0 +1,44 @@
+require "net/pop"
+
+class POP3PollingEnabledSettingValidator
+
+  def initialize(opts={})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    # only validate when enabling polling
+    return true if val == "f"
+    # ensure we can authenticate
+    SiteSetting.pop3_polling_host.present? &&
+    SiteSetting.pop3_polling_username.present? &&
+    SiteSetting.pop3_polling_password.present? &&
+    authentication_works?
+  end
+
+  def error_message
+    if SiteSetting.pop3_polling_host.blank?
+      I18n.t("site_settings.errors.pop3_polling_host_is_empty")
+    elsif SiteSetting.pop3_polling_username.blank?
+      I18n.t("site_settings.errors.pop3_polling_username_is_empty")
+    elsif SiteSetting.pop3_polling_password.blank?
+      I18n.t("site_settings.errors.pop3_polling_password_is_empty")
+    else
+      I18n.t("site_settings.errors.pop3_polling_authentication_failed")
+    end
+  end
+
+  private
+
+    def authentication_works?
+      @authentication_works ||= begin
+        pop3 = Net::POP3.new(SiteSetting.pop3_polling_host, SiteSetting.pop3_polling_port)
+        pop3.enable_ssl(OpenSSL::SSL::VERIFY_NONE) if SiteSetting.pop3_polling_ssl
+        pop3.auth_only(SiteSetting.pop3_polling_username, SiteSetting.pop3_polling_password)
+      rescue Net::POPAuthenticationError
+        false
+      else
+        true
+      end
+    end
+end