Log delivery errors in ActionMailer::LogSubscriber

In case of mail delivery errors, the old ActionMailer::LogSubscriber
erroneously logged a "Delivered mail ..." line.
With this change it checks the passed exception object and logs a
"Failed delivery of mail" line instead.

Co-authored-by: Christian Esser <c_esser@yahoo.de>

actionmailer/lib/action_mailer/log_subscriber.rb

@@ -11,8 +11,9 @@ class LogSubscriber < ActiveSupport::LogSubscriber
     # An email was delivered.
     def deliver(event)
       info do
-        perform_deliveries = event.payload[:perform_deliveries]
-        if perform_deliveries
+        if exception = event.payload[:exception_object]
+          "Failed delivery of mail #{event.payload[:message_id]} error_class=#{exception.class} error_message=#{exception.message.inspect}"
+        elsif event.payload[:perform_deliveries]
           "Delivered mail #{event.payload[:message_id]} (#{event.duration.round(1)}ms)"
         else
           "Skipped delivery of mail #{event.payload[:message_id]} as `perform_deliveries` is false"

actionmailer/test/log_subscriber_test.rb

@@ -19,6 +19,15 @@ def receive(mail)
     end
   end
 
+  class BogusDelivery
+    def initialize(*)
+    end
+
+    def deliver!(mail)
+      raise "failed"
+    end
+  end
+
   def set_logger(logger)
     ActionMailer::Base.logger = logger
   end
@@ -50,4 +59,17 @@ def test_deliver_message_when_perform_deliveries_is_false
   ensure
     BaseMailer.deliveries.clear
   end
+
+  def test_deliver_message_when_exception_happened
+    previous_delivery_method = BaseMailer.delivery_method
+    BaseMailer.delivery_method = BogusDelivery
+
+    assert_raises(RuntimeError) { BaseMailer.welcome(message_id: "123@abc").deliver_now }
+    wait
+
+    assert_equal(1, @logger.logged(:info).size)
+    assert_equal('Failed delivery of mail 123@abc error_class=RuntimeError error_message="failed"', @logger.logged(:info).first)
+  ensure
+    BaseMailer.delivery_method = previous_delivery_method
+  end
 end