Only wrap subscriber exceptions if there is more than one

Ref: rails#43282
Ref: rails#43561

It can be legitimate for subscriber to want to bubble up some exception
to the caller, so wrapping it change the exception class which might break
the calling code expecting a specific error.

We can minimize this by only using InstrumentationSubscriberError
when more than one subscriber raised.

Author: byroot

activerecord/test/cases/adapter_test.rb

@@ -213,11 +213,9 @@ def test_numeric_value_out_of_ranges_are_translated_to_specific_exception
     def test_exceptions_from_notifications_are_not_translated
       original_error = StandardError.new("This StandardError shouldn't get translated")
       subscriber = ActiveSupport::Notifications.subscribe("sql.active_record") { raise original_error }
-      wrapped_error = assert_raises(ActiveSupport::Notifications::InstrumentationSubscriberError) do
+      actual_error = assert_raises(StandardError) do
         @connection.execute("SELECT * FROM posts")
       end
-      actual_error = wrapped_error.exceptions.first
-
       assert_equal original_error, actual_error
 
     ensure

activerecord/test/cases/query_cache_test.rb

@@ -489,13 +489,11 @@ def test_query_cache_does_not_allow_sql_key_mutation
       payload[:sql].downcase!
     end
 
-    error = assert_raises ActiveSupport::Notifications::InstrumentationSubscriberError do
+    assert_raises FrozenError do
       ActiveRecord::Base.cache do
         assert_queries(1) { Task.find(1); Task.find(1) }
       end
     end
-
-    assert error.exceptions.first.is_a?(FrozenError)
   ensure
     ActiveSupport::Notifications.unsubscribe subscriber
   end

activesupport/lib/active_support/notifications/fanout.rb

@@ -93,8 +93,16 @@ def iterate_guarding_exceptions(listeners)
           exceptions ||= []
           exceptions << e
         end
-      ensure
-        raise InstrumentationSubscriberError.new(exceptions) unless exceptions.nil?
+
+        if exceptions
+          if exceptions.size == 1
+            raise exceptions.first
+          else
+            raise InstrumentationSubscriberError.new(exceptions), cause: exceptions.first
+          end
+        end
+
+        listeners
       end
 
       def listeners_for(name)

activesupport/test/notifications/evented_notification_test.rb

@@ -92,18 +92,23 @@ def test_listen_to_everything
         ], listener.events
       end
 
-      def test_listen_start_exception_consistency
+      def test_listen_start_multiple_exception_consistency
         notifier = Fanout.new
         listener = Listener.new
         notifier.subscribe nil, BadStartListener.new
+        notifier.subscribe nil, BadStartListener.new
         notifier.subscribe nil, listener
 
-        assert_raises InstrumentationSubscriberError do
+        error = assert_raises InstrumentationSubscriberError do
           notifier.start  "hello", 1, {}
         end
-        assert_raises InstrumentationSubscriberError do
+        assert_instance_of BadListenerException, error.cause
+
+        error = assert_raises InstrumentationSubscriberError do
           notifier.start  "world", 1, {}
         end
+        assert_instance_of BadListenerException, error.cause
+
         notifier.finish  "world", 1, {}
         notifier.finish  "hello", 1, {}
 
@@ -116,20 +121,24 @@ def test_listen_start_exception_consistency
         ], listener.events
       end
 
-      def test_listen_finish_exception_consistency
+      def test_listen_finish_multiple_exception_consistency
         notifier = Fanout.new
         listener = Listener.new
         notifier.subscribe nil, BadFinishListener.new
+        notifier.subscribe nil, BadFinishListener.new
         notifier.subscribe nil, listener
 
         notifier.start  "hello", 1, {}
         notifier.start  "world", 1, {}
-        assert_raises InstrumentationSubscriberError do
+        error = assert_raises InstrumentationSubscriberError do
           notifier.finish  "world", 1, {}
         end
-        assert_raises InstrumentationSubscriberError do
+        assert_instance_of BadListenerException, error.cause
+
+        error = assert_raises InstrumentationSubscriberError do
           notifier.finish  "hello", 1, {}
         end
+        assert_instance_of BadListenerException, error.cause
 
         assert_equal 4, listener.events.length
         assert_equal [