Non-documented breaking change of channel-executor configuration

[ok-ul-ch]

Version Information
Version of Akka.NET? 1.4.32
Which Akka.NET Modules? Akka

Describe the bug
After update from 1.4.20 to 1.4.32 channel-executor configuration (see doc) is not valid anymore

To Reproduce
Steps to reproduce the behavior:

1. Create a console app with Akka 1.4.29+
2. Execute:

using Akka.Actor;

namespace ChannelExecutorIssue
{
    internal class Program
    {
        static void Main(string[] args)
        {
            // Following: https://getakka.net/articles/actors/dispatchers.html#channelexecutor
            var config = @"
                          akka.actor.default-dispatcher = { 
                                 executor = channel-executor
                          }";
            
            // Throws NRE in 1.4.29+
            var actorSystem = ActorSystem.Create("test", config);
        }
    }
}

3. Observe

Expected behavior
ActorSystem created successfully

Actual behavior
NRE is thrown:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at Akka.Dispatch.ChannelExecutorConfigurator..ctor(Config config, IDispatcherPrerequisites prerequisites)
   at Akka.Dispatch.MessageDispatcherConfigurator.ConfigureExecutor()
   at Akka.Dispatch.DispatcherConfigurator..ctor(Config config, IDispatcherPrerequisites prerequisites)
   at Akka.Dispatch.Dispatchers.ConfiguratorFrom(Config cfg)
   at Akka.Dispatch.Dispatchers.LookupConfigurator(String id)
   at Akka.Dispatch.Dispatchers.Lookup(String dispatcherName)
   at Akka.Dispatch.Dispatchers..ctor(ActorSystem system, IDispatcherPrerequisites prerequisites, ILoggingAdapter logger)
   at Akka.Actor.Internal.ActorSystemImpl.ConfigureDispatchers()
   at Akka.Actor.Internal.ActorSystemImpl..ctor(String name, Config config, ActorSystemSetup setup, Nullable`1 guardianProps)
   at Akka.Actor.ActorSystem.CreateAndStartSystem(String name, Config withFallback, ActorSystemSetup setup)
   at Akka.Actor.ActorSystem.Create(String name, Config config)
   at ChannelExecutorIssue.Program.Main(String[] args)

The issue is in the default config for default-dispatcher that has no channel-executor.priority set. Introduced in #5403

Sidenote: default channel-scheduler configuration is expected to be at: akka.channel-scheduler

akka.net/src/core/Akka/Dispatch/ChannelSchedulerExtension.cs

Line 63 in ccb4670

var config = system.Settings.Config.GetConfig("akka.channel-scheduler");

But it is actually located under akka.actor.channel-scheduler
Current workaround:

using Akka.Actor;

namespace ChannelExecutorIssue
{
    internal class Program
    {
        static void Main(string[] args)
        {
            //Copy akka.actor.channel-scheduler from Akka/Configuration/Pigeon.conf
            var config = @"
                          akka.channel-scheduler {
                              parallelism-min = 4    #same as for ForkJoinDispatcher
                              parallelism-factor = 1 #same as for ForkJoinDispatcher
                              parallelism-max = 64   #same as for ForkJoinDispatcher
                              work-max = 10          #max executed work items in sequence until priority loop
                              work-interval = 500    #time target of executed work items in ms
                              work-step = 2          #target work item count in interval / burst
                          }
                          akka.actor.default-dispatcher = { 
                              executor = channel-executor
                              channel-executor.priority = normal
                          }";
            
            var actorSystem = ActorSystem.Create("test", config);
        }
    }
}

[Aaronontheweb]

@eaba can you take this one?

[Aaronontheweb]

@mralexes thanks for reporting this - I totally missed that during the review.

[ok-ul-ch]

Might be related to #5498

[eaba]

@eaba can you take this one?

Ok @Aaronontheweb

[eaba]

The issue is in the default config for default-dispatcher that has no channel-executor.priority set

The default channel-executor.priority is normal if not set by the user:

akka.net/src/core/Akka/Dispatch/AbstractDispatcher.cs

Line 124 in 405dfc5

Priority = (TaskSchedulerPriority)Enum.Parse(typeof(TaskSchedulerPriority), cfg.GetString("priority", "normal"), true);

[ok-ul-ch]

@eaba yes, but only if cfg variable is not null, which is null in this case)