Handle SIGTERMs for graceful shutdown

[halter73]

@stepro pointed out it should now be possible to handle SIGTERMs in addition to the SIGINTs (CTRL+C) that we already hook for graceful shutdown.

The managed API for SIGTERM is exposed by the AssemblyLoadContext.Unloading event, which is raised when the AppDomain.ProcessExit event is raised. I know this only because I’ve seen it used in the VSTS agent codebase (see this, line 44).
You can see how coreclr maps SIGTERM to shutting down here, line 170.

This should be especially useful for running in docker containers.

[davidfowl]

Do we want to handle both SIGTERM and SIGINT? Should they mean the same thing? Here are related issues:

• SIGTERM is not handled on Linux #516
• Have consistent way to terminate server across platforms #59

[muratg]

cc @glennc

[stepro]

As it stands, .NET Core handles both SIGTERM and SIGINT (as well as most of the other Linux signals) in the sense that it registers for the signals. Whether it actually does something useful is up to whether the application listens on the necessary events and handles them appropriately.

Given this, ASP .NET should definitely gracefully stop the application on SIGTERM because the alternative (effectively, SIGTERM equals SIGKILL) is never going to be the desired behavior.

Aside from potential differences on what is output to a live console, SIGINT and SIGTERM should both stop the ASP .NET application. SIGINT works well for a dev scenario where you are locally running the process synchronously; SIGTERM is appropriate when running in the background or unattended in a server scenario.

[davidfowl]

I can't get this to work in a simple console app on OSX:

public class Program
{
    public static void Main(string[] args)
    {
        var ended = new ManualResetEventSlim();
        var starting = new ManualResetEventSlim();

        AssemblyLoadContext.Default.Unloading += ctx =>
        {
            System.Console.WriteLine("Unloding fired");
            starting.Set();
            System.Console.WriteLine("Waiting for completion");
            ended.Wait();
        };

        System.Console.WriteLine("Waiting for signals");
        starting.Wait();

        System.Console.WriteLine("Received signal gracefully shutting down");
        Thread.Sleep(5000);
        ended.Set();
    }
}

I tried using:

kill -SIGTERM {PID}

It just ends without firing any events.

[muratg]

@davidfowl Does a console app catch SIGINT, though?

[davidfowl]

Handling Console.CancelKeyPress handles SIGINT but not this.

[muratg]

Bringing the bug back to 1.1 for further investigation.

[stepro]

The code I got to work pulls the AssemblyLoadContext in a different way:

AssemblyLoadContext.GetLoadContext(typeof(Program).GetTypeInfo().Assembly)

[halter73]

I just tried your program verbatim on Debian 8.2. Worked fine there. Maybe the issue you're running into is mac specific.

 ~/SigtermApp$ dotnet run
Project SigtermApp (.NETCoreApp,Version=v1.1) was previously compiled. Skipping compilation.
Waiting for signals
Unloding fired
Waiting for completion
Received signal gracefully shutting down
~/SigtermApp$

[halter73]

BTW kill <PID> and kill -SIGTERM <PID> are the same.