ChangeToken.OnChange fires twice when listening for configuration changes

[magnusbakken]

When I use ChangeToken.OnChange to listen to configuration changes as described in #432, my handler usually gets called twice in quick succession.

To reproduce:

1. Create a new ASP.NET Core targeting .NET Framework project using the Web API template (I haven't tested targeting .NET Core).
2. Add this line in the Configure method in Startup.cs: ChangeToken.OnChange(Configuration.GetReloadToken, () => loggerFactory.CreateLogger<Startup>().LogWarning("Configuration changed"));
3. Start the app.
4. While the app is running, make a change in appsettings.json and save the file.

The line "Configuration changed" shows up twice in the console output.

I've tried to make the file change from multiple programs (Visual Studio, Notepad, emacs), and by copying and replacing the file in its entirety. At least once I think I saw the change only being logged once, but usually you get it twice with just a few milliseconds between the log events.

I've attached the call stacks I see from the debugger for the first and second call respectively:
first-callstack.txt
second-callstack.txt

The only difference is the following five lines in the middle of the second stack:

mscorlib.dll!System.Threading.CancellationTokenSource.InternalRegister(System.Action<object> callback, object stateForCallback, System.Threading.SynchronizationContext targetSyncContext, System.Threading.ExecutionContext executionContext)	Unknown
mscorlib.dll!System.Threading.CancellationToken.Register(System.Action<object> callback, object state, bool useSynchronizationContext, bool useExecutionContext)	Unknown
mscorlib.dll!System.Threading.CancellationToken.Register(System.Action<object> callback, object state)	Unknown
Microsoft.Extensions.Primitives.dll!Microsoft.Extensions.Primitives.CancellationChangeToken.RegisterChangeCallback(System.Action<object> callback, object state)	Unknown
Microsoft.Extensions.Primitives.dll!Microsoft.Extensions.Primitives.ChangeToken.OnChange.AnonymousMethod__0(object s)	Unknown

[hcoona]

Reproduced also on .NET Framework 4.5.1

[HaoK]

Yeah this is just how change tokens work, what we did to somewhat mitgate this when we are listening for changes was to add a delay before invoking callback so the changes hopefully will be finished:

https://github.com/aspnet/Configuration/blob/dev/src/Microsoft.Extensions.Configuration.FileExtensions/FileConfigurationProvider.cs#L35

                ChangeToken.OnChange(
                    () => Source.FileProvider.Watch(Source.Path),
                    () => {
                        Thread.Sleep(Source.ReloadDelay);
                        Load(reload: true);
                    });

[HaoK]

You likely need to do something similar if you don't want to get called multiple times for the same underlying change.

[Tratcher]

This issue was moved to aspnet/FileSystem#287