[API] Timer.UnsafeCreate

[benaadams]

Suggested in dotnet/corefx#26071 (comment)

namespace System.Threading
{
    partial class Timer
    {
        static Timer UnsafeCreate(TimerCallback callback, 
                                  object state, 
                                  TimeSpan dueTime, 
                                  TimeSpan period);
    }
}

Creates a timer that doesn't capture AsyncLocal state (for lazy init for global timers from partially unrelated code)

Related fixes dotnet/corefx#26065, dotnet/corefx#26066, dotnet/corefx#26071, dotnet/corefx#26077, dotnet/corefx#26068

[jnm2]

Is Unsafe in the context of threading always about AsyncLocal? Seeing the title, I immediately wondered where this was coming from and thought of UnsafeOnCompleted.

[stephentoub]

Is Unsafe in the context of threading always about AsyncLocal?

It's typically about not flowing ExecutionContext, e.g. ThreadPool.UnsafeQueueUserWorkItem is the same as QueueUserWorkItem but doesn't flow EC, UnsafeOnCompleted is the same as OnCompleted but need not flow EC, etc. Timer.UnsafeCreate would be similar, the same as a Timer's ctor but without flowing EC.

(AsyncLocal data flows as part of EC.)

[benaadams]

Related issues dotnet/coreclr#18670, aspnet/HttpClientFactory#171

[benaadams]

Another instance where it would be useful aspnet/SignalR#2904

[benaadams]

Current (ugly) workaround is:

bool restoreFlow = false;
try
{
    if (!ExecutionContext.IsFlowSuppressed())
    {
        ExecutionContext.SuppressFlow();
        restoreFlow = true;
    }
    
    // Create timer here
}
finally
{
    if (restoreFlow)
    {
        ExecutionContext.RestoreFlow();
    }
}

[benaadams]

Another instance where it would be useful aspnet/Razor#2565

[stephentoub]

Alternate / augmented proposal in https://github.com/dotnet/corefx/issues/32866.

[terrajobst]

API looks reasonable.

[terrajobst]

Just noticed @stephentoub's alternative. Looks like @vancem and @stephentoub should first decide which avenue to pursue.

[stephentoub]

This is effectively a subset of dotnet/corefx#32866 (the only difference as far as this proposal is concerned is an extra optional argument added in the other proposal). We should do dotnet/corefx#32866.

[terrajobst]

Since we approved dotnet/corefx#32866 we can close this.