[Announcement] AllowSynchronousIO disabled in all servers

[Tratcher]

AllowSynchronousIO is a option in each server that enables or disables sync IO APIs like HttpReqeuest.Body.Read, HttpResponse.Body.Write, Stream.Flush, etc.. These APIs have long been a source of thread starvation and application hangs. Starting in 3.0.0-preview3 these are disabled by default.

Affected servers:

• Kestrel
• HttpSys
• IIS in-process
• TestServer

Expect errors similar to:

• Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.
• Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.
• Synchronous operations are disallowed. Call FlushAsync or set AllowSynchronousIO to true instead.

Each server has a AllowSynchronousIO option that controls this behavior and the default for all of them is now false.

The behavior can also be overridden on a per request basis as a temporary mitigation.

var syncIOFeature = HttpContext.Features.Get<IHttpBodyControlFeature>();
if (syncIOFeature != null)
{
    syncIOFeature.AllowSynchronousIO = true;
}

If you have trouble with TextWriters or other streams calling sync APIs in Dispose, call the new DisposeAsync API instead.

[kieronlanning]

As-per dotnet/AspNetCore.Docs#10983, will this be back ported to 2.x's feature set?

Currently setting this to false in 2.2 breaks response compression.

[davidfowl]

No, it won't be back ported. It's a massive breaking change.

[Tratcher]

Note response compression was fixed in 3.0 using the new DisposeAsync API.

[GusBeare]

I have been working on porting an angularJS app to .Net Core and just upgraded to Preview 3. After upgrade some of the client GET requests now blow with this error:

InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.

I don't get it because by default the angular requests are async and why do just some of them break?

I am using VS 2019 Preview and will be hosting IIS in process.

Any ideas?

[Tratcher]

@GusBeare what's the full stack trace?

[GusBeare]

I get the following html in the response.

error.txt