AllowSynchronousIO disabled in all servers

[HaoK]

AllowSynchronousIO disabled in all servers

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.

See dotnet/aspnetcore#7644 for discussion.

Version introduced

3.0

Old behavior

HttpReqeuest.Body.Read, HttpResponse.Body.Write, Stream.Flush were allowed by default.

New behavior

These synchronous APIs are disallowed by default:

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.

Reason for change

These APIs have long been a source of thread starvation and application hangs. Starting in 3.0.0-preview3 these are disabled by default.

Recommended action

Use the async versions of the methods, also 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;
}

Category

• ASP.NET Core

Affected APIs

HttpRequest.Body.Read
HttpResponse.Body.Flush
HttpResponse.Body.Write

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Issue metadata

• Issue type: breaking-change