Synchronous request and response IO to be disallowed by default

[halter73]

Starting in ASP.NET Core 2.0.0 RTM, synchronous request and response IO will be disallowed by default.

For example, HttpContext.Request.Body.Read and HttpContext.Response.Body.Write will both throw InvalidOperationExceptions with a message communicating that either the equivalent async API should be called or synchronous IO should be explicitly allowed using IHttpBodyControlFeature.AllowSynchronousIO.

Synchronous IO hasn't been disallowed by default yet. After 2.0.0-preview2, there will be changes to both Kestrel and HttpSysServer to disallow this. Both servers will have a property added to their respective Options classes to globally allow synchronous IO.

Note: This change only impacts the request and response Stream APIs.

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Please use aspnet/ServerTests#86 for further discussion.

[halter73]

We're closing this issue/announcement for now due to incompatibilities this introduces with existing BCL types that are either difficult or impossible to work around without fully buffering. We hope to introduce this breaking change in a future major release.

Feel free to continue using aspnet/ServerTests#86 for discussion.