XmlSerializer.Deserialize causing Kestrel failure in .NET Core 3.0 Preview 5

[ghost]

I have been using the XmlSerializer for years, and I am using it in a project that I upgraded to .NET Core 3.0 Preview 5 today. After the upgrade, I receive the following exception when I try to XmlSerialize.Deserialize (Stream) the Request stream in a custom middleware component:

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

Prior to the upgrade, everything was working fine, so there is definitely something broken in in this serializer. If I update the Kestrel options as follows, it works fine:

.UseKestrel((context, options) =>
{
	// Temporary? The XmlSerializer is only performing synchronous operations.
	options.AllowSynchronousIO = true;
})

[kevingosse]

What has changed isn't the XmlSerializer but Kestrel. The default value of AllowSynchronousIO was true, it has been changed to false in 3.0: dotnet/aspnetcore@93a24b0
XmlSerializer has never supported async operations.

[ghost]

So is changing it to true the recommended action? Or should we be doing something different?

Reference aspnet/AspNetCore#7644

[jkotas]

Answered in dotnet/aspnetcore#7644 (comment)