Calling Yield() without Reacquire() then returning from a task hangs MSBuild

[rainersigwald]

Here's a simple, buggy task embedded in a project:

<Project>
  <UsingTask
    TaskName="YieldAndSleep"
    TaskFactory="RoslynCodeTaskFactory"
    AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll" >
    <ParameterGroup />
    <Task>
      <Code Type="Fragment" Language="cs">
<![CDATA[
BuildEngine3.Yield();
System.Threading.Thread.Sleep(1_000);
// BuildEngine3.Reacquire();
]]>
      </Code>
    </Task>
  </UsingTask>

  <Target Name="Entrypoint">
    <YieldAndSleep />
  </Target>

</Project>

Note that Reacquire is commented out--but the task completes successfully.

MSBuild just hangs when this happens

❯ msbuild .\yield.proj
Microsoft (R) Build Engine version 16.9.0-preview-20630-02+a2c42ce9f for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

Build started 1/5/2021 2:41:15 PM.

And if you ctrl-c,

Attempting to cancel the build...

Unhandled Exception: Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: One or more errors occurred.
=============
System.AggregateException: One or more errors occurred. ---> Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: One or more errors occurred.
=============
System.AggregateException: One or more errors occurred. ---> Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: MSB0001: Internal MSBuild Error: Entry must be active before it can be Completed successfully.  Config: 2 State: Waiting
=============
Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: Entry must be active before it can be Completed successfully.  Config: 2 State: Waiting
   at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Object[] args)
   at Microsoft.Build.Shared.ErrorUtilities.VerifyThrow(Boolean condition, String unformattedMessage, Object arg0, Object arg1)
   at Microsoft.Build.BackEnd.BuildRequestEntry.Complete(BuildResult result)
   at Microsoft.Build.BackEnd.RequestBuilder.ReportResultAndCleanUp(BuildResult result)
   at Microsoft.Build.BackEnd.RequestBuilder.<BuildAndReport>d__50.MoveNext()

This is not catastrophic because it's a task-authoring error: you're supposed to Yield() and Reacquire() in matched sets. But it sure is awkward, when we could either

1. Implicitly reacquire when ITask.Execute() returns, or
2. Replace the manual yield/reacquire business with a using-compatible resource.