Asynchronous printing hangs the server

Prerequisites

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Description

Printing text asynchronously (e.g., a IO.println run through IO.asTask) causes the Lean server to hang.

Steps to Reproduce

Run the following code via the Lean server (e.g., in VSCode) to observe the server hanging:

#eval show IO _ from do 
  IO.wait <| ← IO.asTask <| IO.println "test"

Expected behavior:

To print "test" either as a diagnostic or in the output panel.

Actual behavior:

The server hangs.

Reproduces how often:

Always.

Versions

Windows 10 20H2
Lean (version 4.0.0-nightly-2021-10-23, commit a741a3dfd422, Release)

Additional Information

I suspect this is actually due to the same problem as #640. That is, I suspect that the asynchronous print is not being properly redirected (due to being in a separate thread) and is thus printing to normal stdout,. This, as #640 shows, causes the server to hang.

Run the provided Lean snippet through the Lean server, for example in VSCode, and compare the behavior with the related issue #640. Trace how asynchronous IO.println output is redirected or handled by the server. Done means the snippet prints "test" as a diagnostic or in the output panel without hanging the server.