Fix test output streaming on Windows

[owenv]

The AsyncProcess implementation on Windows was effectively buffering all output until process exit. Vendor over some utilities from Swift Build to stream the output, and attempt to avoid the bug addressed in #8047 (review)

[owenv]

@swift-ci test

[owenv]

@swift-ci test

[owenv]

@swift-ci test Windows

[dschaefer2]

Yay! Yes, the only proper way to read stdout/stderr on Windows is multi-threaded. I do wonder if creating dedicated threads for this instead of using the Task thread pool would be safer.

[jakepetroules]

This is fine, it's all non-blocking through Dispatch.

[owenv]

@swift-ci test

[owenv]

@swift-ci test

[bripeticca]

@swift-ci please test windows

[owenv]

@swift-ci test

[owenv]

@swift-ci test Windows

[rconnell9]

Should this be a continuation (e.g. cont.finish() here)? I'm not sure what the implications are of returning nil are for clients of this stream.

[owenv]

returning nil here is intended to terminate the stream, this initializer doesn't use a continuation

[owenv]

@swift-ci test

[owenv]

@swift-ci test Windows

[owenv]

@swift-ci test

[owenv]

@swift-ci test Windows

[ahoppen]

Just so you are aware, because we are using DispatchIO.read(offset:length:queue:), we are seeing one thread spinning on Windows in SourceKit-LSP (lengthy discussion here and here and here).

You are using read(fromFileDescriptor:maxLength:runningHandlerOn:handler:), which seems to take a different code path at first but I wouldn’t be surprised if it ends up with a very similar issue in the end. If you don’t, all the better.