Add support for start /B and FreeConsole

[lhecker]

2 new ConPTY APIs were added as part of this commit:

• ClosePseudoConsoleTimeout
  Complements ClosePseudoConsole, allowing users to override the INFINITE
  wait for OpenConsole to exit with any arbitrary timeout, including 0.
• ConptyReleasePseudoConsole
  This releases the \Reference handle held by the HPCON. While it makes
  launching further PTY clients via PROC_THREAD_ATTRIBUTE_PSEUDOCONSOLE
  impossible, it does allow conhost/OpenConsole to exit naturally once all
  console clients have disconnected. This makes it unnecessary to having to
  monitor the exit of the spawned shell/application, just to then call
  ClosePseudoConsole, while carefully continuing to read from the output
  pipe. Instead users can now just read from the output pipe until it is
  closed, knowing for sure that no more data will come or clients connect.
  This is especially useful in combination with ClosePseudoConsoleTimeout
  and a timeout of 0, to allow conhost/OpenConsole to exit asynchronously.

These new APIs are used to fix an array of bugs around Windows Terminal exiting
either too early or too late. They also make usage of the ConPTY API simpler in
most situations (when spawning a single application and waiting for it to exit).

Depends on #13882, #14041, #14160, #14282

Closes #4564
Closes #14416
Closes MSFT-42244182

Validation Steps Performed

• Calling FreeConsole in a handoff'd application closes the tab ✅
• Create a .bat file containing only start /B cmd.exe.
  If WT stable is the default terminal the tab closes instantly ✅
  With these changes included the tab stays open with a cmd.exe prompt ✅
• New ConPTY tests ✅

[DHowett]

Before merge, make sure to fill out the TBA/TBD/TBE

[DHowett]

And note that for the final commit message, each closed issue must be on its own line beginning with Closes or Fixes; GitHub will not handle comma-delimited lists of issues :(

[lhecker]

I've just restarted the PR with a rebase and a much more correct and simpler approach around the new ConptyReleasePseudoConsole API method. It now works correctly under all circumstances and I've tested it with over 200 rounds of cmd.exe /c start /b ... tests in both debug and release mode each.

[zadjii-msft]

Okay Dustin explained most of this to me today and I get it, and it makes more sense now.

Timing like this is hard, and I'm glad you sorted out this wacky, wild edge case that must have just always existed in this API.

I know it's the EOY and we don't have time to write down what all is going on here, but we REALLY should better document what these changes did, and how a terminal should best use these APIs now. Just something we can refer to in 11 months when there's inevitably some new bug and we're trying to put this all back together again 😅

[DHowett]

all said, a signoff

[DHowett]

we used to check whether this was true because it would let us perform closing actions only once. Are these remaining actions safe to do multiple times?

[lhecker]

Actually it's the other way around: We used to potentially perform these actions multiple times and now we don't. This is because the old code ran past a _transitionToState() in ConptyConnection::_ClientTerminated() without checking the return value. So it might try to extract the _hOutputThread and wait on it, even though the main thread might be concurrently running ConptyConnection::Close() and trying to do the exact same thing.

The new code is more robust, because the background thread(s) don't close any system objects anymore. Only the main thread does. Technically we don't need _transitionToState() anymore. The fact that the pipe returned ERROR_BROKEN_PIPE is indication enough that we may exit.

Actually now that I said that I realize that our CancelSynchronousIo() code is a bit racy. If we call it when the output thread is currently not stuck in ReadFile() it might miss the signal to exit, which is problematic because Close() blocks the UI thread.

[lhecker]

Fixed in my latest commit!

[ghost]

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[ghost]

🎉Windows Terminal Preview v1.17.1023 has been released which incorporates this pull request.:tada:

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