Mechanism to cancel reading from standard input

[alexrp]

For input processing we will need to constantly be reading standard in. But we also need a way to cancel that reading so the input thread doesn't just sit there forever.

Investigate how we can do this on each supported platform. Might be worth checking what, if anything, Notcurses does.

[alexrp]

Setting a flag and writing some garbage with ioctl(TIOCSTI) could potentially be an option. The equivalent on Windows should simply be WriteConsole on the input handle (probably, maybe). Or WriteConsoleInput.

[alexrp]

Setting a flag and writing some garbage with ioctl(TIOCSTI) could potentially be an option.

Then again, setting a flag and sending a signal should work just as well and seems less hacky. No such mechanism on Windows, though. Would also require knowing the thread ID of the reading thread.

[alexrp]

Here's an approach I think is workable and way less hacky.

TerminalReader.Read (and APIs that use it) should be changed to accept an optional CancellationToken.

We will create a pipe on startup. Instead of trying to read immediately, we will poll on the input FD and pipe read FD. When poll returns, we check if the pipe had data written. If yes, we drain it and throw an OperationCanceledException (leaving any potential data in the input FD unread). If no, proceed to read input as normal.

The TerminalReader will hook into the cancellation token with UnsafeRegister. The callback will simply write a byte to the pipe write FD to indicate that cancellation is requested.

This scheme should in principle work the same on Windows - just replace pipe with CreatePipe, poll with WaitForMultipleObjects, read with ReadConsole/ReadFile, etc.

[alexrp]

Bad news: WaitForMultipleObjects doesn't work on pipes. So the Unix approach won't translate neatly to Windows.

[alexrp]

Mostly done in fe53706 (with a kinda gross but functional workaround for Windows).

Still need to see if cancellation can somehow be plumbed through for ReadLine in addition to ReadRaw.

[alexrp]

Still need to see if cancellation can somehow be plumbed through for ReadLine in addition to ReadRaw.

It will probably be possible in .NET 7; see:

• Support cancellation in ReadLine/ReadLineAsync methods (.NET 7) #49

Closing this.