From type/Proc/Async

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method close-stdin(Proc::Async:D: --> True)

Closes the standard input stream of the external program. Programs that read from STDIN often only terminate when their input stream is closed. So if waiting for the promise from .start hangs (for a program opened for writing), it might be a forgotten close-stdin.

The Proc::Async object must be created for writing (with Proc::Async.new(:w, $path, @args)). Otherwise an X::Proc::Async::OpenForWriting exception will the thrown.

start must have been called before calling method close-stdin, otherwise an X::Proc::Async::MustBeStarted exception is thrown.