python-launcher | Python launcher for Unix | Command Line Interface library

Every process is in a process group. It either inherits its parent's group, or it gets created as the leader of a new group via the CREATE_NEW_PROCESS_GROUP creation flag. As far as I know, GenerateConsoleCtrlEvent is the only API that uses process groups, and there's no API to query the process group ID. You could grab it from the ProcessParameters in the PEB, but that involves using undocumented internal structures. No one should do that, so GenerateConsoleCtrlEvent should only send to either group 0 to broadcast an event or to a child process that you know was created as a new group.

The problem that you've uncovered here is that sending an event to a process that's attached to the console but not the leader of a group gets silently handled as if the target were group 0. Specifically in your case you're starting py.exe as the group leader and then trying to send CTRL_C_EVENT to python.exe, i.e. os.getpid(). You'd have to send to os.getppid() in this case.

This problem is common with Python scripts on Windows because of the confusing implementation of os.kill. It conflates process IDs and process group IDs. It would have been less confusing had GenerateConsoleCtrlEvent been used for os.killpg (currently not implemented on Windows) and TerminateProcess alone used for os.kill.

Experiencing random hangs when calling os.kill(os.getpid(), signal.CTRL_C_EVENT); time.sleep(10) may be due to a race condition. time.sleep is implemented by pysleep in Modules/timemodule.c. On Windows, when called in the main thread, it waits on an event that gets set by the signal handler for SIGINT (but not SIGBREAK for some reason). The possible race here is that pysleep resets the event before waiting on it. The signal handler executes on a new thread, and occasionally it may have already set the event before pysleep resets it. This is conceivable since executing CPython bytecode is relatively slow. That said, I'd expect it to be a close race because there are a lot of steps involved to create the control handler thread, as the following overview shows for Windows 10.