Debugger does not kill all processes when terminated

[jahan01]

Environment data

• VS Code version: 1.47.2
• Extension version (available under the Extensions sidebar): v2020.7.94776
• OS and version: MacOS 10.15.5
• Python version (& distribution if applicable, e.g. Anaconda): Anaconda 3.6
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): conda

Expected behaviour

All process are killed when debugger is stopped / terminated.

Actual behaviour

Not all processes are stopped when debugger is terminated. I observed this pattern only when debugging multiprocess application like Flask, Celery. It leaves one zombie process running behind. I'll have to manually kill this process.

Steps to reproduce:

1. Code

from flask import Flask
app = Flask(__name__)
app.run(host="0.0.0.0", port=5000, threaded=True, debug=True)

2. launch.json

        {
            "name": "Python: Current File",
            "type": "python",
            "request": "launch",
            "program": "${file}",
            "subProcess": true,
        },

3. Start the debugger and terminate after it starts. The below process is left behind:

501 46232     1   0  9:52PM ??         0:11.50 xxxx/bin/python xxxx/.vscode/extensions/ms-python.python-2020.7.94776/pythonFiles/lib/python/debugpy/_vendored/pydevd/pydevd.py --module --port 50912 --ppid 46228 --client 127.0.0.1 --client-access-token xxxx --multiprocess --json-dap-http --file test_debugpy

4. When you try to debug the same file again without killing the zombie process, you will get Address in use exception.
5. This happens when debugging other multiprocessing applications/libraries like celery.

[int19h]

#359 should take care of this, as well.