Play nicer with scripts that spawn children processes

Summary

When Streamlit interrupts (and terminates) a script that has spawned children processes, the children processes don’t appear to be properly handled causing the session to hang and ultimately become unusable. The user must refresh the browser page to continue using the app; the administrator must send SIGKILL signals the streamlit process (and its children) when trying to stop the streamlit server.

Steps to reproduce

Create a script using with muiltiprocessing.pool.Pool() as pool: and submit some tasks to the pool that require some time to finish. Before the pool has been closed and joined, click Stop or Rerun for the script. Notice the session is now hung and the children processes that persist.

Expected behavior:

Streamlit should probably send some king of a signal to the script so it can attempt to terminate itself before Streamlit kills it.

Actual behavior:

Session is seemingly corrupted and children processes are orphaned.

Is this a regression?

No

Debug info

• Streamlit version: 0.53.0
• Python version: 3.8.1
• Using Conda? PipEnv? PyEnv? Pex? Just a normal virtual environment from python -m venv
• OS version: CentOS Linux 7
• Browser version: Firefox 72.0.1

Additional information

More here discuss.streamlit.io

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[tconkling]

I wonder if we could add something like st.signal_handler(handler_function) that would allow app scripts to register a “Streamlit signal handler” to be called when the script is being terminated early, so that multi-threaded or multi-process Streamlit apps can handle cleanup tasks properly.

[arkanoid87]

I’m also facing this. Streamlit is unable to handle child procs.

Streamlit process cannot be killed with SIGTERM (just prints stopping ...) and I see child procs spinning at 100% cpu even if no browser is open. system.exit() or quit won’t do, only SIGKILL will return to shell