[Bug]: Python Beam does not fail a job if the main session was present could not be unpickled

[psobot]

What happened?

When launching a Beam pipeline in Python, the --save_main_session flag can be used to serialize the local session's state and load it on the workers. However, if the deserialization of this main session fails, a log line indicating "Could not load main session" is printed, but the pipeline continues to execute - just without its global state, causing NameErrors in various places.

These NameError exceptions don't hint at the original root cause (which is that the main session state failed to deserialize, for whatever reason) which causes a large amount of confusion, especially for those unfamiliar with the intricacies of how Beam works under the hood.

I resolved a similar issue back in Beam 2.31 (#14706) that failed to stop a job if the main session was corrupted; this seems similar.

Expected behaviour: if a main session is present but fails to deserialize, this is considered a "hard error" that stops the entire Beam job from running and forces the programmer to deal with the cause of the failure, rather than proceeding to run the job's code in a half-working state.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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[tvalentyn]

Thanks for reporting, sounds like a fixable usability issue. we should probably fail in this case, as long as the main session is provided and not empty. Do you have a repro by chance?

[psobot]

I can put together a tiny repro, but in general this can be triggered by:

• Creating a Python Beam pipeline that's a single script
• Adding import <any_third_party_module> at the top of the script
• Not including anything in install_requires in setup.py
• Launching with --save_main_session

[tvalentyn]

Got it, I don't think we need a repro, reproduction steps are clear. We can improve the logic

beam/sdks/python/apache_beam/runners/worker/sdk_worker_main.py

Line 125 in 011296c

'Could not load main session: %s', exception_details, exc_info=True)

and fail with an actionable error. I'll send a PR.