fix(lcb-service): use the fork start method explicitly - #433
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Python 3.14 changed the default multiprocessing start method on Linux from fork to forkserver. The grading pipeline (pool workers forking a per-problem mp.Process + mp.Manager) only works with fork: under forkserver the grading children die at startup, every result comes back as an error, and execute_code_single_suppressed_errors turns that into all-failed tests, so the service sits at 0/N forever. Pin the fork context for the executor, the per-problem Process and its Manager. Also raise if every subprocess reported an execution error -- that means the judge is broken, not that all samples failed -- and log those errors at error level instead of warning. Seen on a python 3.14 lcb-service image: 0/349 after 3.5h, one defunct child per pool worker. Same inputs with fork forced: done in 6 min. The repo pins 3.12 so CI won't hit this, but shipped images have.
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Python 3.14 changed the default multiprocessing start method on Linux from fork to forkserver. The grading pipeline (pool workers forking a per-problem mp.Process + mp.Manager) only works with fork: under forkserver the grading children die at startup, every result comes back as an error, and execute_code_single_suppressed_errors turns that into all-failed tests, so the service sits at 0/N forever.
Pin the fork context for the executor, the per-problem Process and its Manager. Also raise if every subprocess reported an execution error -- that means the judge is broken, not that all samples failed -- and log those errors at error level instead of warning.
Seen on a python 3.14 lcb-service image: 0/349 after 3.5h, one defunct child per pool worker. Same inputs with fork forced: done in 6 min. The repo pins 3.12 so CI won't hit this, but shipped images have.
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