The gateway-embedded dispatcher (default since `kanban.dispatch_in_gateway
= true`) is the parent of every spawned kanban worker. `_default_spawn`
calls `subprocess.Popen(..., start_new_session=True)` and returns the
pid — `start_new_session` detaches the controlling tty but does not
reparent to init, so the gateway keeps each worker as a child until it
`wait()`s for them.
Nothing in the dispatch loop ever calls `waitpid`. Result: every
completed worker becomes a `<defunct>` zombie that lingers until the
gateway exits. We hit ~430 zombies on a single hermes-agent container
after ~40 days of steady kanban traffic, approaching process-table
exhaustion on the host.
Fix: add a non-blocking reap loop at the top of `dispatch_once`, so
every dispatcher tick (default 60s) drains zombies that accumulated
since the last tick. WNOHANG keeps the call non-blocking; ChildProcessError
means no children to reap.
Why here, not a SIGCHLD handler:
- signal.signal requires the main thread; gateway threading model makes
that placement non-trivial.
- Bounded staleness: at default interval=60s the maximum live zombie
count is one tick's worth of worker completions.
- No interaction with detect_crashed_workers: that function only inspects
rows where status='running', and rows reach 'done' (and stop being
inspected) before their workers exit.
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