tbench2 env server: enable idle-session reaper to reclaim leaked slots

[Shi-Dong]

What

The tbench2_env server now enables the core idle-session reaper by passing an explicit ConcurrencyConfig(max_concurrent_envs=..., session_timeout=...) to create_app, instead of the bare max_concurrent_envs= argument. A new SESSION_IDLE_TIMEOUT_S env var (default 3600) controls the reap interval.

Why

The /mcp WebSocket handler releases its session slot and Docker container only in a finally block, which runs solely on a clean socket close. Half-open or uncleanly-dropped connections never raise WebSocketDisconnect, so receive_text() blocks forever, the finally never runs, and the session slot + its Docker container leak.

Under sustained load these leaks accumulate over roughly a day until all MAX_CONCURRENT_ENVS slots are permanently held. New episodes then deadlock on CAPACITY_REACHED and the only recovery is a manual env-server restart.

Passing a bare max_concurrent_envs maps to ConcurrencyConfig(session_timeout=None) in the core server, which disables _reap_idle_sessions. Supplying an explicit session_timeout re-enables the reaper so stale sessions are evicted and their containers reclaimed automatically, without operator intervention.

Notes

• Default SESSION_IDLE_TIMEOUT_S=3600 preserves existing behavior for short-lived sessions while bounding leaked slots.
• Single-file change scoped to envs/tbench2_env/server/app.py; no change to the core server or to the on-the-wire protocol.

---

Note

Low Risk
Single-file server bootstrap change with a conservative default timeout; no protocol or core server edits.

Overview
The tbench2_env server now passes an explicit ConcurrencyConfig to create_app (with max_concurrent_envs and session_timeout) instead of only max_concurrent_envs, which turns on the core idle-session reaper that was previously off when session_timeout was unset.

A new SESSION_IDLE_TIMEOUT_S env var (default 3600) sets how long a session can sit idle before it is reaped so its WebSocket slot and Docker container are released. That targets leaks from half-open /mcp WebSocket connections that never hit a clean disconnect, which could otherwise hold all MAX_CONCURRENT_ENVS slots until a manual restart.

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[bot-ci-comment]

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[nblintao]

LGTM.