[4/6] Abstract MCP stdio server launching

[aibrahim-oai]

• Move local MCP stdio process startup behind a runtime trait.
• Preserve existing local stdio behavior while making transport creation explicit.

[chatgpt-codex-connector]

💡 Codex Review

codex/codex-rs/exec-server/src/local_process.rs

Line 214 in 3842a64

let (event_tx, _event_rx) = broadcast::channel(PROCESS_EVENT_CHANNEL_CAPACITY);

Buffer local process events before subscribers attach

The event channel is created with _event_rx immediately dropped, so there are initially zero receivers. stream_output/watch_exit ignore broadcast::Sender::send errors, which means early Output/Exited/Closed events are discarded for short-lived commands. A caller that subscribes right after start() can miss terminal events and wait forever.

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codex/codex-rs/exec-server/src/client.rs

Line 458 in 3842a64

let (event_tx, _event_rx) = broadcast::channel(PROCESS_EVENT_CHANNEL_CAPACITY);

Preserve remote events emitted before first subscription

SessionState::new also drops the initial broadcast receiver, leaving no listeners until user code calls subscribe_events(). Notifications published in that window are silently lost because publish_event ignores send failures. Fast remote processes can emit Closed/Exited before subscription, causing event-driven consumers to miss termination and hang.

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