## Why
Interrupted `shell_command` calls can race with the outer tool-dispatch
cancellation path. When that happens, the runtime future may be dropped
before the spawned process gets a chance to run `SIGTERM` cleanup. For
bwrapd-backed Linux sandbox commands, that can leave synthetic
protected-path mount bookkeeping such as `.git/.codex` registrations
under `/tmp` behind after a TUI interruption.
The relevant cancellation points are the outer dispatch race in
[`core/src/tools/parallel.rs`](https://github.com/openai/codex/blob/bd184ba84703cc924921ed883f0cf17d3dba60ff/codex-rs/core/src/tools/parallel.rs#L91-L132)
and the process shutdown logic in
[`core/src/exec.rs`](https://github.com/openai/codex/blob/bd184ba84703cc924921ed883f0cf17d3dba60ff/codex-rs/core/src/exec.rs#L1367-L1393).
## What changed
- Keep `shell_command` dispatch alive long enough for the runtime to
finish cancellation cleanup instead of immediately returning the
synthetic aborted response.
- Fold shell-turn cancellation into the existing `ExecExpiration` path
in
[`core/src/tools/runtimes/shell.rs`](https://github.com/openai/codex/blob/bd184ba84703cc924921ed883f0cf17d3dba60ff/codex-rs/core/src/tools/runtimes/shell.rs#L267-L274),
so cancellation and timeout behavior stay centralized.
- On cancellation, send `SIGTERM` first, wait briefly for cleanup to
run, then hard-kill any remaining descendants in the original process
group.
- Treat `ESRCH` as an already-gone process-group cleanup case in
`codex-utils-pty`, which keeps best-effort teardown from surfacing a
stale-process race as an error.
## Verification
- `cargo test -p codex-core cancellation`
- Added regression coverage for:
- `shell_tool_cancellation_waits_for_runtime_cleanup`
- `process_exec_tool_call_cancellation_allows_sigterm_cleanup`
This PR changes stdio MCP child processes to run in their own process
group
* Add guarded teardown in codex-rmcp-client: send SIGTERM to the group
first, then SIGKILL after a short grace period.
* Add terminate_process_group helper in process_group.rs.
* Add Unix regression test in process_group_cleanup.rs to verify wrapper
+ grandchild are reaped on client drop.
Addresses reported MCP process/thread storm: #10581