The background-process registry signalled host PIDs (recovery adoption,
detached-session kill, tree-kill) using a number captured at spawn, guarded
only by a bare liveness check. Once a session's process exits and is reaped the
kernel recycles that PID onto an unrelated process, so an alive-but-different
PID passed the check and got tree-killed.
Observed in the wild: a recycled background-session PID landed on Firefox's
session leader; a later kill/refresh walked its process tree and SIGTERMed
every tab — Firefox "closing" at irregular intervals with no crash/coredump.
This is the same PID/PGID-recycling class fixed for the MCP orphan reaper in
7bd1f8a2d, but the process_registry subsystem was never guarded — so the bug
persisted.
Fix: record each host process's kernel start time (/proc/<pid>/stat field 22)
at spawn, persist it in the checkpoint, and re-validate it before every signal
via `_host_pid_is_ours`. A PID whose start time no longer matches — or that is
gone — is never signalled:
- recover_from_checkpoint: a recycled PID is not adopted as a session.
- _refresh_detached_session: a recycled detached PID is marked exited.
- kill_process / _terminate_host_pid: refuse to tree-kill a stranger.
Legacy checkpoints and platforms without /proc (no baseline) degrade to the
prior best-effort liveness behaviour, so nothing else changes.
Adds TestPidReuseGuard: real-process tests proving a mismatched start time
refuses termination while a matching one still kills, plus recovery/refresh
recycling paths. 74 registry + 22 MCP-stability tests green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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