What: the POSIX build launcher (_OWN_GROUP -> _REAP_TREE) no longer starts node in a session of
its own. Node stays in the caller's process group; the launcher holds the checkout lock fd and,
once node exits, kills every remaining descendant: on Linux it is a child subreaper
(PR_SET_CHILD_SUBREAPER) and SIGKILLs + reaps its own children until none is left; elsewhere
(macOS) it records node's descendant tree from ps while node runs and kills the recorded
processes whose start time still matches. SIGINT/SIGTERM/SIGHUP are forwarded to node (killed
10 s later if still running), then the descendants are killed the same way.
Why: start_new_session=True moved node out of the caller's group, so every existing group kill
(update_completion's killpg on Ctrl-C, Desktop's kill(-pid), killHandoffTree) reached only the
launcher; node was orphaned and its npm/esbuild descendants kept writing after the checkout lock
was free (review L1, probe review/lock/custody/p1: grandchild wrote after abort, lock free).
The old killpg after p.wait() also signalled a reaped leader's group (pid-reuse window, review
L6): the launcher now signals only its own unreaped children (Linux) or start-time-verified
pids (ps walk), never a group.
Tests (RED on bbfa23b137d, GREEN here): a group kill of the caller-visible group stops node and
its child; a detached (setsid) grandchild outliving node is killed before the build returns, on
the subreaper and the ps-walk paths; SIGTERM to the launcher exits 143 and leaves no writer.
ab8a00d2be