The register path builds each profile-gateway slot in a sibling staging
dir under /run/service (the scandir s6-svscan watches), then atomically
renames it to the live gateway-<profile> name. The staging dir was named
gateway-<profile>.tmp — a NON-dotfile — so a concurrent `s6-svscanctl -a`
rescan (fired by the cont-init reconciler registering gateway-default, or
by a sibling register) would supervise the half-built slot the moment it
had a valid type/run: s6-supervise spawns AS ROOT and mkdirs supervise/
root-owned 0700, then the in-flight _seed_supervise_skeleton early-returns
on the now-existing supervise/ and the next `mkdir supervise/event` hits
PermissionError.
That is the arm64-only CI flake on
test_s6_unregister_removes_service_dir_in_live_container
(PermissionError: /run/service/gateway-phase3test.tmp/supervise/event) —
arm64-only because the native-arm runner's wider scheduling jitter lets
the rescan land inside the ~ms seed window; amd64 ran 30/30 clean.
Fix: dot-prefix the staging dir (.gateway-<profile>.tmp) in both register
paths (S6ServiceManager.register_profile_gateway and
container_boot._register_service). s6-svscan skips any scandir entry whose
name begins with '.', so the half-built slot can never be supervised
mid-build. The atomic rename to the dotless live name is unchanged.
Verified on a real s6 image (amd64): a non-dotted staging dir is picked up
by an svscanctl -a rescan (SUPERVISED owner=root) while a dot-prefixed one
is ignored (NOT-SUPERVISED). Added a docker-harness regression test that
asserts both, plus a unit test that the staging dir is dot-prefixed.
Ben Barclay
f53ba9bb54