fix(shim): use per-container id for TaskOOM routing (fixes #12838)

[kpurdon]

Summary

Fixes #12838 for real. On aarch64 (and any architecture where the containerd race between TaskOOM and TaskExit routinely resolves OOM-last), OOM-killed pods run by gVisor are reported by kubelet with reason:Error instead of reason:OOMKilled.

PR #12843 added a synchronous cgroup check at init exit to close the async OOM-vs-exit race. That part works — but it also publishes TaskOOM{ContainerID: s.id}, and s.id is empty in the normal shim spawn path. So on aarch64 the symptom survives #12843 because the bug is routing, not just timing.

Root cause

1. pkg/shim/v1/manager.go:newCommand() spawns the shim binary with -namespace, -address, and -publish-binary only — no -id.
2. In the spawned shim, containerd's vendored shim framework parses the flags; -id defaults to "" (runtime/v2/shim/shim.go: flag.StringVar(&id, "id", "", ...)). With manager == nil, that empty id is threaded into runsc.NewTaskService(ctx, id, ...), so runscService.id = "".
3. Every TaskOOM{ContainerID: s.id} — from the async oom_v2.go's run() and from the sync service.go's checkProcesses() added by fix(shim): synchronously check cgroup for OOM at container exit #12843 — ships with ContainerID="".
4. containerd's CRI handler (internal/cri/server/events.go) routes TaskOOM via containerStore.Get(e.ContainerID). Get("") returns ErrEmptyPrefix (from internal/truncindex). That is not IsNotFound, so handleEvent wraps and returns the error; containerd logs can't find container for TaskOOM event: prefix can't be empty, and the container's Status.Reason is never set.
5. TaskExit is unaffected because containerd routes TaskExit by e.ID (init process id), not e.ContainerID — different field, different lookup.

Fix

Don't rely on s.id for TaskOOM routing. Use the per-container id that's already available at the point the OOM poller is wired up and at the point the exit is observed.

• CreateWithFSRestore: register the cgroup with rfs.Create.ID (the per-container id from the CreateTaskRequest) rather than s.id. The id stored in the watcher's cgroups and lastOOM maps is now non-empty and correctly keyed per container, so both the async (EventChan -> run) and sync (isOOM at exit) paths publish with a real ContainerID.
• checkProcesses: locate the container whose process matched the exit event, use that container's id for isOOM, and use it for both TaskOOM.ContainerID and TaskExit.ContainerID.

oom_v2.go is unchanged — with add() keyed by rfs.Create.ID, the existing async publish path already emits TaskOOM with the correct id via i.id.

Validation

End-to-end in a prod-matching environment (AL2023 aarch64, kernel 6.12, containerd 2.2.1, kubelet 1.34, runsc built from this branch):

• Before: OOM'd pods show reason:Error; containerd journal shows TaskOOM event (trailing space = empty ContainerID) and Failed to handle backOff event for error="can't find container for TaskOOM event: prefix can't be empty".
• After: OOM'd pods show reason:OOMKilled; journal shows TaskOOM event container_id:"<real-64-hex>" and zero prefix can't be empty errors.

Also soaked on staging at Semgrep with real workloads across both x86_64 and aarch64 nodepools (multi-container gvisor pods, forced OOM on a 128Mi-limited python container alongside a busybox sidecar). Main-container OOMs now consistently report reason:OOMKilled on both arches.

Scope

This PR touches only the cgroups v2 init-exit path (CreateWithFSRestore and checkProcesses in runsc/service.go). The cgroups v1 poller (epoll.go) has the same structural reliance on s.id and should get the equivalent treatment — happy to file a follow-up once this lands.

Follow-ups noted in this work (separate PRs)

• cgroups v1 shim (epoll.go) has the same empty-id TaskOOM path.
• pkg/shim/v1/runsccmd/utils.go:FormatShimLogPath documents %ID% substitution but the implementation only appends a filename when the path ends in /. As a result the shim currently writes all container logs into a literal /var/log/runsc/%ID%/ directory rather than per-container directories. Noticed while gathering logs for this PR.

[milantracy]

Thanks! I'll happy to review when it is ready

[kpurdon]

@milantracy validate this fully today, should be good to go!

[milantracy]

thanks @kpurdon ! i added some comments, but most of change LGTM