`docker stack deploy` drops the per-service `runtime:` key, so a swarm worker
can only carry the sandbox as its daemon-wide `default-runtime`. That was never
set: the role reached the manager only, no node was labelled, and one of three
agent replicas executed model-generated code under plain runc.
Setting it from the base render is not possible either. The container role
writes `daemon.json` before the runtime binary exists, and dockerd refuses to
start when a runtime path does not resolve ("stat /usr/local/bin/runsc: no such
file or directory"). The role's own attempt to re-render afterwards was a silent
no-op, because `include_role: sys-svc-container` hits the
`run_once_sys_svc_container` guard.
Move the probes plus the render into `daemon_config.yml` and let the role
re-enter it via `tasks_from` after installing runsc. Only runtimes whose binary
the probe found are registered, and `default-runtime` is written only for a
present shim on a non-stack host, so the stack host keeps the shared kernel for
the platform's own services.
The placement constraint compares against a fixed literal because Docker
constraints have no presence test: label the node `kata-capable=true` rather
than with the runtime name, otherwise a Kata node fails a `== runsc`
comparison. Without a worker outside the stack host no node is eligible at all,
so refuse the deploy instead of leaving the service pending forever.
`select_runtime.yml` is gone: hermes and openclaw included it by absolute path
out of a foreign role, and one probe in the container role now serves every
consumer.
Verified live on a three node cluster: worker `DefaultRuntime=runsc`, both
workers labelled, manager unlabelled, service constrained, and the running
replica reports `runtime=runsc`.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Agent workloads execute model-generated code, so they need a kernel
boundary on top of the app's own sandboxing: Kata plus Firecracker where
hardware virtualisation is available, the gVisor userspace kernel
otherwise, never plain runc.
runsc is fetched as the upstream static release binary with its published
sha512 and the host architecture in the URL, because Debian ships only a
2024 build and Arch and Fedora ship none at all; the same path serves the
arm64 Raspberry Pi target.
select_runtime.yml is a reusable task file: it picks kata only when
/dev/kvm, the kata shim and a bare host all hold. /dev/kvm is visible
inside a Docker-in-Docker daemon through host passthrough, but the microVM
still cannot start there (fuse-overlayfs, no containerd block devices), so
DinD is excluded explicitly and degrades to runsc.
daemon.json gains a runtimes block gated on group membership. Under DinD
runsc additionally needs --network=host: its netstack cannot pluck
interfaces inside a nested netns and aborts with 'cannot run with network
enabled in root network namespace'. The flag keeps the syscall sandbox and
reaches the container netns through real sockets.
The role bootstraps before the service-loader preload, alongside
svc-storage and svc-registry, so the runtime is registered and the daemon
restarted before the first workload compose file renders.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
kevinveenbirkenbachandClaude Opus 5