fabric-s6-container-supervision

name	fabric-s6-container-supervision
description	Modify, debug, or extend the s6-overlay supervision tree inside the Fabric Docker image — adding new services, debugging profile gateways, understanding the Architecture B main-program pattern.
version	1.0.0
author	Fabric
license	MIT
platforms	["linux"]
environments	["s6"]
metadata	{"fabric":{"tags":["docker","s6","supervision","gateway","profiles"],"related_skills":["fabric-agent","fabric-agent-dev"]}}

Fabric s6-overlay Container Supervision

When to use this skill

Load this skill when you're working on:

• Adding or removing a static service in the Fabric Docker image
• Diagnosing why a per-profile gateway isn't starting, restarting, or surviving docker restart
• Understanding why the container's CMD is /opt/fabric/docker/main-wrapper.sh and how leading-dash args reach the user's program
• Modifying cont-init.d boot scripts (UID remap, volume seeding, profile reconciliation)
• Changing the rendered run-script for per-profile gateways (Phase 4)

If you're running Fabric and want to use Docker, see website/docs/user-guide/docker.md instead.

Architecture at a glance

/init                                  ← PID 1 (s6-overlay v3.2.3.0)
├── cont-init.d                        ← oneshot setup, runs as root
│   ├── 01-fabric-setup                ← docker/stage2-hook.sh
│   │   ├── UID/GID remap
│   │   ├── chown /opt/data
│   │   ├── chown /opt/data/profiles (every boot)
│   │   ├── seed .env / config.yaml / SOUL.md
│   │   └── skills_sync.py
│   └── 02-reconcile-profiles          ← fabric_cli.container_boot
│       ├── chown /run/service (fabric-writable for runtime register)
│       └── walk $FABRIC_HOME/profiles/<name>/gateway_state.json
│           → recreate /run/service/gateway-<name>/
│           → auto-start only those with prior_state == "running"
│
├── s6-rc.d (static services, in /etc/s6-overlay/s6-rc.d/)
│   └── main-fabric/run                ← exec sleep infinity (no-op slot)
│
├── /run/service (s6-svscan watches; tmpfs)
│   ├── gateway-coder/                 ← runtime-registered per-profile
│   │   ├── type        ("longrun")
│   │   ├── run         ("#!/command/with-contenv sh ... exec s6-setuidgid fabric fabric -p coder gateway run")
│   │   ├── down        (marker — present means "registered but don't auto-start")
│   │   └── log/run     (s6-log → $FABRIC_HOME/logs/gateways/coder/current)
│   └── ...
│
└── CMD ("main program")               ← /opt/fabric/docker/main-wrapper.sh
    └── routes user args: bare exec | fabric subcommand | fabric (no args)
        — exec'd by /init with stdin/stdout/stderr inherited (TTY for --tui)

Key files

Path	Role
Dockerfile	s6-overlay install + cont-init.d wiring + ENTRYPOINT ["/init", "/opt/fabric/docker/main-wrapper.sh"]
docker/stage2-hook.sh	Container initialization — UID remap, chown, seed, and skills sync. Runs as cont-init.d/01-fabric-setup.
docker/cont-init.d/02-reconcile-profiles	Calls fabric_cli.container_boot on every boot to restore profile gateway slots from the persistent volume.
docker/main-wrapper.sh	The container's CMD. Routes user args, drops to fabric via s6-setuidgid, exec's the chosen program.
docker/s6-rc.d/main-fabric/run	No-op sleep infinity — slot exists so the s6-rc user bundle is valid; the Fabric process runs as the CMD, not as a supervised service.
fabric_cli/service_manager.py	S6ServiceManager: register_profile_gateway, unregister_profile_gateway, start/stop/restart/is_running, list_profile_gateways.
fabric_cli/container_boot.py	reconcile_profile_gateways() — walks persistent profiles, regenerates s6 slots, emits container-boot.log.
fabric_cli/gateway.py::_dispatch_via_service_manager_if_s6	Intercepts fabric gateway start/stop/restart and routes to s6 when running in a container.

Why Architecture B (CMD as main program, not s6-supervised)

The original plan (v1–v3) called for the main Fabric process to run as a supervised s6-rc service. Two real s6-overlay v3 mechanics blocked that:

1. cont-init.d scripts receive no CMD args — so the stage2 hook cannot pass docker run <image> chat -q "hi" arguments to a service run script.
2. /run/s6/basedir/bin/halt does NOT propagate the exit code written to /run/s6-linux-init-container-results/exitcode. Containers always exit 143 (SIGTERM) regardless. Confirmed by skarnet (s6 author) in issue #477: "if you want a container shutdown, you need to either have your CMD exit, or, if you have no CMD, write the container exit code you want then call halt".

So we use the s6-overlay-native CMD pattern: ENTRYPOINT ["/init", "/opt/fabric/docker/main-wrapper.sh"]. /init prepends the wrapper to user args automatically — so docker run <image> --version becomes /init main-wrapper.sh --version, and --version doesn't get intercepted by /init's POSIX shell. The wrapper drops to fabric via s6-setuidgid, then exec's the chosen program. The program's exit code becomes the container exit code, exactly matching the pre-s6 tini contract.

Trade-off: the main Fabric process is unsupervised under s6. That exactly matches its behavior under tini (the pre-s6 image), while per-profile gateways under /run/service/ get full supervision.

Quick recipes

Verify s6 is PID 1 in a running container

docker exec <c> sh -c 'cat /proc/1/comm; readlink /proc/1/exe'
# Expect: s6-svscan or init / /package/admin/s6/.../s6-svscan

Inspect a profile gateway service

# /command/ isn't on docker-exec PATH — use absolute path
docker exec <c> /command/s6-svstat /run/service/gateway-<name>
# "up (pid …) … seconds"            → running
# "down (exitcode N) … seconds, normally up, want up, …" → s6 wants it up but the process keeps exiting (crash loop)
# "down … normally up, ready …"     → user stopped it

Bring a service up/down manually

docker exec <c> /command/s6-svc -u /run/service/gateway-<name>   # up
docker exec <c> /command/s6-svc -d /run/service/gateway-<name>   # down
docker exec <c> /command/s6-svc -t /run/service/gateway-<name>   # SIGTERM (restart)

Watch the cont-init reconciler log

docker exec <c> tail -n 50 /opt/data/logs/container-boot.log
# 2026-05-21T06:18:05+0000 profile=coder prior_state=running action=started
# 2026-05-21T06:18:05+0000 profile=writer prior_state=stopped action=registered

Add a new static service

1. Create docker/s6-rc.d/<name>/type with longrun\n and docker/s6-rc.d/<name>/run (use #!/command/with-contenv sh + # shellcheck shell=sh).
2. Drop to fabric via s6-setuidgid fabric at the top of run (unless you specifically need root).
3. Create empty docker/s6-rc.d/<name>/dependencies.d/base so it waits for the base bundle.
4. Create empty docker/s6-rc.d/user/contents.d/<name> so it joins the user bundle.
5. The COPY docker/s6-rc.d/ in the Dockerfile picks it up automatically — no other changes.

Change the per-profile gateway run command

Edit S6ServiceManager._render_run_script in fabric_cli/service_manager.py. The function is also called by fabric_cli/container_boot.py::_register_service during boot reconciliation, so it's the single source of truth. Update the corresponding assertion in tests/fabric_cli/test_service_manager.py::test_s6_register_creates_service_dir_and_triggers_scan.

Run the docker test harness

docker build -t fabric-agent-harness:latest .
scripts/run_tests.sh tests/docker/ -v
# Expect 19 passed, 0 xfailed against the s6 image

The harness lives in tests/docker/ and skips when Docker isn't available. The per-test timeout is bumped to 180s (see tests/docker/conftest.py).

Common pitfalls

"command not found" via docker exec

/command/ (where s6-overlay puts its binaries) is on PATH only for processes spawned by the supervision tree — services, cont-init.d, main-wrapper.sh. docker exec <c> s6-svstat … will fail with "command not found"; always use the absolute path /command/s6-svstat. The fabric command works because the Dockerfile adds /opt/fabric/.venv/bin to the runtime ENV PATH.

Profile directory ownership

The cont-init reconciler runs as fabric (s6-setuidgid fabric in 02-reconcile-profiles). If a profile dir ends up root-owned (e.g. because docker exec <c> fabric profile create … ran as root by default), the reconciler can't read SOUL.md and fails with PermissionError. Mitigation: stage2-hook.sh chowns $FABRIC_HOME/profiles to fabric on every boot, idempotently. Don't remove that block.

Files written by docker exec are root-owned

docker exec defaults to root. Either pass --user fabric or rely on the stage2 chown sweep next reboot. Don't write files under $FABRIC_HOME/profiles/<name>/ as root manually — the next reconcile pass will sweep them but in-flight operations may hit permission errors.

Service slot exists but s6-svstat says "s6-supervise not running"

The service directory is on tmpfs and was wiped on container restart. Either the cont-init reconciler hasn't run yet (give it a moment after docker restart) or it failed. Check docker logs <c> | grep '02-reconcile'.

Gateway starts then immediately exits (down (exitcode 1) in svstat)

Most likely the profile has no model or auth configured. The service slot is correct — the gateway itself is unconfigured. Run fabric -p <profile> setup first. The s6 supervisor will keep restarting it; that's the desired behavior (when you fix the config, the next attempt succeeds and stays up).

Reconciler skipped a profile

The reconciler keys on the presence of SOUL.md as the "real profile" marker. fabric profile create always seeds it. If a profile dir is missing SOUL.md (stray directory, partial restore, backup-in-progress), the reconciler skips it intentionally. Add a SOUL.md (even empty) to opt back in.

"Help, the container exits 143!"

Check whether something is invoking s6-svscanctl -t or /run/s6/basedir/bin/halt — both cause /init to begin stage 3 shutdown but return 143 (SIGTERM) rather than the desired exit code. This was the Phase 2 architecture pivot from A to B. For container shutdown with a real exit code, you must let the CMD (main-wrapper.sh) exit normally; do not try to control exit from a finish script.

Related skills

• fabric-agent-dev: General fabric-agent codebase navigation