ci: backend-agnostic integration guards + per-backend preflight

Integration tests now select their backend from BOT_BOTTLE_BACKEND and
skip on the capability that backend actually needs, instead of gating
every backend on unrelated Docker availability.

Task 1 — backend-agnostic guards (tests/_backend.py):
- Capability probes: docker_capability() (reachable daemon) and
  firecracker_capability() (accessible /dev/kvm + firecracker on PATH,
  Docker-independent). backend_capability()/selected_backend() resolve
  the target from BOT_BOTTLE_BACKEND (default docker).
- skip_unless_selected_backend_available() for backend-agnostic tests
  (test_sandbox_escape) — runs through whichever backend is selected and
  checks that backend's real capability.
- skip_unless_backend("docker") for Docker-implementation tests
  (DockerBroker, DockerGateway, backend.docker.*) — they no-op under a
  non-Docker run rather than testing internals that run doesn't target.
- Retires tests/_docker.py; the KVM job no longer needs SKIP_DOCKER_TESTS
  to steer Docker-only classes.

Task 2 — explicit per-backend skip visibility:
- tests/backend_preflight.py prints a clear PASS/FAIL capability line and
  exits non-zero when the selected backend is missing.
- Both integration jobs run it as a preflight, so absent infrastructure
  is surfaced at the job level instead of hidden among unittest.skip
  lines. The docker job replaces its soft "Show environment" step; the
  firecracker job keeps its richer backend-status check.

Docs (tests/README.md, docs/ci.md) updated; unit coverage for the probes,
guards, and preflight in test_backend_skip_guards.py.

Closes #414

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -103,14 +103,15 @@ jobs:
       - name: Install coverage
         run: python3 -m pip install --break-system-packages coverage
 
-      - name: Show environment
+      # Fail loudly if the backend this job promises isn't actually usable,
+      # rather than letting every test silently `unittest.skip` and the job
+      # go green on zero coverage. Same capability probe the skip guards use.
+      - name: Preflight — Docker backend is ready
+        env:
+          BOT_BOTTLE_BACKEND: docker
         run: |
           python3 --version
-          if command -v docker >/dev/null 2>&1; then
+          python3 -m tests.backend_preflight docker
-            docker version || true
-          else
-            echo "docker not on PATH — integration tests will skip"
-          fi
 
       - name: Run integration tests (docker) with coverage
         env:
@@ -157,10 +158,13 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Preflight — Firecracker host is ready
+        env:
+          BOT_BOTTLE_BACKEND: firecracker
         run: |
-          command -v firecracker >/dev/null || {
+          # Standardized capability line — same probe (and PASS/FAIL wording)
-            echo "firecracker not on PATH — provision the runner (README: Firecracker on Linux)"; exit 1; }
+          # the integration skip guards use, so a missing backend is visible
-          test -e /dev/kvm || { echo "/dev/kvm missing — KVM not available on this runner"; exit 1; }
+          # at the job level rather than hidden among `unittest.skip` lines.
+          python3 -m tests.backend_preflight firecracker
           # `backend status` exits non-zero unless the TAP pool is up + no
           # range overlap; it prints the exact `backend setup` fix.
           python3 cli.py backend status --backend=firecracker

docs/ci.md

@@ -1,13 +1,23 @@
 # CI
 
 The test workflow lives at [`.gitea/workflows/test.yml`](../.gitea/workflows/test.yml).
-It runs `tests/run_tests.py` (full suite — unit + integration) on:
+It runs the unit suite plus one integration job per backend
+(`integration-docker`, `integration-firecracker`) on:
 
 - every push to a branch with an open pull request, and
 - every push to `main`.
 
-Integration tests need Docker on the runner; they skip cleanly via
+Each integration job selects its backend via `BOT_BOTTLE_BACKEND` and
-`tests/_docker.skip_unless_docker` when no daemon is reachable.
+runs a **preflight** (`python3 -m tests.backend_preflight <backend>`)
+that prints a clear PASS/FAIL capability line and fails the job when the
+backend is missing — so absent infrastructure is visible at the job level
+rather than hidden among per-test `unittest.skip` lines. The same
+capability probes back the skip guards in
+[`tests/_backend.py`](../tests/_backend.py): backend-agnostic tests use
+`skip_unless_selected_backend_available()` and run through whichever
+backend is selected (checking, e.g., `/dev/kvm` for Firecracker rather
+than unrelated Docker availability); Docker-implementation tests use
+`skip_unless_backend("docker")` and no-op under a non-Docker run.
 
 A small subset of integration tests skip when running specifically
 under Gitea Actions (`GITEA_ACTIONS=true`), because `act_runner` runs

tests/README.md

@@ -2,14 +2,16 @@
 
 Plain-Python test suite using stdlib `unittest`. No external
 dependencies. Unit tests run anywhere Python 3 is present; integration
-tests need Docker and skip cleanly otherwise.
+tests run through the backend named by `BOT_BOTTLE_BACKEND` (default
+`docker`) and skip cleanly when that backend isn't available on the host.
 
 ## Layout
 
 ```
 tests/
   fixtures.py                       # JSON manifest builders (shared)
-  _docker.py                        # docker-availability skip helper (shared)
+  _backend.py                       # backend capability probes + skip guards
+  backend_preflight.py              # `python -m tests.backend_preflight` (CI)
   unit/
     test_egress.py
     test_egress_addon_core.py
@@ -73,7 +75,7 @@ BOT_BOTTLE_RUN_CANARIES=1 python -m unittest discover -t . -s tests/canaries -v
 ## Adding a test
 
 1. Pick the directory: `tests/unit/` for a pure unit test,
-   `tests/integration/` for one that needs Docker.
+   `tests/integration/` for one that needs a backend.
 2. Filename: `test_<topic>.py`.
 3. Boilerplate:
    ```python
@@ -88,5 +90,16 @@ BOT_BOTTLE_RUN_CANARIES=1 python -m unittest discover -t . -s tests/canaries -v
    if __name__ == "__main__":
        unittest.main()
    ```
-4. For Docker-dependent tests, decorate the class with
+4. Skip guards live in `tests._backend`:
-   `@skip_unless_docker()` from `tests._docker`.
+   - Backend-agnostic tests (go through `get_bottle_backend()`) decorate
+     the class with `@skip_unless_selected_backend_available()` — the test
+     runs against whichever backend `BOT_BOTTLE_BACKEND` selects and skips
+     unless that backend's capability is present (a reachable Docker daemon,
+     or an accessible `/dev/kvm` + `firecracker` for Firecracker).
+   - Backend-specific tests (exercise `DockerBroker`, `DockerGateway`,
+     `backend.docker.*`, …) decorate with `@skip_unless_backend("docker")`
+     so they no-op under a run targeting a different backend.
+
+   The same capability probes back the CI preflight,
+   `python -m tests.backend_preflight [<backend>]`, which prints a clear
+   PASS/FAIL line and exits non-zero when the selected backend is missing.

tests/_backend.py

@@ -0,0 +1,153 @@
+"""Backend-aware capability probes and skip guards for the integration suite.
+
+Every integration test selects its backend from ``BOT_BOTTLE_BACKEND``
+(default ``docker``) and exercises the full test through that backend. Skip
+guards check the *capability* the selected backend actually needs — a
+reachable Docker daemon for ``docker``, an accessible ``/dev/kvm`` plus a
+``firecracker`` binary for ``firecracker`` — rather than gating every backend
+on unrelated Docker availability.
+
+The same probes back the CI preflight (``python3 -m tests.backend_preflight``)
+so a job surfaces missing infrastructure at the job level with a clear
+PASS/FAIL line instead of turning every test into a silent ``unittest.skip``.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import subprocess
+import unittest
+from dataclasses import dataclass
+
+# Default when ``BOT_BOTTLE_BACKEND`` is unset. Docker preserves the historical
+# Docker-backed CI path (and mirrors the pin in ``test_sandbox_escape``).
+DEFAULT_BACKEND = "docker"
+
+# `/dev/kvm` must exist and be openable by the invoking user for the
+# Firecracker backend to boot a guest (mirrors
+# ``bot_bottle.backend.firecracker.util``).
+_KVM_DEVICE = "/dev/kvm"
+
+
+def selected_backend() -> str:
+    """The backend this test run targets, from ``BOT_BOTTLE_BACKEND``.
+
+    Mirrors the CLI's env selector; unset means ``docker`` so an
+    unconfigured run behaves exactly as the suite did before backends were
+    pluggable.
+    """
+    return os.environ.get("BOT_BOTTLE_BACKEND") or DEFAULT_BACKEND
+
+
+@dataclass(frozen=True)
+class Capability:
+    """Outcome of a backend capability probe.
+
+    ``ok`` gates the tests; ``detail`` is a one-line human string reused for
+    both preflight output and ``unittest.skip`` reasons so the same wording
+    shows up at the job level and next to a skipped test.
+    """
+
+    backend: str
+    ok: bool
+    detail: str
+
+
+def docker_capability() -> Capability:
+    """Whether a Docker daemon is reachable (and not opted out)."""
+    name = "docker"
+    if os.environ.get("SKIP_DOCKER_TESTS"):
+        return Capability(name, False, "SKIP_DOCKER_TESTS is set")
+    if shutil.which("docker") is None:
+        return Capability(name, False, "docker not on PATH")
+    try:
+        returncode = subprocess.run(
+            ["docker", "info"],
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL,
+            check=False,
+            timeout=5,
+        ).returncode
+    except subprocess.TimeoutExpired:
+        return Capability(name, False, "docker info timed out")
+    if returncode != 0:
+        return Capability(name, False, "docker daemon unreachable")
+    return Capability(name, True, "docker daemon reachable")
+
+
+def firecracker_capability() -> Capability:
+    """Whether this host can boot a Firecracker guest: an accessible
+    ``/dev/kvm`` and a ``firecracker`` binary on ``PATH``.
+
+    Deliberately does not probe unrelated Docker availability — the
+    Firecracker backend is Docker-independent at runtime.
+    """
+    name = "firecracker"
+    if not os.path.exists(_KVM_DEVICE):
+        return Capability(name, False, f"{_KVM_DEVICE} missing — KVM unavailable")
+    if not os.access(_KVM_DEVICE, os.R_OK | os.W_OK):
+        return Capability(
+            name, False, f"{_KVM_DEVICE} not accessible — add your user to the kvm group"
+        )
+    if shutil.which("firecracker") is None:
+        return Capability(name, False, "firecracker not on PATH")
+    return Capability(name, True, f"{_KVM_DEVICE} accessible; firecracker on PATH")
+
+
+_PROBES = {
+    "docker": docker_capability,
+    "firecracker": firecracker_capability,
+}
+
+
+def backend_capability(backend: str | None = None) -> Capability:
+    """Probe ``backend`` (default: the selected backend) for readiness."""
+    backend = backend or selected_backend()
+    probe = _PROBES.get(backend)
+    if probe is None:
+        return Capability(backend, False, f"no capability probe for backend {backend!r}")
+    return probe()
+
+
+# ---- back-compat boolean probe -------------------------------------------
+
+
+def docker_available() -> bool:
+    """Boolean Docker probe (kept for callers that only need the flag)."""
+    return docker_capability().ok
+
+
+# ---- skip guards ---------------------------------------------------------
+
+
+def skip_unless_backend(backend: str):
+    """Skip a backend-specific test unless the selected backend matches AND
+    that backend's capability is present.
+
+    Docker-implementation tests (``DockerBroker``, ``DockerGateway``,
+    ``backend.docker.*``) use ``skip_unless_backend("docker")`` so they no-op
+    under a Firecracker run instead of testing Docker internals that run
+    doesn't target — the guard reads ``BOT_BOTTLE_BACKEND`` rather than
+    "is Docker installed".
+    """
+    sel = selected_backend()
+    if sel != backend:
+        return unittest.skip(
+            f"backend {backend!r} not selected (BOT_BOTTLE_BACKEND={sel})"
+        )
+    cap = backend_capability(backend)
+    return unittest.skipUnless(cap.ok, f"{backend} backend unavailable: {cap.detail}")
+
+
+def skip_unless_selected_backend_available():
+    """Skip a backend-agnostic test unless the *selected* backend can run it.
+
+    The test then exercises whichever backend ``BOT_BOTTLE_BACKEND`` names,
+    checking that backend's real capability (e.g. ``/dev/kvm`` for
+    Firecracker) rather than unrelated Docker availability.
+    """
+    cap = backend_capability()
+    return unittest.skipUnless(
+        cap.ok, f"selected backend {cap.backend!r} unavailable: {cap.detail}"
+    )

tests/_docker.py

@@ -1,45 +0,0 @@
-"""Docker availability check used by integration tests."""
-
-from __future__ import annotations
-
-import os
-import shutil
-import subprocess
-import unittest
-
-
-def docker_available() -> bool:
-    if os.environ.get("SKIP_DOCKER_TESTS"):
-        return False
-    if shutil.which("docker") is None:
-        return False
-    try:
-        return (
-            subprocess.run(
-                ["docker", "info"],
-                stdout=subprocess.DEVNULL,
-                stderr=subprocess.DEVNULL,
-                check=False,
-                timeout=5,
-            ).returncode
-            == 0
-        )
-    except subprocess.TimeoutExpired:
-        return False
-
-
-def skip_unless_docker(reason: str = "docker unreachable"):
-    return unittest.skipUnless(docker_available(), reason)
-
-
-def skip_unless_docker_or_firecracker(
-    reason: str = "neither Docker nor Firecracker selected",
-):
-    """Skip a backend-agnostic test unless one supported backend can run.
-
-    Firecracker does not require the host Docker daemon. The KVM coverage job
-    deliberately sets ``SKIP_DOCKER_TESTS`` to exclude Docker-only integration
-    classes while still exercising this path.
-    """
-    firecracker_selected = os.environ.get("BOT_BOTTLE_BACKEND") == "firecracker"
-    return unittest.skipUnless(firecracker_selected or docker_available(), reason)

tests/backend_preflight.py

@@ -0,0 +1,27 @@
+"""Backend capability preflight for the integration CI jobs.
+
+Run as ``python3 -m tests.backend_preflight`` (optionally with a backend name
+argument; default: ``BOT_BOTTLE_BACKEND`` or ``docker``). Prints a single
+clear PASS/FAIL line for the selected backend's capability and exits non-zero
+on failure, so a job surfaces missing infrastructure at the job level instead
+of hiding it among ``unittest.skip`` lines further down the output.
+"""
+
+from __future__ import annotations
+
+import sys
+
+from tests._backend import backend_capability, selected_backend
+
+
+def main(argv: list[str] | None = None) -> int:
+    argv = sys.argv[1:] if argv is None else argv
+    backend = argv[0] if argv else selected_backend()
+    cap = backend_capability(backend)
+    status = "PASS" if cap.ok else "FAIL"
+    print(f"[preflight] backend={backend} {status}: {cap.detail}")
+    return 0 if cap.ok else 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tests/integration/test_gateway_image.py

@@ -25,14 +25,14 @@ import os
 import subprocess
 import unittest
 
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 
 _IMAGE = "bot-bottle-gateway-test:chunk1"
 _DOCKERFILE = "Dockerfile.gateway"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true",
     "skipped under act_runner: multi-stage build pulls a 200+MB "

tests/integration/test_multitenant_isolation.py

@@ -31,7 +31,7 @@ from bot_bottle.backend.docker.gateway_net import next_free_ip
 from bot_bottle.orchestrator.client import OrchestratorClient
 from bot_bottle.orchestrator.gateway import GATEWAY_IMAGE, GATEWAY_NAME, GATEWAY_NETWORK
 from bot_bottle.orchestrator.lifecycle import OrchestratorService
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 # One upstream reachable under two names; reflects the Authorization header so
 # the probe can see exactly what the gateway injected (or didn't).
@@ -72,7 +72,7 @@ _PROBE_SRC = (
 )
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true",
     "skipped under act_runner: the orchestrator container bind-mounts the repo "

tests/integration/test_orchestrator_docker_broker.py

@@ -13,12 +13,12 @@ import unittest
 
 from bot_bottle.orchestrator.broker import LaunchRequest, sign_request
 from bot_bottle.orchestrator.docker_broker import DockerBroker, container_name
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 IMAGE = "busybox"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestDockerBrokerIntegration(unittest.TestCase):
     def setUp(self) -> None:
         self.secret = secrets.token_bytes(16)

tests/integration/test_orchestrator_docker_control_plane_auth.py

@@ -28,7 +28,7 @@ from pathlib import Path
 from bot_bottle.orchestrator.client import OrchestratorClient
 from bot_bottle.orchestrator.lifecycle import OrchestratorService
 from bot_bottle.paths import host_control_plane_token
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 # Fixed (not per-run-suffixed) so repeated runs reuse the same layer-cached
 # image instead of leaking a new dangling tag on every invocation.
@@ -37,7 +37,7 @@ _TEST_GATEWAY_IMAGE = "bot-bottle-gateway:itest"
 _TEST_INFRA_IMAGE = "bot-bottle-infra:itest"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true",
     "skipped under act_runner: the orchestrator container bind-mounts the repo "

tests/integration/test_orchestrator_docker_gateway.py

@@ -11,12 +11,12 @@ import subprocess
 import unittest
 
 from bot_bottle.orchestrator.gateway import DockerGateway
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 IMAGE = "busybox"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestDockerGatewayIntegration(unittest.TestCase):
     def setUp(self) -> None:
         self.name = "bot-bottle-orch-gateway-itest-" + secrets.token_hex(4)

tests/integration/test_orchestrator_docker_gateway_build.py

@@ -12,12 +12,12 @@ import subprocess
 import unittest
 
 from bot_bottle.orchestrator.gateway import DockerGateway
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 IMAGE = "busybox"
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestDockerGatewayImageExists(unittest.TestCase):
     def test_image_exists_true_for_present_false_for_absent(self) -> None:
         # Ensure the tiny image is present (build_if_missing is disabled here

tests/integration/test_orphan_cleanup.py

@@ -18,10 +18,10 @@ from bot_bottle.backend.docker.network import (
     network_create_internal,
     network_remove,
 )
-from tests._docker import skip_unless_docker
+from tests._backend import skip_unless_backend
 
 
-@skip_unless_docker()
+@skip_unless_backend("docker")
 class TestOrphanCleanup(unittest.TestCase):
     def setUp(self):
         self.slug = f"cb-test-orphan-{os.getpid()}"

tests/integration/test_sandbox_escape.py

@@ -31,7 +31,7 @@ from pathlib import Path
 from bot_bottle.backend import BottleSpec, get_bottle_backend
 from bot_bottle.bottle_state import cleanup_state
 from bot_bottle.manifest import ManifestIndex
-from tests._docker import skip_unless_docker_or_firecracker
+from tests._backend import skip_unless_selected_backend_available
 
 
 # Secrets planted in the bottle env as literals (agents substitute via
@@ -67,7 +67,7 @@ _DUMMY_HOST_KEY = (
 )
 
 
-@skip_unless_docker_or_firecracker()
+@skip_unless_selected_backend_available()
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true"
     and os.environ.get("BOT_BOTTLE_BACKEND") != "firecracker",

tests/unit/test_backend_skip_guards.py

@@ -0,0 +1,188 @@
+"""Tests for the backend-aware capability probes and skip guards
+(``tests/_backend.py``) and the CI preflight (``tests/backend_preflight.py``).
+
+The probes shell out to Docker / inspect ``/dev/kvm``; here they are driven
+entirely through mocks so the unit job asserts the decision logic without
+needing either backend present on the runner.
+"""
+
+from __future__ import annotations
+
+import os
+import unittest
+from unittest.mock import patch
+
+from tests import _backend, backend_preflight
+from tests._backend import (
+    Capability,
+    backend_capability,
+    docker_capability,
+    firecracker_capability,
+    selected_backend,
+    skip_unless_backend,
+    skip_unless_selected_backend_available,
+)
+
+
+def _skipped(decorated: type) -> bool:
+    return getattr(decorated, "__unittest_skip__", False)
+
+
+def _new_case() -> type:
+    return type("Case", (unittest.TestCase,), {})
+
+
+class TestSelectedBackend(unittest.TestCase):
+    def test_defaults_to_docker_when_unset(self):
+        with patch.dict(os.environ, {}, clear=True):
+            self.assertEqual("docker", selected_backend())
+
+    def test_reads_env(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}, clear=True):
+            self.assertEqual("firecracker", selected_backend())
+
+
+class TestDockerCapability(unittest.TestCase):
+    def test_opt_out_via_skip_docker_tests(self):
+        with patch.dict(os.environ, {"SKIP_DOCKER_TESTS": "1"}, clear=True):
+            cap = docker_capability()
+        self.assertFalse(cap.ok)
+        self.assertIn("SKIP_DOCKER_TESTS", cap.detail)
+
+    def test_not_on_path(self):
+        with patch.dict(os.environ, {}, clear=True), \
+                patch("tests._backend.shutil.which", return_value=None):
+            cap = docker_capability()
+        self.assertFalse(cap.ok)
+        self.assertIn("PATH", cap.detail)
+
+    def test_daemon_reachable(self):
+        with patch.dict(os.environ, {}, clear=True), \
+                patch("tests._backend.shutil.which", return_value="/usr/bin/docker"), \
+                patch("tests._backend.subprocess.run") as run:
+            run.return_value.returncode = 0
+            cap = docker_capability()
+        self.assertTrue(cap.ok)
+
+    def test_daemon_unreachable(self):
+        with patch.dict(os.environ, {}, clear=True), \
+                patch("tests._backend.shutil.which", return_value="/usr/bin/docker"), \
+                patch("tests._backend.subprocess.run") as run:
+            run.return_value.returncode = 1
+            cap = docker_capability()
+        self.assertFalse(cap.ok)
+        self.assertIn("unreachable", cap.detail)
+
+
+class TestFirecrackerCapability(unittest.TestCase):
+    def test_kvm_missing(self):
+        with patch("tests._backend.os.path.exists", return_value=False):
+            cap = firecracker_capability()
+        self.assertFalse(cap.ok)
+        self.assertIn("KVM", cap.detail)
+
+    def test_kvm_inaccessible(self):
+        with patch("tests._backend.os.path.exists", return_value=True), \
+                patch("tests._backend.os.access", return_value=False):
+            cap = firecracker_capability()
+        self.assertFalse(cap.ok)
+        self.assertIn("kvm group", cap.detail)
+
+    def test_binary_missing(self):
+        with patch("tests._backend.os.path.exists", return_value=True), \
+                patch("tests._backend.os.access", return_value=True), \
+                patch("tests._backend.shutil.which", return_value=None):
+            cap = firecracker_capability()
+        self.assertFalse(cap.ok)
+        self.assertIn("firecracker not on PATH", cap.detail)
+
+    def test_ready(self):
+        with patch("tests._backend.os.path.exists", return_value=True), \
+                patch("tests._backend.os.access", return_value=True), \
+                patch("tests._backend.shutil.which", return_value="/usr/bin/firecracker"):
+            cap = firecracker_capability()
+        self.assertTrue(cap.ok)
+
+
+class TestBackendCapability(unittest.TestCase):
+    def test_unknown_backend(self):
+        cap = backend_capability("qemu")
+        self.assertFalse(cap.ok)
+        self.assertIn("no capability probe", cap.detail)
+
+    def test_defaults_to_selected(self):
+        def probe() -> Capability:
+            return Capability("docker", True, "ok")
+
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "docker"}, clear=True), \
+                patch.dict("tests._backend._PROBES", {"docker": probe}):
+            cap = backend_capability()
+        self.assertEqual("docker", cap.backend)
+        self.assertTrue(cap.ok)
+
+
+class TestSkipUnlessBackend(unittest.TestCase):
+    def test_skips_when_other_backend_selected(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}, clear=True):
+            decorated = skip_unless_backend("docker")(_new_case())
+        self.assertTrue(_skipped(decorated))
+
+    def test_runs_when_selected_and_capable(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "docker"}, clear=True), \
+                patch("tests._backend.backend_capability",
+                      return_value=Capability("docker", True, "ok")):
+            decorated = skip_unless_backend("docker")(_new_case())
+        self.assertFalse(_skipped(decorated))
+
+    def test_skips_when_selected_but_incapable(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "docker"}, clear=True), \
+                patch("tests._backend.backend_capability",
+                      return_value=Capability("docker", False, "docker daemon unreachable")):
+            decorated = skip_unless_backend("docker")(_new_case())
+        self.assertTrue(_skipped(decorated))
+
+
+class TestSkipUnlessSelectedBackendAvailable(unittest.TestCase):
+    def test_runs_when_selected_backend_capable(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}, clear=True), \
+                patch("tests._backend.backend_capability",
+                      return_value=Capability("firecracker", True, "ok")):
+            decorated = skip_unless_selected_backend_available()(_new_case())
+        self.assertFalse(_skipped(decorated))
+
+    def test_skips_when_selected_backend_incapable(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "firecracker"}, clear=True), \
+                patch("tests._backend.backend_capability",
+                      return_value=Capability("firecracker", False, "/dev/kvm missing")):
+            decorated = skip_unless_selected_backend_available()(_new_case())
+        self.assertTrue(_skipped(decorated))
+
+
+class TestPreflightMain(unittest.TestCase):
+    def test_pass_exits_zero(self):
+        with patch.object(backend_preflight, "backend_capability",
+                          return_value=Capability("docker", True, "docker daemon reachable")):
+            rc = backend_preflight.main(["docker"])
+        self.assertEqual(0, rc)
+
+    def test_fail_exits_nonzero(self):
+        with patch.object(backend_preflight, "backend_capability",
+                          return_value=Capability("firecracker", False, "/dev/kvm missing")):
+            rc = backend_preflight.main(["firecracker"])
+        self.assertEqual(1, rc)
+
+    def test_defaults_to_selected_backend(self):
+        with patch.dict(os.environ, {"BOT_BOTTLE_BACKEND": "docker"}, clear=True), \
+                patch.object(backend_preflight, "backend_capability",
+                             return_value=Capability("docker", True, "ok")) as cap:
+            rc = backend_preflight.main([])
+        self.assertEqual(0, rc)
+        cap.assert_called_once_with("docker")
+
+    def test_module_exposes_selected_backend(self):
+        # Import-surface guard: the preflight re-exports the selector it uses.
+        self.assertIs(backend_preflight.selected_backend, _backend.selected_backend)
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/unit/test_docker_test_helpers.py

@@ -1,35 +0,0 @@
-"""Tests for integration-test backend selection helpers."""
-
-from __future__ import annotations
-
-import os
-import unittest
-from unittest.mock import patch
-
-from tests._docker import skip_unless_docker_or_firecracker
-
-
-class TestSkipUnlessDockerOrFirecracker(unittest.TestCase):
-    def test_firecracker_runs_when_docker_tests_are_disabled(self):
-        with patch.dict(
-            os.environ,
-            {"BOT_BOTTLE_BACKEND": "firecracker", "SKIP_DOCKER_TESTS": "1"},
-            clear=True,
-        ):
-            decorated = skip_unless_docker_or_firecracker()(type("Case", (), {}))
-
-        self.assertFalse(getattr(decorated, "__unittest_skip__", False))
-
-    def test_non_firecracker_still_skips_when_docker_tests_are_disabled(self):
-        with patch.dict(
-            os.environ,
-            {"BOT_BOTTLE_BACKEND": "docker", "SKIP_DOCKER_TESTS": "1"},
-            clear=True,
-        ):
-            decorated = skip_unless_docker_or_firecracker()(type("Case", (), {}))
-
-        self.assertTrue(getattr(decorated, "__unittest_skip__", False))
-
-
-if __name__ == "__main__":
-    unittest.main()