feat(firecracker): pull the infra rootfs as a prebuilt artifact (PRD 0069 Stage 2)

Stage 2 of the docker-free Firecracker backend (#348): stop building the
fixed infra image on the launch host. The infra VM's rootfs is host- and
bottle-agnostic (authorized_keys + guest IP ride the kernel cmdline, not the
rootfs), so it's built once off-host and published as a versioned, ready-to-
boot ext4; the launch host downloads + verifies + boots it — no Docker, no
image tooling, just HTTP + gunzip.

- infra_artifact.py: version = content hash of the rootfs inputs (the shipped
  bot_bottle package + the three Dockerfiles + the init), so a launch host
  pulls the artifact matching its code and a content change can't silently
  boot a stale rootfs. Pull + sha256-verify (fail-closed) + gunzip from a
  Gitea generic package; base/owner/token configurable, default this Gitea.
- infra_vm.ensure_built/boot default to the pull path; BOT_BOTTLE_INFRA_BUILD=
  local keeps the docker build-from-source path for iterating on Dockerfiles.
- publish_infra.py: the off-host half — builds the images with Docker, mke2fs
  the rootfs (with buildah slack), gzips, and PUTs it to the generic package.

Rollout note: default=pull means a launch 404s until an artifact is published;
until the Gitea packages endpoint is enabled + an artifact published, use
BOT_BOTTLE_INFRA_BUILD=local. Freeze/migrate's remaining docker use is a
separate PR.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

bot_bottle/backend/firecracker/infra_artifact.py

@@ -0,0 +1,189 @@
+"""Prebuilt infra-VM rootfs, pulled as an artifact (PRD 0069 Stage 2).
+
+The Firecracker infra VM boots a fixed rootfs (orchestrator control plane +
+gateway + buildah, control-plane init as PID 1) that does not vary per launch —
+the per-boot bits (authorized_keys, guest IP) ride the kernel cmdline, so one
+rootfs boots on any host. Instead of building that rootfs on the launch host
+with Docker, we build it **off-host** and publish it as a versioned, ready-to-
+boot ext4 (gzip-compressed) to a Gitea **generic package**; the launch host
+downloads + verifies + boots it. No Docker, no image tooling on the launch
+host — just an HTTP fetch and gunzip.
+
+  publish (off-host, see publish_infra.py):
+    docker build -> rootfs dir -> mke2fs -> gzip -> PUT generic package
+  pull (this module, launch host):
+    GET .../rootfs.ext4.gz (+ .sha256) -> verify -> gunzip -> boot
+
+The artifact **version** is a content hash of everything baked into the rootfs
+(the shipped bot_bottle package, the three Dockerfiles, and the init), so a
+launch host always pulls the artifact matching its code and a content change
+can't silently boot a stale rootfs. A checksum mismatch fails closed.
+
+Set `BOT_BOTTLE_INFRA_BUILD=local` to skip the pull and build the rootfs
+locally with Docker (dev iteration on the Dockerfiles) — see `infra_vm`.
+"""
+
+from __future__ import annotations
+
+import gzip
+import hashlib
+import os
+import shutil
+import urllib.error
+import urllib.request
+from pathlib import Path
+
+from ...log import die, info
+from . import util
+
+# Bump if the on-disk artifact *format* changes (compression, layout) so a new
+# scheme can't collide with a cached/published artifact of the old one.
+_ARTIFACT_FORMAT = "1"
+
+_REPO_ROOT = Path(__file__).resolve().parents[3]
+_DOCKERFILES = ("Dockerfile.orchestrator", "Dockerfile.gateway", "Dockerfile.infra")
+
+_DEFAULT_BASE = "https://gitea.dideric.is"
+_DEFAULT_OWNER = "didericis"
+_PACKAGE = "bot-bottle-infra"
+
+# Streaming copy chunk for the (hundreds-of-MB) download.
+_CHUNK = 1 << 20
+
+
+def local_build_requested() -> bool:
+    """True when the operator opted into the dev Docker-build path instead of
+    pulling the published artifact (`BOT_BOTTLE_INFRA_BUILD=local`)."""
+    return os.environ.get("BOT_BOTTLE_INFRA_BUILD", "").strip().lower() == "local"
+
+
+def infra_artifact_version(init_script: str) -> str:
+    """Content hash (16 hex) of everything baked into the infra rootfs: the
+    whole shipped `bot_bottle` package (the infra image `COPY`s it wholesale),
+    the three fixed Dockerfiles, and the guest init. Deterministic across the
+    publish host and the launch host when both run the same checkout, so the
+    tag the launch host pulls is exactly the tag publish produced."""
+    h = hashlib.sha256()
+    h.update(f"format={_ARTIFACT_FORMAT}\n".encode())
+    pkg = _REPO_ROOT / "bot_bottle"
+    for path in sorted(pkg.rglob("*.py")):
+        if "__pycache__" in path.parts:
+            continue
+        h.update(str(path.relative_to(_REPO_ROOT)).encode())
+        h.update(path.read_bytes())
+    for name in _DOCKERFILES:
+        p = _REPO_ROOT / name
+        h.update(name.encode())
+        h.update(p.read_bytes())
+    h.update(b"init\0")
+    h.update(init_script.encode())
+    return h.hexdigest()[:16]
+
+
+def _config() -> tuple[str, str, str]:
+    """(base_url, owner, token) for the generic-package endpoint. Base + owner
+    are overridable for other deployments / mirrors; the token (optional — a
+    public package needs none) reuses the shared Gitea token."""
+    base = os.environ.get("BOT_BOTTLE_INFRA_ARTIFACT_BASE", _DEFAULT_BASE).rstrip("/")
+    owner = os.environ.get("BOT_BOTTLE_INFRA_ARTIFACT_OWNER", _DEFAULT_OWNER)
+    token = os.environ.get("BOT_BOTTLE_INFRA_ARTIFACT_TOKEN") or os.environ.get(
+        "BOT_BOTTLE_CLAUDE_GITEA_TOKEN", ""
+    )
+    return base, owner, token
+
+
+def artifact_url(version: str, filename: str) -> str:
+    """The generic-package download URL for one file of this version's
+    artifact (`rootfs.ext4.gz` / `rootfs.ext4.gz.sha256`)."""
+    base, owner, _ = _config()
+    return f"{base}/api/packages/{owner}/generic/{_PACKAGE}/{version}/{filename}"
+
+
+_GZ_NAME = "rootfs.ext4.gz"
+_SHA_NAME = "rootfs.ext4.gz.sha256"
+
+
+def _cache_root(version: str) -> Path:
+    return util.cache_dir() / "infra-artifact" / version
+
+
+def _open(url: str) -> urllib.request.Request:
+    _, _, token = _config()
+    req = urllib.request.Request(url)
+    if token:
+        req.add_header("Authorization", f"token {token}")
+    return req
+
+
+def _download(url: str, dest: Path) -> None:
+    """Stream `url` to `dest` (atomic via a `.part` sibling)."""
+    tmp = dest.with_suffix(dest.suffix + ".part")
+    try:
+        with urllib.request.urlopen(_open(url)) as resp, open(tmp, "wb") as out:
+            shutil.copyfileobj(resp, out, _CHUNK)
+    except urllib.error.HTTPError as e:
+        tmp.unlink(missing_ok=True)
+        if e.code == 404:
+            die(
+                f"infra artifact not published for this code version.\n"
+                f"    missing: {url}\n"
+                f"    publish it from a build host (Docker):\n"
+                f"        python3 -m bot_bottle.backend.firecracker.publish_infra\n"
+                f"    or build the rootfs locally: BOT_BOTTLE_INFRA_BUILD=local"
+            )
+        die(f"downloading infra artifact failed (HTTP {e.code}): {url}")
+    except urllib.error.URLError as e:
+        tmp.unlink(missing_ok=True)
+        die(f"infra artifact registry unreachable: {url} ({e.reason})")
+    tmp.replace(dest)
+
+
+def _sha256_file(path: Path) -> str:
+    h = hashlib.sha256()
+    with open(path, "rb") as fh:
+        for chunk in iter(lambda: fh.read(_CHUNK), b""):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def ensure_artifact_gz(version: str) -> Path:
+    """The verified, cached `rootfs.ext4.gz` for `version` — downloading it (and
+    its `.sha256`) once, then reusing it. Fail-closed on a checksum mismatch:
+    the partial is removed and we die rather than boot an unverified rootfs."""
+    root = _cache_root(version)
+    root.mkdir(parents=True, exist_ok=True)
+    gz = root / _GZ_NAME
+    ok = root / ".verified"
+    if gz.is_file() and ok.is_file():
+        return gz
+
+    info(f"pulling infra rootfs artifact {_PACKAGE}/{version}")
+    _download(artifact_url(version, _GZ_NAME), gz)
+    sha = root / _SHA_NAME
+    _download(artifact_url(version, _SHA_NAME), sha)
+
+    expected = sha.read_text().split()[0].strip().lower()
+    actual = _sha256_file(gz)
+    if actual != expected:
+        gz.unlink(missing_ok=True)
+        sha.unlink(missing_ok=True)
+        die(
+            f"infra artifact checksum mismatch for {version}:\n"
+            f"    expected {expected}\n"
+            f"    actual   {actual}\n"
+            f"    refusing to boot an unverified rootfs."
+        )
+    ok.write_text("ok\n")
+    return gz
+
+
+def materialize_ext4(version: str, dest: Path) -> None:
+    """Ensure the verified artifact is cached, then gunzip it to `dest` — a
+    fresh, writable per-boot rootfs (the VM mutates it; the cached `.gz` stays
+    pristine). Atomic via a `.part` sibling."""
+    gz = ensure_artifact_gz(version)
+    tmp = dest.with_suffix(dest.suffix + ".part")
+    info(f"expanding infra rootfs -> {dest}")
+    with gzip.open(gz, "rb") as src, open(tmp, "wb") as out:
+        shutil.copyfileobj(src, out, _CHUNK)
+    tmp.replace(dest)