docs(research): firecracker image remote store

Investigates storage candidates, retention policy design, and
image-size reduction techniques in response to the PR #459 comment
on a secure remote store for committed Firecracker snapshots.

.gitea/workflows/test.yml

@@ -1,4 +1,5 @@
-# Run the project's test suite on every PR push and on push to main.
+# Run the project's test suite when package or runtime inputs change on a PR
+# or on push to main.
 #
 # The suite uses stdlib `unittest` discovery — no external Python
 # dependencies are required to execute it. Tests are split by directory:
@@ -23,22 +24,34 @@ on:
     branches:
       - main
     paths:
-      - '**.py'
-      - '.gitea/workflows/**.yml'
-      - 'scripts/**'
-      - 'README.md'
-      # Dockerfiles and pyproject.toml are baked into the infra rootfs; a
-      # change here alters what the integration/coverage jobs build locally.
+      - 'bot_bottle/**'
+      - 'tests/**/*.py'
+      - 'cli.py'
+      - 'scripts/coverage.sh'
+      - 'scripts/critical-modules.txt'
+      - 'scripts/diff_coverage.py'
+      - 'scripts/tracker_policy.py'
+      - 'scripts/firecracker-netpool.sh'
       - 'Dockerfile*'
       - 'pyproject.toml'
+      - 'requirements-dev.txt'
+      - '.coveragerc'
+      - '.dockerignore'
   pull_request:
     paths:
-      - '**.py'
-      - '.gitea/workflows/**.yml'
-      - 'scripts/**'
-      - 'README.md'
+      - 'bot_bottle/**'
+      - 'tests/**/*.py'
+      - 'cli.py'
+      - 'scripts/coverage.sh'
+      - 'scripts/critical-modules.txt'
+      - 'scripts/diff_coverage.py'
+      - 'scripts/tracker_policy.py'
+      - 'scripts/firecracker-netpool.sh'
       - 'Dockerfile*'
       - 'pyproject.toml'
+      - 'requirements-dev.txt'
+      - '.coveragerc'
+      - '.dockerignore'
   workflow_dispatch:
 
 jobs:

.gitea/workflows/update-badges.yml

@@ -21,7 +21,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           fetch-depth: 0
-          token: ${{ secrets.GITHUB_TOKEN }}
+          token: ${{ secrets.BADGE_PUSH_TOKEN }}
 
       # No actions/setup-python: the runner image ships Python 3.12 and older
       # act_runner engines mishandle setup-python's PATH. Install into the

README.md

@@ -5,7 +5,7 @@
 # bot-bottle
 
 [![test](https://gitea.dideric.is/didericis/bot-bottle/actions/workflows/test.yml/badge.svg?branch=main)](https://gitea.dideric.is/didericis/bot-bottle/actions?workflow=test.yml)
-[![coverage](https://img.shields.io/badge/coverage-81%25-brightgreen)](https://coverage.readthedocs.io/)
+[![coverage](https://img.shields.io/badge/coverage-83%25-brightgreen)](https://coverage.readthedocs.io/)
 [![core coverage](https://img.shields.io/badge/core%20coverage-94%25-brightgreen)](https://gitea.dideric.is/didericis/bot-bottle/src/branch/main/docs/decisions/0004-coverage-policy.md)
 
 **Problem:** Developer wants to run a coding agent without supervision, but they don't want a prompt injected or misbehaving agent wrecking their environment or exfiltrating sensitive data.

bot_bottle/backend/firecracker/cleanup.py

@@ -1,8 +1,23 @@
 """Cleanup for the Firecracker backend.
 
-Orphans are: firecracker VMM processes whose config lives under our run
-dir, and the per-bottle run dirs. TAP slots free themselves (the flock
-drops when the launcher exits), so there is nothing to reclaim there.
+Reaps *orphans* only — resources with no live VM behind them:
+
+  * orphan run dirs: a per-bottle run dir (holding the ~1G rootfs.ext4)
+    whose firecracker process has exited. These leak when a launch is
+    hard-killed before its teardown runs (host OOM/crash, a cancelled CI
+    job, `kill -9`); the clean-exit path already removes its own dir in
+    launch.py.
+  * orphan VM pids: a firecracker process whose run dir is already gone
+    — a VMM left lingering after its dir was removed.
+
+A run dir with a *live* firecracker process is a running bottle and is
+left strictly alone: it is neither killed nor removed. (The backend's
+`enumerate_active` registry is still a stub — #354 — so a live process
+is the only reliable "this bottle is in use" signal we have. Once the
+registry lands, registry-orphaned-but-running VMs can be reaped too.)
+
+TAP slots free themselves (the flock drops when the launcher exits), so
+there is nothing to reclaim there.
 """
 
 from __future__ import annotations
@@ -22,38 +37,73 @@ def _run_root() -> Path:
     return util.cache_dir() / "run"
 
 
-def _orphan_vm_pids() -> list[int]:
-    """firecracker processes whose --config-file is under our run dir."""
-    run_root = str(_run_root())
+def _run_dir_of(cmd: str, run_root: Path) -> Path | None:
+    """The bottle run dir a firecracker cmdline belongs to, or None.
+
+    A bottle VM is launched with `--config-file <run_root>/<slug>/config.json`,
+    so the run dir is the config file's parent when it sits directly under
+    the run root. Anything else (a builder VM, the infra VM elsewhere) is
+    not ours to reap here.
+    """
+    toks = cmd.split()
+    for i, tok in enumerate(toks):
+        if tok == "--config-file" and i + 1 < len(toks):
+            parent = Path(toks[i + 1]).parent
+            if parent.parent == run_root:
+                return parent
+    return None
+
+
+def _scan_processes(run_root: Path) -> tuple[set[str], list[int]]:
+    """Inspect running firecracker VMs under ``run_root``.
+
+    Returns ``(live_run_dirs, orphan_pids)``:
+      * ``live_run_dirs`` — run dirs backed by a running VM (never reaped);
+      * ``orphan_pids`` — firecracker pids whose run dir no longer exists
+        (a lingering VMM to kill).
+    """
     result = subprocess.run(
         ["pgrep", "-a", "firecracker"],
         capture_output=True, text=True, check=False,
     )
     if result.returncode != 0:
-        return []
-    pids: list[int] = []
+        return set(), []
+    live: set[str] = set()
+    orphan_pids: list[int] = []
     for line in result.stdout.splitlines():
         parts = line.split(None, 1)
-        if len(parts) != 2 or run_root not in parts[1]:
+        if len(parts) != 2:
             continue
         try:
-            pids.append(int(parts[0]))
+            pid = int(parts[0])
         except ValueError:
             continue
-    return pids
+        run_dir = _run_dir_of(parts[1], run_root)
+        if run_dir is None:
+            continue
+        if run_dir.is_dir():
+            live.add(str(run_dir))
+        else:
+            orphan_pids.append(pid)
+    return live, orphan_pids
 
 
-def _run_dirs() -> list[str]:
-    run_root = _run_root()
+def _orphan_run_dirs(run_root: Path, live: set[str]) -> list[str]:
+    """Run dirs with no live VM behind them — the leaked ones to remove."""
     if not run_root.is_dir():
         return []
-    return sorted(str(p) for p in run_root.iterdir() if p.is_dir())
+    return sorted(
+        str(p) for p in run_root.iterdir()
+        if p.is_dir() and str(p) not in live
+    )
 
 
 def prepare_cleanup() -> FirecrackerBottleCleanupPlan:
+    run_root = _run_root()
+    live, orphan_pids = _scan_processes(run_root)
     return FirecrackerBottleCleanupPlan(
-        vm_pids=tuple(_orphan_vm_pids()),
-        run_dirs=tuple(_run_dirs()),
+        vm_pids=tuple(orphan_pids),
+        run_dirs=tuple(_orphan_run_dirs(run_root, live)),
     )
 
 

bot_bottle/backend/firecracker/launch.py

@@ -26,6 +26,7 @@ from __future__ import annotations
 
 import dataclasses
 import os
+import shutil
 from contextlib import ExitStack, contextmanager
 from pathlib import Path
 from typing import Callable, Generator
@@ -167,6 +168,10 @@ def launch(
         # Step 6: build the per-bottle rootfs + SSH key, then boot.
         run_dir = util.cache_dir() / "run" / plan.slug
         run_dir.mkdir(parents=True, exist_ok=True)
+        # Remove the run dir on teardown so the per-bottle rootfs.ext4 (~1G)
+        # doesn't leak. Registered before vm.terminate below so it runs *after*
+        # it (ExitStack is LIFO): the VM is gone before we rm its rootfs.
+        stack.callback(lambda: shutil.rmtree(run_dir, ignore_errors=True))
         rootfs = run_dir / "rootfs.ext4"
         util.build_rootfs_ext4(agent_base, rootfs)
         private_key, pubkey = util.generate_keypair(run_dir)

docs/research/firecracker-image-remote-store.md

@@ -0,0 +1,178 @@
+# Firecracker Image Remote Store
+
+**Date:** 2026-07-22  
+**Context:** PR #459 (run-dir leak fix) surfaced that committed Firecracker snapshots
+currently live only on the host machine. Once a host is wiped or a run-dir is
+evicted, a user's preserved bottle is gone. This note investigates a secure remote
+store so committed images survive host turnover and can be restored without the
+user having to pre-flag which sessions to keep.
+
+## Verdict
+
+Backblaze B2 + Cloudflare CDN is the cost-optimal choice for most deployments.
+Cloudflare R2 is the simpler zero-config option at slightly higher storage cost.
+Self-hosted MinIO is the right call for air-gapped or on-premises installs.
+
+On the image-size front, zstd-compressing the committed tar before upload
+produces roughly a 60–70% reduction with negligible impact on restore latency.
+OverlayFS (for in-flight working rootfs, not for the committed artifact) cuts
+per-instance disk use to ~10–50 MB per extra bottle sharing the same base.
+
+---
+
+## What Gets Stored
+
+The Firecracker backend produces two artifact types:
+
+| Artifact | Created by | Size | Lifetime |
+|----------|-----------|------|---------|
+| `rootfs/agent-<digest>/` (dir) | `image_builder.py` → `mke2fs -d` | ~1 GB as ext4 | Cached per Dockerfile hash; evictable |
+| `committed/<slug>/rootfs.tar` | `FirecrackerFreezer._freeze` via SSH tar | 500 MB–1 GB | User-preserved; must survive host wipe |
+
+The committed artifact is a tar of the guest's live filesystem streamed out over
+SSH (`freezer.py:58–90`). At resume time `launch.py` calls `mke2fs -d` to
+rebuild a fresh ext4 from this tar. The tar — not the ext4 — is what needs to be
+pushed to remote storage and pulled back at restore time.
+
+The base image cache (`rootfs/agent-<digest>/`) is derivable from the Dockerfile
+and can be rebuilt on demand; it is lower priority for remote storage.
+
+---
+
+## Storage Candidates
+
+### Object storage
+
+| Provider | Storage | Egress | Notes |
+|----------|---------|--------|-------|
+| **Backblaze B2** | $0.006/GB | Free (via Cloudflare Bandwidth Alliance) | Cheapest storage; pairs with Cloudflare CDN to eliminate egress |
+| **Cloudflare R2** | $0.015/GB | $0 always | Zero-config egress; no lifecycle transitions (limitation) |
+| **Wasabi** | $0.0069/GB | Free (1:1 ratio) | 90-day minimum retention; good for archival; lifecycle evaluated daily |
+| **AWS S3** | $0.023/GB | $0.09/GB | Richest lifecycle support; expensive at scale; avoid unless already in AWS |
+| **MinIO** (self-hosted) | Host cost only | None | S3-compatible; best for private/on-prem deployments |
+
+**B2 + Cloudflare CDN** is effectively $0.006/GB with zero egress — about 18×
+cheaper than S3 for restore-heavy workloads. **R2** is the zero-config choice
+($0 egress by default, no Bandwidth Alliance pairing needed) at a slightly
+higher storage rate.
+
+**Cloudflare R2's missing lifecycle support** is the main caveat: auto-eviction
+rules (evict images older than N days) cannot currently be expressed natively in
+R2. Wasabi and S3 both support declarative lifecycle policies.
+
+### Retention policy recommendation
+
+The comment proposes:
+- Retain images for ~1 week by default
+- Warn when approaching a capacity threshold
+- Auto-evict oldest images once threshold is exceeded
+
+This maps cleanly to an application-level policy (not a provider lifecycle rule),
+which avoids the R2 limitation and works consistently across providers:
+
+1. On `commit`: upload tar, record `(slug, size_bytes, uploaded_at)` in a local
+   or remote manifest file.
+2. On startup / on `list`: scan the manifest, warn if total stored size exceeds
+   e.g. 80% of the configured threshold.
+3. On eviction run (CLI or cron): delete objects older than `retention_days`
+   (default 7) that push total over `max_capacity`; oldest-first.
+
+This keeps the policy logic in bot-bottle and the storage provider as a dumb
+object store — no vendor-specific lifecycle API required.
+
+---
+
+## Image Size Reduction
+
+### Current artifact sizes
+
+A typical committed tar for a Claude Code agent image is 500 MB–1 GB uncompressed.
+The per-run ext4 (copy of the base, written at `start`) adds another ~1 GB of
+local disk. The leak fix in this PR addresses the ext4 copies; the remote store
+addresses the committed tars.
+
+### Compression
+
+zstd compression of the committed tar before upload is the highest-leverage
+single change:
+
+| Codec | Typical size (1 GB rootfs) | Compress speed | Decompress speed |
+|-------|---------------------------|---------------|-----------------|
+| gzip | 350–430 MB | ~100 MB/s | ~500 MB/s |
+| **zstd (default)** | **330–360 MB** | **~400 MB/s** | **~2 GB/s** |
+| xz | 290–320 MB | ~20 MB/s | ~200 MB/s |
+
+**zstd is the best trade-off**: 65–67% size reduction, near-instantaneous
+decompression. The `tar` call in `freezer.py` could pipe through `zstd` before
+writing to disk and to the remote; `resume` decompresses on the way back. A
+`.tar.zst` suffix marks compressed artifacts so old tars remain restorable
+without the codec.
+
+### SquashFS for the base image cache
+
+The `rootfs/agent-<digest>/` directory (the buildah-exported tree) is rebuilt by
+`image_builder.py` and turned into per-run ext4 by `mke2fs -d`. Storing the
+base as a SquashFS image instead of a flat directory tree would reduce it from
+~1 GB to ~330–360 MB and make the cache remote-friendly. Firecracker does not
+directly boot SquashFS, but the existing `mke2fs -d` path reads a directory tree
+— a SquashFS mount could serve as the source. This is a larger change and lower
+priority than tar compression.
+
+### OverlayFS for per-run rootfs
+
+Multiple simultaneous bottles sharing the same agent image today each get a full
+`mke2fs -d` copy (~1 GB). OverlayFS (read-only base + writable sparse overlay)
+would reduce this to ~10–50 MB per instance beyond the first:
+
+- Mount the base image directory as read-only lower layer
+- Attach a sparse ext4 or tmpfs writable layer per bottle
+- Pass the merged overlay to Firecracker as the block device
+
+E2B's public write-up on Firecracker + OverlayFS confirms this approach works
+at scale. The `launch.py` changes would be non-trivial (device mapper or
+`fuse-overlayfs` plumbing), so this is a follow-up rather than a prerequisite
+for the remote store.
+
+---
+
+## Recommended Approach
+
+**Phase 1 — remote store with zstd (tight scope, actionable now)**
+
+1. Add `--zstd` to the `tar` call in `FirecrackerFreezer._freeze`; name the
+   artifact `rootfs.tar.zst`. Keep uncompressed restore path for legacy tars.
+2. Add a `bb firecracker upload <slug>` / `bb firecracker pull <slug>` pair that
+   pushes/fetches the compressed tar to the configured object store (S3-compatible
+   API, so B2, R2, MinIO, and Wasabi all work with the same client).
+3. Store a `manifest.json` in the bucket (or a local mirror) tracking slug →
+   `{size, uploaded_at}`. Use it for threshold warnings and eviction.
+4. Default retention: 7 days, configurable via `firecracker.image_retention_days`
+   in `~/.config/bot-bottle/config.toml` (or equivalent).
+5. Warn at 80% of `max_capacity` (default e.g. 50 GB); evict oldest on commit
+   once at 100%.
+
+**Storage recommendation:** Cloudflare R2 for hosted deployments (zero egress,
+zero config), MinIO for private/on-premises.
+
+**Phase 2 — base image cache compression**
+
+Compress the `agent-<digest>` cache dir as a `.tar.zst` to save ~65% on repeated
+image uploads. Low urgency since the base image is rebuildable.
+
+**Phase 3 — OverlayFS per-run disk**
+
+Replace the full per-run ext4 copy with an OverlayFS sparse layer. Largest disk
+impact (~90–95% savings per concurrent bottle) but highest implementation
+complexity. Track as a separate PRD.
+
+---
+
+## Open Questions
+
+- Does the host have a configured object-store credential path, or should the
+  remote store be an opt-in with an explicit `bb config set image-store.url ...`?
+- Should `commit` automatically upload, or should upload be an explicit step to
+  avoid surprise egress?
+- What is the acceptable cold-start latency for a restore from remote? A 330 MB
+  zstd tar at 100 Mbit/s takes ~26 s; at 1 Gbit/s, ~2.6 s. This bounds the
+  retention strategy (evict from local after successful upload vs keep local copy).

tests/unit/test_firecracker_cleanup.py

@@ -10,7 +10,9 @@ classmethods forward to their module.
 from __future__ import annotations
 
 import subprocess
+import tempfile
 import unittest
+from pathlib import Path
 from unittest.mock import patch
 
 from bot_bottle.backend.firecracker import cleanup as fc_cleanup
@@ -23,32 +25,69 @@ def _proc(stdout: str = "", returncode: int = 0) -> "subprocess.CompletedProcess
     return subprocess.CompletedProcess([], returncode, stdout=stdout, stderr="")
 
 
-class TestOrphanEnumeration(unittest.TestCase):
-    def test_orphan_vm_pids_filters_by_run_dir(self):
-        run_root = str(fc_cleanup._run_root())
-        out = (
-            f"111 firecracker --config-file {run_root}/dev-a/config.json\n"
-            "222 firecracker --config-file /somewhere/else/config.json\n"
-            "notanint firecracker --config-file " + run_root + "/x\n"
+class TestProcessScan(unittest.TestCase):
+    def test_run_dir_of_matches_only_direct_children(self):
+        run_root = Path("/cache/run")
+        self.assertEqual(
+            Path("/cache/run/dev-a"),
+            fc_cleanup._run_dir_of(
+                f"firecracker --config-file {run_root}/dev-a/config.json", run_root
+            ),
+        )
+        # infra/builder VMs elsewhere, or nested paths, are not ours.
+        self.assertIsNone(
+            fc_cleanup._run_dir_of("firecracker --config-file /elsewhere/config.json", run_root)
+        )
+        self.assertIsNone(
+            fc_cleanup._run_dir_of("firecracker --no-config", run_root)
         )
-        with patch.object(fc_cleanup.subprocess, "run", return_value=_proc(out)):
-            self.assertEqual([111], fc_cleanup._orphan_vm_pids())
 
-    def test_orphan_vm_pids_empty_when_pgrep_fails(self):
+    def test_scan_splits_live_dirs_from_orphan_pids(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            run_root = Path(tmp)
+            (run_root / "live-a").mkdir()  # dir present -> live VM, protected
+            # "gone-b" dir intentionally absent -> lingering VMM, orphan pid
+            out = (
+                f"111 firecracker --config-file {run_root}/live-a/config.json\n"
+                f"222 firecracker --config-file {run_root}/gone-b/config.json\n"
+                "333 firecracker --config-file /elsewhere/config.json\n"
+                "notanint firecracker --config-file x\n"
+            )
+            with patch.object(fc_cleanup.subprocess, "run", return_value=_proc(out)):
+                live, orphan_pids = fc_cleanup._scan_processes(run_root)
+        self.assertEqual({str(run_root / "live-a")}, live)
+        self.assertEqual([222], orphan_pids)
+
+    def test_scan_empty_when_pgrep_fails(self):
         with patch.object(fc_cleanup.subprocess, "run", return_value=_proc(returncode=1)):
-            self.assertEqual([], fc_cleanup._orphan_vm_pids())
+            self.assertEqual((set(), []), fc_cleanup._scan_processes(Path("/x")))
 
-    def test_run_dirs_empty_when_absent(self):
-        with patch.object(fc_cleanup.util, "cache_dir") as cache:
-            cache.return_value.__truediv__.return_value.is_dir.return_value = False
-            self.assertEqual([], fc_cleanup._run_dirs())
+    def test_orphan_run_dirs_excludes_live_and_missing_root(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            run_root = Path(tmp)
+            (run_root / "live-a").mkdir()
+            (run_root / "dead-b").mkdir()
+            live = {str(run_root / "live-a")}
+            self.assertEqual(
+                [str(run_root / "dead-b")],
+                fc_cleanup._orphan_run_dirs(run_root, live),
+            )
+        # absent run root -> nothing to reap
+        self.assertEqual([], fc_cleanup._orphan_run_dirs(Path("/nope/run"), set()))
 
-    def test_prepare_cleanup_assembles_plan(self):
-        with patch.object(fc_cleanup, "_orphan_vm_pids", return_value=[7]), \
-             patch.object(fc_cleanup, "_run_dirs", return_value=["/run/x"]):
-            plan = fc_cleanup.prepare_cleanup()
-        self.assertEqual((7,), plan.vm_pids)
-        self.assertEqual(("/run/x",), plan.run_dirs)
+    def test_prepare_cleanup_reaps_orphans_only(self):
+        """The live VM's dir is never in the plan; the dead one is."""
+        with tempfile.TemporaryDirectory() as tmp:
+            run_root = Path(tmp)
+            (run_root / "live-a").mkdir()
+            (run_root / "dead-b").mkdir()
+            out = f"111 firecracker --config-file {run_root}/live-a/config.json\n"
+            with patch.object(fc_cleanup, "_run_root", return_value=run_root), \
+                 patch.object(fc_cleanup.subprocess, "run", return_value=_proc(out)):
+                plan = fc_cleanup.prepare_cleanup()
+        self.assertEqual((), plan.vm_pids)
+        self.assertEqual((str(run_root / "dead-b"),), plan.run_dirs)
+        self.assertNotIn(str(run_root / "live-a"), plan.run_dirs)
 
 
 class TestCleanupRemoval(unittest.TestCase):