feat(secrets): pluggable secret backend with OpenBao read (Phase 4)

- secret_backends.py: SecretBackend protocol, FileSecretBackend (historical),
  OpenBaoBackend (KV-v2 read + file fallback), build_backend() env-selected
- _read_secret delegates to the active backend; default is file, so production
  is unchanged until CASTLE_SECRET_BACKEND=openbao
- OpenBao token bootstraps from the file backend; missing/unreachable falls back
- tests: file hit/miss, backend selection, unreachable + empty-token fallback

Read path verified live against castle-openbao. Write path, auto-unseal-on-boot,
and TLS hardening documented as remaining for full OpenBao production use.

core/src/castle_core/config.py

@@ -319,10 +319,15 @@ def resolve_env_vars(
 
 
 def _read_secret(name: str) -> str:
-    """Read a secret from ~/.castle/secrets/<name>. Returns placeholder if not found."""
-    secret_path = SECRETS_DIR / name
-    if secret_path.exists():
-        return secret_path.read_text().strip()
+    """Resolve a secret via the active backend (file by default; OpenBao opt-in).
+
+    Returns a ``<MISSING_SECRET:...>`` placeholder if unresolved (never raises).
+    """
+    from castle_core.secret_backends import build_backend
+
+    value = build_backend(SECRETS_DIR).read(name)
+    if value is not None:
+        return value
     return f"<MISSING_SECRET:{name}>"
 
 

core/src/castle_core/secret_backends.py

@@ -0,0 +1,90 @@
+"""Pluggable secret backends for ``${secret:NAME}`` resolution.
+
+Default is the **file** backend (``~/.castle/secrets/<name>``) — identical to the
+historical behavior, so nothing changes unless a backend is explicitly selected
+via ``CASTLE_SECRET_BACKEND``.
+
+The **openbao** backend reads from an OpenBao/Vault KV-v2 mount and falls back to
+the file backend, which is also how it bootstraps: the OpenBao *token* itself is a
+file secret (it can't live in the vault it unlocks).
+
+Selection (env, so it works in both the CLI and the systemd-run API):
+  CASTLE_SECRET_BACKEND        file | openbao        (default: file)
+  CASTLE_OPENBAO_ADDR          http://localhost:8200
+  CASTLE_OPENBAO_MOUNT         castle                (kv-v2 mount path)
+  CASTLE_OPENBAO_TOKEN_SECRET  OPENBAO_TOKEN         (file secret holding the token)
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import urllib.request
+from pathlib import Path
+from typing import Protocol
+
+
+class SecretBackend(Protocol):
+    def read(self, name: str) -> str | None: ...
+
+
+class FileSecretBackend:
+    """Reads ``<secrets_dir>/<name>`` (the historical behavior)."""
+
+    def __init__(self, secrets_dir: Path) -> None:
+        self._dir = secrets_dir
+
+    def read(self, name: str) -> str | None:
+        path = self._dir / name
+        if path.exists():
+            return path.read_text().strip()
+        return None
+
+
+class OpenBaoBackend:
+    """Reads from an OpenBao/Vault KV-v2 mount; falls back to ``fallback``.
+
+    A missing key, an auth failure, or an unreachable server all fall through to
+    the fallback — so a partly-migrated vault and the bootstrap token both resolve.
+    """
+
+    def __init__(
+        self, addr: str, token: str, mount: str, fallback: SecretBackend
+    ) -> None:
+        self._addr = addr.rstrip("/")
+        self._token = token
+        self._mount = mount
+        self._fallback = fallback
+
+    def read(self, name: str) -> str | None:
+        value = self._read_bao(name)
+        if value is not None:
+            return value
+        return self._fallback.read(name)
+
+    def _read_bao(self, name: str) -> str | None:
+        if not self._token:
+            return None
+        url = f"{self._addr}/v1/{self._mount}/data/{name}"
+        req = urllib.request.Request(url, headers={"X-Vault-Token": self._token})
+        try:
+            with urllib.request.urlopen(req, timeout=5) as resp:  # noqa: S310
+                data = json.load(resp)
+            return data["data"]["data"].get("value")
+        except Exception:
+            return None
+
+
+def build_backend(secrets_dir: Path) -> SecretBackend:
+    """Construct the active secret backend from the environment."""
+    file_backend = FileSecretBackend(secrets_dir)
+    kind = os.environ.get("CASTLE_SECRET_BACKEND", "file").lower()
+    if kind == "openbao":
+        addr = os.environ.get("CASTLE_OPENBAO_ADDR", "http://localhost:8200")
+        mount = os.environ.get("CASTLE_OPENBAO_MOUNT", "castle")
+        token_secret = os.environ.get("CASTLE_OPENBAO_TOKEN_SECRET", "OPENBAO_TOKEN")
+        token = file_backend.read(token_secret) or os.environ.get(
+            "CASTLE_OPENBAO_TOKEN", ""
+        )
+        return OpenBaoBackend(addr, token, mount, fallback=file_backend)
+    return file_backend

core/tests/test_secret_backends.py

@@ -0,0 +1,54 @@
+"""Tests for the pluggable secret backends (file default, OpenBao opt-in)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from castle_core.secret_backends import (
+    FileSecretBackend,
+    OpenBaoBackend,
+    build_backend,
+)
+
+
+def test_file_backend_read_hit(tmp_path: Path) -> None:
+    (tmp_path / "MY_SECRET").write_text("value\n")
+    assert FileSecretBackend(tmp_path).read("MY_SECRET") == "value"
+
+
+def test_file_backend_read_miss(tmp_path: Path) -> None:
+    assert FileSecretBackend(tmp_path).read("ABSENT") is None
+
+
+def test_build_backend_defaults_to_file(tmp_path: Path, monkeypatch) -> None:
+    monkeypatch.delenv("CASTLE_SECRET_BACKEND", raising=False)
+    assert isinstance(build_backend(tmp_path), FileSecretBackend)
+
+
+def test_build_backend_openbao_selected(tmp_path: Path, monkeypatch) -> None:
+    monkeypatch.setenv("CASTLE_SECRET_BACKEND", "openbao")
+    assert isinstance(build_backend(tmp_path), OpenBaoBackend)
+
+
+def test_openbao_falls_back_to_file_when_unreachable(tmp_path: Path) -> None:
+    """An unreachable vault (or empty token) resolves via the file fallback."""
+    (tmp_path / "ONLY_IN_FILE").write_text("from-file")
+    backend = OpenBaoBackend(
+        addr="http://127.0.0.1:1",  # nothing listening
+        token="dummy",
+        mount="castle",
+        fallback=FileSecretBackend(tmp_path),
+    )
+    assert backend.read("ONLY_IN_FILE") == "from-file"
+    assert backend.read("NOT_ANYWHERE") is None
+
+
+def test_openbao_empty_token_uses_fallback(tmp_path: Path) -> None:
+    (tmp_path / "K").write_text("v")
+    backend = OpenBaoBackend(
+        addr="http://127.0.0.1:8200",
+        token="",  # no token → never hits the network
+        mount="castle",
+        fallback=FileSecretBackend(tmp_path),
+    )
+    assert backend.read("K") == "v"

docs/fleet-mesh-plan.md

@@ -1,7 +1,8 @@
 # Fleet Mesh Plan — OpenBao + NATS
 
 **Status:** in progress on branch `feat/fleet-mesh-nats-openbao`.
-Phases 0–2 complete + single-node verified (live). Phases 3–4 pending (need 2nd node).
+Phases 0–2 + Phase 4 (secret-read backend) complete + single-node verified (live).
+Phase 3 (cross-node routing/breaker) and Phase 4 hardening pending the 2nd node.
 
 ## Context
 
@@ -218,6 +219,34 @@ second node is proven on NATS.
 **Pending second node:** the follower-side reconcile (watch `castle-config` →
 `castle apply`) is wired as an SSE hook but only meaningful with a peer.
 
+### Phase 4 — secret-read backend DONE + verified + tested (2026-07-07)
+
+- New `core/castle_core/secret_backends.py`: `SecretBackend` protocol,
+  `FileSecretBackend` (the historical behavior), `OpenBaoBackend` (KV-v2 read with
+  file fallback), and `build_backend()` selecting via `CASTLE_SECRET_BACKEND`
+  (default **file** — production is byte-for-byte unchanged until opted in).
+- `_read_secret` (the single chokepoint, `config.py`) now delegates to the active
+  backend. `${secret:NAME}` syntax untouched.
+- OpenBao token bootstraps from the file backend (it can't live in the vault it
+  unlocks); a missing key / auth failure / unreachable server all fall through to
+  file, so a partly-migrated vault keeps working.
+- Verified live against the running `castle-openbao`: a secret stored in the vault
+  resolves through `${secret:...}` in openbao mode; file-only secrets resolve via
+  fallback; missing → placeholder; **default file mode unchanged**.
+- **Tests:** `core/tests/test_secret_backends.py` (file hit/miss, backend
+  selection, unreachable-vault + empty-token fallback). Suites: **206 core + 93 api**.
+
+**Remaining for full OpenBao production use (documented, not blocking — nothing
+uses the backend until `CASTLE_SECRET_BACKEND=openbao`):**
+1. **Write path** — `castle-api/secrets.py` CRUD still writes to files; in openbao
+   mode dashboard-set secrets land in file (resolve via fallback) rather than the
+   vault. Add `write`/`delete` to the backends + route the API writer through them.
+2. **Auto-unseal on boot** (Phase 0 deferral) — OpenBao re-seals on container
+   restart. Wire an unseal step (manifest `exec_start_post` or a companion
+   oneshot reading `OPENBAO_UNSEAL_KEY`). Not urgent while the backend is unused.
+3. **TLS hardening** — NATS + OpenBao are plaintext localhost. Required before
+   cross-network or moving real secrets: NATS mTLS + auth, OpenBao TLS listener.
+
 ## Decisions (resolved)
 
 1. **Discovery — both.** Keep mDNS for zero-config LAN peer discovery *and*