# Task 02 — The agent serves a protected TLS web server from one installed bundle

**Phase:** `15_agent_mtls` · **Source:** `TODO.md:3` — "I want all exporters to be protected web servers. The collector should generate and manage a root certificate that needs to be installed with each exporter. Use the certs as auth to each exporter."

## Objective

`host_agent.py` gains a TLS mode that makes the web server protected by certificate: one installed bundle holds the server cert + key, the root CA, a bundle revision, and a revocation list; the agent requires a client cert signed by the root on every path except `/health` (mirroring the token-mode `/health` exemption, so an installer probe can reach `/health` with the CA only), and it reports the bundle revision it loaded so the dashboard can tell a stale bundle from a fresh one. The token mode is byte-identical to today.

## Work

1. `agent/host_agent.py` — stdlib only (an `ssl` import; no new third-party dependency):
   - a new flag `--cert-bundle <file>` (env `AGENT_CERT_BUNDLE`): one PEM text containing the server cert, the server key, the root CA cert, a `# BUNDLE-REV <n>` line, and `# REVOKED <serial>` lines. `resolve_bundle(path) -> (context, bundle_rev, revoked: frozenset[str], error)` is a pure function (the `resolve_token` idiom): parse the PEM blocks by text, extract the three blocks, write them to a temp dir for `load_cert_chain` (into `/run/dashboards-agent` when present and writable — the unit's `RuntimeDirectory`, task 06 — else `tempfile.mkdtemp`), and build the context.
   - The TLS context: `ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)`, `load_cert_chain`, `load_verify_locations(ca)`, `verify_mode = ssl.CERT_OPTIONAL` — the handshake is optional and the **handler** enforces it: on every path except `/health`, `self.connection.getpeercert()` must carry a verified certificate (subject present), else `403 {"ok": false, "detail": "client certificate required"}`; that cert's serial must not be in the bundle's `# REVOKED` set, else `403 {"ok": false, "detail": "client certificate revoked"}`. The denylist file is re-read on mtime change per request (a small file; a redeployed bundle takes effect without a restart — the reason the `# REVOKED` lines live in the bundle at all).
   - The startup gate: off-loopback with neither a token nor a bundle → the existing exit-2 refusal (its message now names the bundle too); a bundle present → the off-loopback bind is allowed (the client cert is the protection); a token plus a bundle → the token check is skipped on the TLS path (the handshake is the auth).
   - The snapshot payload gains two **additive** top-level fields in TLS mode: `tls: true` and `bundle_rev: <n>` (absent in token mode — an additive field never changes an existing shape); the `serve()` start line says `tls (client cert required, rev <n>)`.
   - `AGENT_VERSION` 1.0.0 → 1.1.0; the docstring's deployment paragraph gains one line for the TLS mode (no flavour text — the AGENTS.md audit rule).
   - ASSUMPTION: `CERT_OPTIONAL` + the per-path check (not `CERT_REQUIRED`) is what keeps `/health` probeable with the CA only — the installer's `--status` and the systemd probe (task 06) cannot present a client cert the installer does not hold, and a probe that cannot health-check is a dead install.
   - ASSUMPTION: the agent never generates or renews certificates (stdlib cannot mint x509); the bundle is the artifact the dashboard signs and the installer places.
2. `tests/test_agent.py` — the existing in-process harness (`serve()` in a thread on `127.0.0.1`, `urllib`/`ssl` clients), with a test CA + leafs generated by `src/dashboards/certs.py` over a dict store (no second generator):
   - a client with a matching client cert → `/snapshot` 200 with `tls: true` and `bundle_rev` equal to the bundle's;
   - a client with no cert → the handshake fails or a 403 on a non-`/health` path; `/health` still 200 with a CA-only context (the probe path);
   - a client signed by a *different* CA → refused;
   - a client whose serial is in the bundle's `# REVOKED` list → 403 "revoked"; rewrite the bundle file without the line (mtime change) → the same client is accepted on the next request (the denylist re-read);
   - a token-mode agent (no bundle): the existing tests unchanged and green (byte-identical behaviour — no `tls`/`bundle_rev` fields);
   - `resolve_bundle` error paths: a missing file, an empty file, a bundle missing the key block, a bundle missing the `# BUNDLE-REV` line (an error — a revless bundle is not one this app signed; ASSUMPTION: the `# BUNDLE-REV` line is mandatory).
3. `deploy/` is untouched here (task 06); the non-systemd fallback (`tempfile.mkdtemp`) keeps dev and tests working.

## Testing & Quality

- the `tests/test_agent.py` matrix above; the existing token-mode tests stay green unchanged.
- Coverage: **>90%** on this task's new/modified code.

## Completion Criteria

- [ ] a TLS-mode agent on a loopback port answers the full handshake matrix (tests) and reports `bundle_rev`
- [ ] a token-mode agent is byte-identical to today (existing tests green, no new fields present)
- [ ] `uv run pytest` green; `uv run ruff check src tests agent` + `uv run pyright` clean