hyperhive/nix/agent-modules/bao.nix

# This agent's own identity at the swarm secret store, and the check that
# proves it works.
#
# `swarm-controller` mints the leaf at agent creation and publishes it; this
# agent's hive collects it under the hive's own certificate and hands it in as
# systemd credentials (`hive_c0re::lifecycle::agent_identity`). Nothing here
# fetches anything from the store — by the time this container boots, its
# identity is already inside it.
#
# ⚠️ The identity arrives as credentials and NOT as a bind mount, and the mode
# is why: the host file is `0600` to the hive daemon and this unit runs as the
# unprivileged agent user. nspawn's `--load-credential` is read by the
# container manager as root and re-exposed under this unit's own `User=`; a
# bind would deliver a private key this user cannot open. Same answer, same
# reason, as ./queue.nix's credential pair.
#
# This is the courier's other end: the hive-side collector has no purpose
# without it, so the two ship together and neither is reachable alone.
#
# 🩸 This unit fails LOUDLY where the hive-side readers degrade quietly, and
# that is the opposite default on purpose. A missing OIDC client secret means
# a hive whose publisher has yet to run; a missing or refused certificate
# means an agent that believes it can reach the store and cannot, which every
# later pull would report as its own unrelated failure. The one place that
# knows the real cause is the login itself, so this is where it is said.
{
  pkgs,
  lib,
  config,
  ...
}:
let
  cfg = config.services.hyperhive.agent.bao;

  # This container's agent name. The same string the hive published the
  # identity under, because the agent's unix user is named for the agent —
  # see ./user.nix.
  agentName = config.services.hyperhive.agent.user.name;

  # The three ids `hive_c0re::lifecycle::agent_identity` forwards under.
  # Neither side can discover the other's spelling, so a rename is a rename
  # there too.
  certCredential = "hive-agent-bao-cert";
  keyCredential = "hive-agent-bao-key";
  serverCaCredential = "hive-agent-bao-server-ca";

  unitName = "hive-agent-bao-identity";

  # Where the identity lives in the store, spelled from the same pieces the
  # publisher uses. `swarm_secret_client::mtls::identity_path` builds
  # `swarm/agents/<agent>/bao-mtls` and `path::MOUNT` is `secret`; this
  # literal is the nix half of that one agreement, exactly as
  # ../host-modules/glue-queue-agent-credential.nix spells its own.
  identityPath = "secret/swarm/agents/${agentName}/bao-mtls";

  # The address is the whole switch — no separate `enable`, the same shape
  # ./queue.nix and ./logs.nix gate themselves with. A hive that has a store
  # forwards its address and every agent on it gets the check; a hive that has
  # none forwards nothing and no agent does. An `enable` beside it would be a
  # knob whose only correct setting is whatever the address already says, and
  # its default would decide whether the hive-side courier delivers into a
  # container that reads what it is given or into one that never looks.
  configured = cfg.addr != null;

  storeRetry = import ../host-modules/lib/store-retry.nix { };
in
{
  options.services.hyperhive.agent.bao = {
    addr = lib.mkOption {
      type = lib.types.nullOr lib.types.str;
      default = null;
      example = "https://bao.example.com:8200";
      description = ''
        Where the swarm secret store listens, as this container reaches it.

        Set by the generated meta flake from the host's own `BAO_ADDR`, which
        is the address this hive already uses, and setting it is what generates
        `${unitName}.service`: at boot that unit logs in with the certificate
        its hive delivered and reads this agent's own path back, failing if
        either step does not succeed. `systemctl status ${unitName}` inside the
        container is then the answer to "can this agent reach the store as
        itself", which nothing else in the tree reports.

        `null` means the hive was given no store. No unit is generated then,
        because an agent whose swarm never minted an identity has nothing to
        log in with, and a failed unit at every boot would say that in the
        loudest possible way about a deployment that never asked for it.
      '';
    };
  };

  config = lib.mkIf configured {
    systemd.services.${unitName} = {
      description = "prove this agent can authenticate to the swarm secret store as itself";
      after = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
      path = [
        pkgs.openbao
        pkgs.coreutils
      ];
      # ../host-modules/lib/store-retry.nix. From an agent the store is reached
      # through the gateway's stream passthrough, so either being down fails
      # the login below.
      inherit (storeRetry) startLimitBurst startLimitIntervalSec;
      serviceConfig = storeRetry.serviceConfig // {
        Type = "oneshot";
        RemainAfterExit = true;
        TimeoutStartSec = 30;
        User = agentName;
        Group = agentName;
        # Bare ids, no paths: the terse `LoadCredential=` form that inherits a
        # credential the service *manager* received, which is what the
        # container manager passed in. ./queue.nix states the same shape.
        LoadCredential = [
          certCredential
          keyCredential
          serverCaCredential
        ];
      };
      environment = {
        BAO_ADDR = cfg.addr;
        # `%d` is `$CREDENTIALS_DIRECTORY`, per-unit and owned by `User=`.
        BAO_CLIENT_CERT = "%d/${certCredential}";
        BAO_CLIENT_KEY = "%d/${keyCredential}";
      };
      script = ''
        set -euo pipefail

        # Absent credentials are the first thing checked, because every later
        # message would blame the store for a delivery that never happened.
        # 🩸 The files are named and never read here: what went wrong is a
        # property of the path, and the bytes at it are a private key.
        missing=
        for id in ${lib.escapeShellArg certCredential} ${lib.escapeShellArg keyCredential}; do
          if [ ! -s "$CREDENTIALS_DIRECTORY/$id" ]; then
            missing="$missing $id"
          fi
        done
        if [ -n "$missing" ]; then
          echo "this agent has no store identity: the hive delivered no$missing." >&2
          echo "swarm-controller publishes it at ${identityPath} when the agent is created, and the hive collects it from there." >&2
          exit 1
        fi

        # Only when one was delivered. Absent means the container verifies the
        # store's listener against its own trust store, which is what a
        # deployment with a real CA wants; pointing BAO_CACERT at a file that
        # is not there would fail the handshake and name the wrong cause.
        if [ -s "$CREDENTIALS_DIRECTORY/${serverCaCredential}" ]; then
          export BAO_CACERT="$CREDENTIALS_DIRECTORY/${serverCaCredential}"
        fi

        err="$(mktemp)"
        trap 'rm -f "$err"' EXIT

        # Cert auth is a login, not a transport setting. The `BAO_CLIENT_*`
        # variables above only decide which certificate the TLS handshake
        # presents; without a token `bao` asks its token helper instead, and
        # that is a `sh` this unit's `path` does not carry. `-token-only`
        # answers on stdout and skips the helper on both sides.
        #
        # No `name=`: a cert role pins both its authority and the common name
        # it accepts, so this agent's leaf matches its own role and no other.
        # Naming the role here would be a third copy of a string
        # `swarm_secret_client::policy::agent_object_name` already owns.
        if ! BAO_TOKEN="$(bao login -method=cert -token-only 2>"$err")"; then
          echo "this agent's certificate was refused by the swarm secret store at $BAO_ADDR." >&2
          if [ -s "$err" ]; then
            cat "$err" >&2
          else
            echo "bao failed without writing a diagnostic." >&2
          fi
          exit 1
        fi
        export BAO_TOKEN

        # A token is not yet an answer: the login proves the certificate, this
        # proves the policy attached to it. Reading this agent's own identity
        # back is the smallest read its document grants, and it is the same
        # check `swarm-controller` runs against the leaf before it reports the
        # creation done — so a policy that drifted apart from the path fails
        # here rather than in whichever pull needed it first.
        #
        # ⚠️ Output discarded, not printed: the field is a certificate and the
        # object beside it is a private key. Nothing about this check needs a
        # value, only whether the read succeeded.
        if ! bao kv get -field=cert ${lib.escapeShellArg identityPath} >/dev/null 2>"$err"; then
          echo "this agent logged in to the swarm secret store but cannot read ${identityPath}, so its policy does not cover its own path." >&2
          if [ -s "$err" ]; then
            cat "$err" >&2
          else
            echo "bao failed without writing a diagnostic." >&2
          fi
          exit 1
        fi

        echo "authenticated to the swarm secret store at $BAO_ADDR as ${agentName} and read ${identityPath}."
      '';
    };
  };
}