swarm-bao-pki: enabling a service mints its client leaf on the same switch

`swarm-bao-pki` was a `RemainAfterExit` oneshot. Once active, nothing
re-ran it: a switch restarts it only when its definition changes, and a
`Requires=`/`Wants=` on an active unit is satisfied without running it.

On muede-lpt2 the 10:57Z switch that added the builder line did change
the unit, so switch-to-configuration stopped it in its stop phase. A
start of `container@swarm-bao` 4 s later (before the daemon-reload at
10:57:35Z) pulled it back in through `swarm-bao-certs` with the OLD
definition, which had no builder line: 48 ms, nothing missing. The start
phase then found it active and did nothing. The 20:14Z switch that
enabled the builder left the unit unchanged, so it was never re-run, and
`container@swarm-builder` crash-looped on the absent `builder.pem`.

The unit now goes inactive after each run. Every switch starts
`multi-user.target`, which runs it again post-reload with the current
definition, and every unit that wants or requires it runs it on its own
start. Each line is idempotent on presence, so an existing leaf is never
re-signed.

The three consumers started from `machines.target` rather than
`multi-user.target` also want it, since `After=` alone does not order
against a job in another transaction: the builder's and the review
classifier's containers (nspawn `--load-credential`) and
`swarm-nats-auth-secrets` (pulled in by the queue's container).

Fixes #5284

nix/host-modules/glue-bao-tls.nix

@@ -133,28 +133,25 @@ in
     # every client certificate already trusting it, so a rebuild that
     # "refreshed" it would lock every reader in the swarm out at once — the
     # same rule the store's TPM PIN unit follows, for a sharper reason.
+    #
+    # ⚠️ Not `RemainAfterExit`: the unit goes inactive after every run, so
+    # each start of a unit that wants or requires it, and each switch's start
+    # of `multi-user.target`, runs it again with the definition loaded at that
+    # moment. That is what mints a leaf a deploy adds, or a consumer it
+    # enables, on that deploy. An active `RemainAfterExit` unit satisfies all
+    # of those without running, and a switch only restarts it when its
+    # definition changes. A run where nothing is missing is one `[ -s ]` test
+    # per leaf.
     systemd.services.swarm-bao-pki = {
       description = "mint the swarm secret store's own CA and leaves";
       before = [ "swarm-bao-certs.service" ];
       requiredBy = [ "swarm-bao-certs.service" ];
-      # 🩸 A target wants this, not only `swarm-bao-certs`, and the reason is
-      # what happens when the LIST of leaves grows. `requiredBy` alone is
-      # satisfied by a unit that already ran: this one is `RemainAfterExit`,
-      # so an activation that adds a leaf here finds it active, pulls nothing,
-      # and the new leaf is never minted — while every sibling granting unit,
-      # each of which carries this line, was re-run by the same activation.
-      # That is how `services-issuer.pem` came to be missing on a host whose
-      # config named it, leaving `swarm-services-cert` to fail on an absent
-      # file until someone restarted this by hand.
       wantedBy = [ "multi-user.target" ];
       path = [
         pkgs.openssl
         pkgs.coreutils
       ];
-      serviceConfig = {
-        Type = "oneshot";
-        RemainAfterExit = true;
-      };
+      serviceConfig.Type = "oneshot";
       script = ''
         set -euo pipefail
         install -d -m 0700 ${pkiDir}

nix/host-modules/swarm-builder.nix

@@ -131,10 +131,17 @@ in
     services.hyperhive.gateway.dns.enable = lib.mkDefault true;
 
     # nspawn fails to start the container when a credential source is
-    # missing, so where this host mints the leaf the container waits for it.
+    # missing, so where this host mints the leaf the container pulls the
+    # minting unit in and waits for it. Ordering alone is not enough: the
+    # container starts from `machines.target`, outside the transaction that
+    # runs the minting unit from `multi-user.target`. Ignored elsewhere, where
+    # the unit does not exist.
     systemd.services."container@${machine}" = lib.mkMerge [
       caTrust.containerOrdering
-      { after = [ "swarm-bao-pki.service" ]; }
+      {
+        after = [ "swarm-bao-pki.service" ];
+        wants = [ "swarm-bao-pki.service" ];
+      }
     ];
 
     containers.${machine} = {

nix/host-modules/swarm-nats.nix

@@ -877,8 +877,13 @@ in
       before = [ "container@swarm-nats.service" ];
       wantedBy = [ "container@swarm-nats.service" ];
       # Where the store's PKI is minted on this host, the responder's leaf is
-      # one of its files. Ordering only: elsewhere the unit does not exist.
+      # one of its files, and the minting unit runs before every copy. Ordering
+      # alone is not enough: the queue's container, which pulls this unit in,
+      # starts from `machines.target`, outside the transaction that runs the
+      # minting unit from `multi-user.target`. Ignored elsewhere, where the unit
+      # does not exist.
       after = [ "swarm-bao-pki.service" ];
+      wants = [ "swarm-bao-pki.service" ];
       serviceConfig = {
         Type = "oneshot";
         RemainAfterExit = true;

nix/host-modules/swarm-review-classifier.nix

@@ -137,10 +137,17 @@ in
     services.hyperhive.gateway.dns.enable = lib.mkDefault true;
 
     # nspawn fails to start the container when a credential source is
-    # missing, so where this host mints the leaf the container waits for it.
+    # missing, so where this host mints the leaf the container pulls the
+    # minting unit in and waits for it. Ordering alone is not enough: the
+    # container starts from `machines.target`, outside the transaction that
+    # runs the minting unit from `multi-user.target`. Ignored elsewhere, where
+    # the unit does not exist.
     systemd.services."container@${machine}" = lib.mkMerge [
       caTrust.containerOrdering
-      { after = [ "swarm-bao-pki.service" ]; }
+      {
+        after = [ "swarm-bao-pki.service" ];
+        wants = [ "swarm-bao-pki.service" ];
+      }
     ];
 
     containers.${machine} = {