Run the bus from its pinned upstream image with the snapshot program and the entrypoint mounted, and write and rehearse the way there and back (issues 1640, 1641, 1642, ADR 0480 clauses 1-3, 5, 6, 11)

modules/nats/program/entrypoint.sh

@@ -0,0 +1,69 @@
+#!/bin/sh
+# nats's entrypoint: run the server, and reload it in place when the mesh rewrites its
+# configuration.
+#
+# **Why this exists inside the module** (novox/hq design 25 §5). The controller composes every
+# account and permission into one file, and that file changes whenever a module is added,
+# reassigned, or a person's access is granted or revoked — which is often, and on the one server
+# everything else depends on. The host has no way to say "reload this container": a
+# container resource has `restart-on` and nothing else, and a container's `restart-on` means
+# *recreate* — every connection dropped and every in-flight JetStream ack lost, mid-flight, for a
+# permission change. `reload-on` is real but it is a *service* field, not a container's.
+#
+# nats-server already reloads its own configuration on SIGHUP — accounts, permissions, everything
+# the mesh composes — without dropping a connection. That is the server's own documented
+# capability, not something built for the mesh. So the configuration is mounted as a directory
+# (a directory's contents are not digest-tracked the way a directly-mounted file's are, novox/hq
+# issue 103), and this watches the one file inside it and signals the server itself. The host's
+# only job is what it already does for any directory: keep the file's content current. Nothing
+# here is declared `restart-on` or `reload-on`.
+set -eu
+
+# **Two files, and only one of them is the mesh's** (novox/hq design 25 §4, task 1.7). CONF is this
+# module's own — ports, TLS, JetStream — declared in its manifest, because those are properties of
+# the container this module raises. USERS is every account and permission, composed by the
+# controller, and CONF includes it. So what is watched here is the mesh's half: the module's own
+# does not change without a new declaration, and that recreates the container anyway.
+CONF="${MESH_NATS_CONF:-/etc/nats/nats.conf}"
+USERS="${MESH_NATS_USERS:-/etc/nats/accounts.conf}"
+POLL="${MESH_NATS_CONF_POLL_SECONDS:-5}"
+
+# Both are written as part of the same declaration that creates this container, but none of the
+# three are ordered against each other. Waiting is correct and starting without them is not:
+# nats-server given a configuration whose include is missing refuses to start, and one given no
+# configuration at all comes up with its compiled-in defaults — no TLS, no accounts, every subject
+# open to anyone who can reach the port. A bus that is briefly open to everything is not a bus that
+# is briefly wrong; it is an open bus.
+for needed in "$CONF" "$USERS"; do
+  while [ ! -s "$needed" ]; do
+    echo "[nats] waiting for the mesh to write $needed"
+    sleep 1
+  done
+done
+
+digest() { sha256sum "$USERS" 2>/dev/null | cut -d' ' -f1; }
+
+nats-server --config "$CONF" "$@" &
+server=$!
+
+# Forward a stop to the server and let it drain, rather than dying and leaving it orphaned as
+# PID 1's child.
+stop() { kill -TERM "$server" 2>/dev/null || true; }
+trap stop TERM INT
+
+last=$(digest)
+while kill -0 "$server" 2>/dev/null; do
+  sleep "$POLL"
+  now=$(digest)
+  # An empty digest means the file is mid-write or briefly gone. Reloading on that would hand the
+  # server a truncated configuration; the next tick sees the finished one.
+  [ -n "$now" ] || continue
+  if [ "$now" != "$last" ]; then
+    last=$now
+    echo "[nats] the mesh's user list changed; reloading in place"
+    kill -HUP "$server" || true
+  fi
+done
+
+# `wait` on an already-exited child still yields its status, which becomes this container's.
+wait "$server"