feat(db): use POSTGRES_USER in pg_backup; document it in release note

- pg_backup.sh: use the db service's POSTGRES_USER (default postgres) for the
  dump/drop/recreate instead of detecting the superuser at runtime, since the
  recipe now sets that env var; one-line the constants comment
- release note: explain the in-place pg_upgrade + the POSTGRES_USER override
- bump PG_BACKUP_VERSION v4

Verified on cctest: backup + restore via the hooks round-trips with POSTGRES_USER.

abra.sh

@@ -1,4 +1,4 @@
-export PG_BACKUP_VERSION=v3
+export PG_BACKUP_VERSION=v4
 export APP_ENTRYPOINT_VERSION=v2
 export APP_INSTALL_SSL_VERSION=v1
 export APP_MIGRATE_UPLOADS_VERSION=v1

pg_backup.sh

@@ -4,23 +4,13 @@
 
 set -e
 
-# constants 
+# dump goes at the volume root so backupbot's backup.sql label finds it
 BACKUP_FILE='/var/lib/postgresql/backup.sql'
 DATADIR="${PGDATA:-/var/lib/postgresql/18/docker}"
 DB_NAME="${POSTGRES_DB:-discourse}"
 
-# The bootstrap superuser (install user, oid 10) differs between deployments
+# bootstrap superuser for the dump/drop/recreate; same POSTGRES_USER the db service sets
-# (`postgres` on bitnami-origin clusters, `discourse` on others). Detect it at
+SU="${POSTGRES_USER:-postgres}"
-# runtime over the local trust socket rather than hard-coding a name.
-detect_superuser() {
-  local u name
-  for u in discourse postgres; do
-    name="$(psql -U "$u" -d "$DB_NAME" -tAc 'select rolname from pg_roles where oid = 10' 2>/dev/null | tr -d '[:space:]')"
-    if [ -n "$name" ]; then echo "$name"; return 0; fi
-  done
-  echo postgres
-}
-SU="$(detect_superuser)"
 
 function backup {
   pg_dump -U "$SU" "$DB_NAME" | gzip > "$BACKUP_FILE"

release/1.0.0+3.5.3

@@ -8,3 +8,13 @@ Rename these in your app's .env (the values carry over):
   DISCOURSE_SMTP_USER     --> DISCOURSE_SMTP_USER_NAME
   DISCOURSE_SMTP_AUTH     --> DISCOURSE_SMTP_AUTHENTICATION
   DISCOURSE_SMTP_PROTOCOL --> DISCOURSE_SMTP_ENABLE_START_TLS (takes a boolean true/false, not the old tls/ssl value, so translate it rather than copying it straight across)
+
+The database now runs the discourse/postgres image, which upgrades your existing
+postgres cluster to a new major version in place on first boot (no dump/restore).
+
+That upgrade runs as the cluster's bootstrap superuser ("install user"), which the
+recipe sets via POSTGRES_USER, defaulting to `postgres`. This default is correct
+for clusters created by this recipe and the old bitnami recipe, so most operators
+need to do nothing. ONLY if your cluster was bootstrapped with a different
+superuser (e.g. `discourse`) must you set POSTGRES_USER in your app's .env before
+upgrading -- otherwise pg_upgrade aborts with an install-user mismatch.