Never emit an empty synchronous_standby_names list

[souravbiswassanto]

Problem

The role scripts build synchronous_standby_names from $REPLICAS, skipping the pod's own index:

for ((i = 0; i < $REPLICAS; i++)); do
    if [[ $self_idx == $i ]]; then echo "skip $i"; else names+="\"$sts_prefix$i\"",; fi
done
names=${names%,}
echo "synchronous_standby_names = '${SYNC_REPLICATION_MODE:-ANY} ${NUM_SYNC_REPLICAS:-1} ("$names")'"

At one replica that loop skips its only iteration and $names is empty, while
SYNC_REPLICATION_MODE / NUM_SYNC_REPLICAS still come from the CR and describe the eventual
topology. The emitted value is 'FIRST 2 ()'.

Postgres does not read an empty list as "no synchronous standbys" — it is a parse error, and the
server refuses to start:

LOG:     invalid value for parameter "synchronous_standby_names": "FIRST 2 ()"
DETAIL:  syntax error at or near ")"
FATAL:   configuration file "/var/pv/data/postgresql.conf" contains errors

scripts/run.sh then re-runs the role script on its one-second loop, and start.sh prepends a
fresh block each pass, so the config also accumulates duplicate entries while the pod sits
not-ready:

$ grep standby /var/pv/data/postgresql.conf
hot_standby = on
synchronous_standby_names = 'FIRST 2 ()'
hot_standby = on
synchronous_standby_names = 'FIRST 2 ()'

How it is reached today

A PITR restore. postgres/pkg/controller/restore.go collapses the CR to a single replica for the
sync and fscopy strategies, while leaving streamingMode: Synchronous in place:

if *db.Spec.Replicas > 1 && (rs == ReplicationStrategyFSCopy || rs == ReplicationStrategySync) {
    ...
    db.Spec.Replicas = ptr.To(int32(1))
}

sync is the default (postgres_helpers.go SetDefaults), so this affects any synchronous
cluster with replicas > 1 being restored — the user need not have chosen a strategy at all. The
restore's WAL replay completes and then the primary never starts; the CR sits in Provisioning.

Reported against KubeDB v2026.1.19 with PG 16.9, streamingMode: Synchronous,
synchronousReplicationConfig: {mode: First, numSyncReplicas: 2}, replicas: 3.

Fix

Guard the empty list. Postgres accepts synchronous_standby_names = '' and treats every
synchronous_commit level as local, so commits wait for local flush only — no error, and no
hang waiting for standbys that do not exist. The next reconcile rewrites the file with a real list
once the peers are up.

Applied to all 40 emitters (majors 9–18 × primary/start.sh, standby/run.sh,
standby/warm_stanby.sh, standby/ha_backup_job.sh). The majors are independent copies, so each
one needs the guard. No behaviour change when the list is non-empty.

Verification

Against a real PostgreSQL 16.15:

value	result
'FIRST 2 ()'	invalid value for parameter ... syntax error at or near ")" — reproduces the report exactly
''	starts
'FIRST 2 ("a","b")'	starts

Replaying the block from role_scripts/16/primary/start.sh:

BEFORE:  REPLICAS=1 -> synchronous_standby_names = 'FIRST 2 ()'
         REPLICAS=3 -> synchronous_standby_names = 'FIRST 2 ("test-pg-1","test-pg-2")'
AFTER:   REPLICAS=1 -> synchronous_standby_names = ''
         REPLICAS=3 -> synchronous_standby_names = 'FIRST 2 ("test-pg-1","test-pg-2")'

• bash -n passes on all 66 scripts in role_scripts/.
• shfmt -l -ci -i 4 reports the same 56 files before and after, so this adds no formatting drift.

Related

https://github.com/kubedb/postgres/pull/938 removes the mismatch at the source by restoring asynchronously while the CR is
collapsed to one pod. Either change alone fixes the reported case; this one is the defensive half
and covers every other path that can produce an empty list.

Shipping this to existing clusters needs a postgres-init release plus a PostgresVersion
catalog bump in kubedb/installer.

🤖 Generated with Claude Code

[coderabbitai]

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