PostgreSQL addon: expose pgbouncer (pgbouncer.ini) tunables via Reconfiguring / cluster API

[weicao]

Problem

Today the PostgreSQL addon ships a per-pod pgbouncer sidecar (port 6432) using addons/postgresql/config/pgbouncer-ini.tpl mounted as pgbouncer-configuration. Operators often need to tune pool sizing (default_pool_size, related pool limits) and other pgbouncer.ini settings for workload shape and connection storms.

KubeBlocks Reconfiguring OpsRequest only targets postgresql.conf (see templates/paramsdef.yaml: fileName: postgresql.conf). The pgbouncer-configuration config entry in templates/cmpd.yaml has no reconfigure action (unlike postgresql-configuration, which includes postgresql.config.reconfigureAction).

So users cannot adjust pgbouncer via the cluster API after deployment; default_pool_size is not even set in the template, so PgBouncer default (20) applies. That is a real product gap (documented as such in kubeblocks-docs PostgreSQL FAQ §7).

Desired behavior

• First-class support to change pgbouncer parameters (at minimum: default_pool_size, ideally pool_mode, max_client_conn override, and other common pgbouncer.ini keys) without forking the chart for every cluster.
• Consistent UX with engine reconfiguration: e.g. OpsRequest type Reconfiguring (or a dedicated operation) with validated parameters and clear reload vs restart semantics for the pgbouncer container.

Suggested implementation sketch

1. Add a parameters definition / schema for pgbouncer.ini (properties format or dedicated constraint), or extend the component config model so pgbouncer keys are valid reconfigures[].parameters for component postgresql.
2. Attach reconfigure lifecycle to pgbouncer-configuration in ComponentDefinition (reload pgbouncer or restart sidecar as appropriate—Bitnami images often support SIGHUP reload for many settings).
3. Ensure generated pgbouncer.ini merges user overrides with templated values (memory-based max_client_conn, etc.).
4. Tests / examples: sample OpsRequest and kbcli path if applicable.

References

• addons/postgresql/templates/cmpd.yaml — configs for postgresql-configuration vs pgbouncer-configuration
• addons/postgresql/templates/paramsdef.yaml — postgresql.conf only
• addons/postgresql/config/pgbouncer-ini.tpl — current template

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If this belongs in the main kubeblocks repo instead (core reconfiguring API), please move/relabel; the addon change still needs the pgbouncer config + reload wiring above.