fix(postgres,mariadb): attach pool 'error' listeners so a dropped idle connection no longer crashes the process

[tianzhou]

Fixes #422

Problem

The PostgreSQL connector creates its pool with new Pool(poolConfig) but never attaches a pool.on("error", ...) listener. pg-pool re-emits an idle client's error (server restart, failover, idle-timeout close) as an 'error' event on the pool. With no listener, Node treats it as an unhandled 'error' event and the whole DBHub process exits:

Emitted 'error' event on BoundPool instance at:
    at Client.idleListener (.../pg-pool/index.js:62:10)

Over stdio, MCP clients (Claude Code, Codex) do not restart the server, so the database tools are gone until the user reconnects manually.

I reproduced this at the driver level: pg-pool 3.10.0 driven with a fake client, released into the idle set, then given an error. Without a listener the process exits with code 1 and the stack in the issue. With a listener the client is purged, the idle count drops to zero, and the next query reconnects normally.

The mariadb driver has the same shape. Its pool keeps minimumIdle connections open in the background (default: connectionLimit) and emits 'error' on the pool from a plain callback when a background reconnect attempt fails, e.g. while the server is restarting. mysql2 handles idle errors internally without emitting on the pool, and mssql emits inside a promise chain (a rejection, not a crash), so those connectors are not changed.

Fix

• src/connectors/postgres/index.ts: attach an 'error' listener right after new Pool(...). The client has already been purged by the time the event fires, so the listener only logs the error with the source id.
• src/connectors/mariadb/index.ts: same, right after createPool(...). The pool retries with backoff on its own. The mariadb typings omit the 'error' overload on Pool, so it is cast to NodeJS.EventEmitter (the runtime class extends it).

Tests

• New src/connectors/__tests__/pool-error-listener.test.ts: drives each connector with an EventEmitter-based fake pool, where emit("error") throws exactly as it would without a listener (the file includes a sanity check for that). Asserts that after connect() the pool has one listener, emitting no longer throws, the log line names the pool and source, and listeners do not stack across reconnects.
• Existing fake pools in connect-failure-cleanup.test.ts and readonly-transaction-strategy.test.ts gain an on stub so their connect paths keep working.

Full unit project: 38 files, 1002 tests passing. Integration suites need Docker, which was not available in this session.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KU7ZccZ8RgLY6rqnnoQbgv

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Generated by Claude Code

[Copilot]

🟢 Approval recommended

No unresolved blocking issues were identified.

Pull request overview

Prevents PostgreSQL and MariaDB pool errors from crashing the process by adding error listeners and regression tests.

Changes:

• Adds pool error logging for PostgreSQL and MariaDB.
• Adds listener behavior tests.
• Updates affected fake pool mocks.

File summaries

File	Summary
src/connectors/postgres/index.ts	Handles PostgreSQL pool errors.
src/connectors/mariadb/index.ts	Handles MariaDB pool errors.
src/connectors/__tests__/readonly-transaction-strategy.test.ts	Updates fake pool support.
src/connectors/__tests__/pool-error-listener.test.ts	Tests listener behavior and logging.
src/connectors/__tests__/connect-failure-cleanup.test.ts	Updates pool mocks.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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