fix(adapter-mariadb): release pooled connections and harden transaction cleanup

[andychinghk01]

Summary

Fixes #28964.

The MariaDB adapter previously called mariadb.Connection#end() to dispose of a transaction connection. For a PoolConnection end() happens to be aliased to release(), so in practice it does return the connection to the pool — but the intent is ambiguous, and the same code path is reachable with a plain mariadb.Connection (e.g. in tests), where end() permanently closes the connection. The transaction failure path also removed the connection's error listener after releasing the connection and without a try/finally, so a throw during teardown would leak the listener onto the next caller.

Changes

• Introduce a small releaseConnection() helper that prefers PoolConnection#release() and falls back to Connection#end().
• Use it from MariaDbTransaction#commit and #rollback.
• In startTransaction's catch path, detach the error listener before releasing the connection and wrap the release in try/finally so the connection is always returned to the pool, even if cleanup throws.

Tests

Added unit tests in packages/adapter-mariadb/src/mariadb.test.ts:

• release() is used for pool connections; end() fallback for non-pool connections.
• The transaction error listener is detached on commit.
• The transaction error listener is detached and the connection is released on startTransaction failure.

All 26 adapter unit tests pass (pnpm --filter @prisma/adapter-mariadb test).

Out-of-band verification

I also ran an end-to-end harness with Docker (mariadb:10.11) covering:

1. connectionLimit=100, 100 concurrent transactions → 100 idle conns after release ✅
2. Two sequential rounds of 100 → pool reuses, stays at 100 ✅
3. 200 concurrent vs connectionLimit=100 → capped at 100, no leak ✅
4. idleTimeout=10s → pool drains to minimumIdle after the window ✅
5. After drain, a new burst grows the pool again ✅

Closes #28964

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: ede67556-70f8-4f48-bc6f-40d06381fa0f

📥 Commits

Reviewing files that changed from the base of the PR and between 42f9102 and 60c9938.

📒 Files selected for processing (2)

• packages/adapter-mariadb/src/mariadb.test.ts
• packages/adapter-mariadb/src/mariadb.ts

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Summary by CodeRabbit

• Bug Fixes
  • Fixed MariaDB transaction handling to properly manage database connections during commit and rollback operations
  • Improved connection reuse and stability in transaction scenarios, preventing connection leaks

Walkthrough

The MariaDB adapter now properly returns transaction connections to the connection pool instead of permanently closing them. A new releaseConnection() helper intelligently uses PoolConnection#release() when available and falls back to end() for non-pool connections. commit() and rollback() release connections via this helper, and error handling in startTransaction ensures listeners are cleaned up and connections are released even when BEGIN fails.

Changes

Transaction connection pool management

Layer / File(s)	Summary
Connection pool release helper packages/adapter-mariadb/src/mariadb.ts	New releaseConnection(conn) internal helper that calls PoolConnection#release() when available, otherwise falls back to conn.end() for non-pool connections.
Transaction commit and rollback pool release packages/adapter-mariadb/src/mariadb.ts	MariaDbTransaction.commit() and rollback() now call releaseConnection(this.client) in their finally blocks instead of calling end(), returning connections to the pool.
Transaction start error handling and listener cleanup packages/adapter-mariadb/src/mariadb.ts	startTransaction detaches the connection error listener with conn.off('error', onError) in the success path cleanup. When BEGIN fails, the error path detaches the listener and calls releaseConnection(conn) inside a try/finally, replacing the prior conn.end() + cleanup() flow.
Transaction pool behavior tests packages/adapter-mariadb/src/mariadb.test.ts	Comprehensive Vitest suite for PrismaMariaDbAdapter transaction behavior: mock connection helper supporting pool operations and listener tracking; tests verifying commit() and rollback() use release(), error listener cleanup after commit(), connection cleanup when BEGIN fails, and fallback to end() for non-pool connections.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main changes: fixing the adapter to properly release pooled connections and improve transaction cleanup error handling.
Description check	✅ Passed	The description is well-related to the changeset, explaining the root cause of issue #28964, the implemented fixes, added tests, and verification steps.
Linked Issues check	✅ Passed	The PR successfully addresses all coding requirements from issue #28964: introduces releaseConnection() helper to properly return connections to the pool, fixes the transaction failure cleanup path with try/finally, adds comprehensive unit tests, and includes out-of-band E2E verification.
Out of Scope Changes check	✅ Passed	All changes directly address issue #28964: the releaseConnection() helper, transaction commit/rollback modifications, startTransaction error handling, and new unit tests are all in-scope improvements to fix the connection pool management.

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[aqrln]

LGTM, thank you!