fix: Cleanup ResourcePool & Include Mutex Lock for refreshPool

[rMaxiQp]

Description

This PR cleans up the resource leakage from TestReopen and TestShrink, which causes flaky tests across ./go/pools/resource_pool_test.go. During my local run, TestIdleTimeout ended up failing with

    resource_pool_test.go:407: 
                Error Trace:    /home/zq2/vitess/go/pools/resource_pool_test.go:407
                Error:          Not equal: 
                                expected: int(2)
                                actual  : int64(6)
                Test:           TestIdleTimeout

Properly closing the resource pool helped to resolve the first issue but surfaces data race from refreshPool:

WARNING: DATA RACE
Write at 0x00c0000b9e68 by goroutine 38:
  runtime.racewrite()
      <autogenerated>:1 +0x1e
  vitess.io/vitess/go/pools.(*poolRefresh).stop()
      /home/zq2/vitess/go/pools/refresh_pool.go:98 +0xc6
  vitess.io/vitess/go/pools.(*ResourcePool).Close()
      /home/zq2/vitess/go/pools/resource_pool.go:147 +0x7e
  vitess.io/vitess/go/pools.TestReopen()
      /home/zq2/vitess/go/pools/resource_pool_test.go:369 +0x30b
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1934 +0x21c
  testing.(*T).Run.gowrap1()
      /usr/local/go/src/testing/testing.go:1997 +0x44

Previous read at 0x00c0000b9e68 by goroutine 42:
  runtime.raceread()
      <autogenerated>:1 +0x1e
  vitess.io/vitess/go/pools.(*poolRefresh).startRefreshTicker()
      /home/zq2/vitess/go/pools/refresh_pool.go:71 +0xf9
  vitess.io/vitess/go/pools.(*ResourcePool).reopen()
      /home/zq2/vitess/go/pools/resource_pool.go:185 +0x258
  vitess.io/vitess/go/pools.(*poolRefresh).startRefreshTicker.func1.gowrap2()
      /home/zq2/vitess/go/pools/refresh_pool.go:82 +0x42
...

Thus, a mutex lock for refreshPool.go is included here as well.

Related Issue(s)

related to #18965

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

This PR was written with the help of Claude Sonnet 4.5

[vitess-bot]

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[Copilot]

Pull request overview

This PR addresses resource leakage and data race issues in the resource pool tests and implementation. The primary goal is to fix flaky tests in resource_pool_test.go by ensuring proper cleanup of resource pools and adding thread-safe access to the refresh ticker mechanism.

Key Changes

• Added proper cleanup (Close() calls) to TestShrinking and TestReopen to prevent resource leaks between tests
• Introduced mutex synchronization (sync.Mutex) in poolRefresh to fix data races when accessing refreshTicker and related fields
• Removed unused resetCount atomic variable from test file

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
go/pools/resource_pool_test.go	Added defer p.Close() in TestShrinking and p.Close() in TestReopen to prevent resource leaks; removed unused resetCount variable
go/pools/refresh_pool.go	Added mutex to protect concurrent access to refreshTicker, refreshStop, and goroutine lifecycle in startRefreshTicker() and stop() methods

---

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[Copilot]

The startRefreshTicker() method doesn't check if a ticker is already running before creating a new one. This can lead to resource leaks (goroutines and tickers not being cleaned up) if startRefreshTicker() is called multiple times. Consider adding a check after acquiring the lock:

pr.mu.Lock()
defer pr.mu.Unlock()

if pr.refreshTicker != nil {
    return // ticker already running
}

pr.refreshTicker = time.NewTicker(pr.refreshInterval)
...

Suggested change

	if pr.refreshTicker != nil {
	return // ticker already running
	}