[release-19.0] Fix a potential connection pool leak. (vitessio#17807) (…

…vitessio#17812)

Signed-off-by: Arthur Schreiber <arthurschreiber@github.com>
Co-authored-by: vitess-bot[bot] <108069721+vitess-bot[bot]@users.noreply.github.com>

go/pools/smartconnpool/stack.go

@@ -48,6 +48,7 @@ func (s *connStack[C]) Pop() (*Pooled[C], bool) {
 
 		newHead := oldHead.next.Load()
 		if s.top.CompareAndSwap(oldHead, popCount, newHead, popCount+1) {
+			oldHead.next.Store(nil)
 			return oldHead, true
 		}
 		runtime.Gosched()

go/pools/smartconnpool/stack_test.go

@@ -0,0 +1,26 @@
+package smartconnpool
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type testPooled struct {
+	Connection
+}
+
+func TestStackPop(t *testing.T) {
+	s := &connStack[testPooled]{}
+
+	first := &Pooled[testPooled]{}
+	s.Push(first)
+
+	second := &Pooled[testPooled]{}
+	s.Push(second)
+
+	c, ok := s.Pop()
+	assert.True(t, ok)
+
+	assert.Nil(t, c.next.Load())
+}