Fix the bug in unit tests about sync.Pool

[SataQiu]

Ⅰ. Describe what this PR did

Callers should not assume any relation between values passed to Put and the values returned by Get.

Ref: https://github.com/golang/go/blob/master/src/sync/pool.go#L116-L151

// Get selects an arbitrary item from the Pool, removes it from the
// Pool, and returns it to the caller.
// Get may choose to ignore the pool and treat it as empty.
// Callers should not assume any relation between values passed to Put and
// the values returned by Get.
//
// If Get would otherwise return nil and p.New is non-nil, Get returns
// the result of calling p.New.
func (p *Pool) Get() interface{} {

Ref: https://github.com/golang/go/blob/master/src/sync/pool_test.go#L28-L42

	// Make sure that the goroutine doesn't migrate to another P
	// between Put and Get calls.
	Runtime_procPin()
	p.Put("a")
	p.Put("b")
	if g := p.Get(); g != "a" {
		t.Fatalf("got %#v; want a", g)
	}
	if g := p.Get(); g != "b" {
		t.Fatalf("got %#v; want b", g)
	}
	if g := p.Get(); g != nil {
		t.Fatalf("got %#v; want nil", g)
	}
	Runtime_procUnpin()

It seems that we should pin the P when do the unit test :)

Ⅱ. Does this pull request fix one issue?

Fixes: #1416

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

make unit-test

Ⅴ. Special notes for reviews

[YesterdayxD]

#1418 @SataQiu

[lowzj]

It's caused by the race detecting of go test -race. It has 1/4 possibility to drop the value in sync.Pool.Put function.

[SataQiu]

Ref: https://github.com/golang/go/blob/master/src/sync/pool_test.go#L5-L6

[lowzj]

It's a workaround solution, but it will not run unit-testing of this file in current CI flow.

[SataQiu]

Maybe we can do something in hack/unit-test.sh, such as adding -race dynamically for go test :)

Dragonfly/hack/unit-test.sh

Lines 25 to 28 in 12cfd62

	for d in $(eval "${cmd}")
	do
	go test -race -coverprofile=profile.out -covermode=atomic "${d}"
	if [ "$?" != "0" ]; then

[SataQiu]

Ref: https://github.com/golang/go/blob/master/src/sync/pool_test.go#L28-L30

[lowzj]

LGTM

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I will merge this to make the CI pass.
If there is a better solution, please submit another pull request.