util.RequestBuffers: Fix test flakiness

[aknuds1]

What this PR does

util.TestRequestBuffers is flaky due to sync.Pool not being 100% deterministic, see example CI failure.

This PR abstracts sync.Pool through an interface, which gets faked in tests for deterministic behaviour.

Which issue(s) this PR fixes or relates to

Checklist

• Tests updated.
• [na] Documentation added.
• [na] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [na] about-versioning.md updated with experimental features.

[dimitarvdimitrov]

the failure seems to be here

34:	assert.Same(t, b1, b)

why do we need to assert that the returned slice is the same as the previous one? Is this part of the API of *RequestBuffers?

[aknuds1]

@dimitarvdimitrov it's just to make sure it uses the pool.

[dimitarvdimitrov]

thanks for fixing it

[dimitarvdimitrov]

this looks like an existing interface, which made me think this is probably a good fit for that pool package, but that's a different topic

mimir/pkg/util/pool/pool.go

Lines 12 to 18 in 942e16c

	// Interface defines the same functions of sync.Pool.
	type Interface interface {
	// Put is sync.Pool.Put().
	Put(x any)
	// Get is sync.Pool.Get().
	Get() any
	}

[aknuds1]

Aha, thanks @dimitarvdimitrov - I was not aware of this package.