pkg/agentctl: FindCardinality, FindSamples do not invoke WAL.Close() hence leaks resources

[odeke-em]

There is a missing defer WAL.Close() in FindCardinality and that’ll look like substantial resources will be leaked per

agent/pkg/agentctl/cardinality.go

Lines 18 to 38 in e83883d

	func FindCardinality(walDir string, job string, instance string) ([]Cardinality, error) {
	w, err := wal.Open(nil, walDir)
	if err != nil {
	return nil, err
	}
	cardinality := map[string]int{}
	err = walIterate(w, func(r *wal.Reader) error {
	return collectCardinality(r, job, instance, cardinality)
	})
	if err != nil {
	return nil, err
	}
	res := make([]Cardinality, 0, len(cardinality))
	for k, v := range cardinality {
	res = append(res, Cardinality{Metric: k, Instances: v})
	}
	return res, nil
	}

ditto for

agent/pkg/agentctl/samples.go

Lines 27 to 35 in e83883d

	func FindSamples(walDir string, selectorStr string) ([]*SampleStats, error) {
	w, err := wal.Open(nil, walDir)
	if err != nil {
	return nil, err
	}
	selector, err := parser.ParseMetricSelector(selectorStr)
	if err != nil {
	return nil, err

given the code in (*WAL).Close

[rfratto]

Thanks for reporting! It's not clear to me how much this actually leaks since they're not being written to, but they should definitely be closed.

[odeke-em]

Thank you for the fix and quick turnaround, @rfratto.