invalid relabel config: invalid relabel config: labelkeep action requires only 'regex', and no other fields

[aglees]

https://github.com/grafana/loki/blob/42eed6ded5e96edde6831f3fc3a010efd9904e85/tools/lambda-promtail/lambda-promtail/relabel.go#L75-L88

I think there's an issue with how Lambda Promtail is setting up Relabelling configurations before they're validated, and it's resulting in the following errors:

invalid relabel config: invalid relabel config: labelkeep action requires only 'regex', and no other fields

A basic relabelling configuration that produces an error is:

[
    {
        "regex": "(.*)",
        "action": "labelkeep"
    }
]

The issues appears to stem from how SourceLabels are being setup for actions that cannot have source_labels set, such as labelkeep & labeldrop:

	sourceLabels := make(model.LabelNames, 0, len(rc.SourceLabels))
	for _, l := range rc.SourceLabels {
		sourceLabels = append(sourceLabels, model.LabelName(l))
	}

	cfg := &relabel.Config{
		SourceLabels: sourceLabels,
		Separator:    separator,
		Regex:        regex,
		Modulus:      rc.Modulus,
		TargetLabel:  rc.TargetLabel,
		Replacement:  replacement,
		Action:       action,
	}

The issue arises when this go is hit in the Validate function, specifically the c.SourceLabels != nil conditional, that evaluates to true because SourceLabels are being set:

	if c.Action == LabelDrop || c.Action == LabelKeep {
		if c.SourceLabels != nil ||
			c.TargetLabel != DefaultRelabelConfig.TargetLabel ||
			c.Modulus != DefaultRelabelConfig.Modulus ||
			c.Separator != DefaultRelabelConfig.Separator ||
			c.Replacement != DefaultRelabelConfig.Replacement {
			return fmt.Errorf("%s action requires only 'regex', and no other fields", c.Action)
		}
	}

[tlipatov-asapp]

Same issue, this is blocking us from using lambda promtail with grafana cloud

[eserte]

This can be easily reproduced by adding the relabeling snippet from the documentation to relabel_test.go:

		{
			name: "labeldrop",
			input: `[{
				"regex": "__aws_.*",
				"action": "labeldrop"
			}]`,
			wantErr: false,
		},

--- FAIL: TestParseRelabelConfigs (0.00s)
    --- FAIL: TestParseRelabelConfigs/labeldrop (0.00s)
        relabel_test.go:111: 
                Error Trace:    /home/user/work/loki/tools/lambda-promtail/lambda-promtail/relabel_test.go:111
                Error:          Received unexpected error:
                                failed to parse RELABEL_CONFIGS: invalid relabel config: invalid relabel config: labeldrop action requires only 'regex', and no other fields
                Test:           TestParseRelabelConfigs/labeldrop

[dmitrii-bdv]

A possible solution might be:

func (rc *RelabelConfig) ToPrometheusConfig() (*relabel.Config, error) {
    var regex relabel.Regexp
    if rc.Regex != "" {
        var err error
        regex, err = relabel.NewRegexp(rc.Regex)
        if err != nil {
            return nil, fmt.Errorf("invalid regex %q: %w", rc.Regex, err)
        }
    } else {
        regex = relabel.DefaultRelabelConfig.Regex
    }

    action := relabel.Action(rc.Action)
    if rc.Action == "" {
        action = relabel.DefaultRelabelConfig.Action
    }

    // Only set allowed fields for labeldrop/labelkeep
    switch action {
    case relabel.LabelDrop, relabel.LabelKeep:
        cfg := &relabel.Config{
            Action: action,
            Regex:  regex,
        }
        if err := cfg.Validate(); err != nil {
            return nil, fmt.Errorf("invalid relabel config: %w", err)
        }
        return cfg, nil
    default:
        separator := rc.Separator
        if separator == "" {
            separator = relabel.DefaultRelabelConfig.Separator
        }

        replacement := rc.Replacement
        if replacement == "" {
            replacement = relabel.DefaultRelabelConfig.Replacement
        }

        sourceLabels := make(model.LabelNames, 0, len(rc.SourceLabels))
        for _, l := range rc.SourceLabels {
            sourceLabels = append(sourceLabels, model.LabelName(l))
        }

        cfg := &relabel.Config{
            SourceLabels: sourceLabels,
            Separator:    separator,
            Regex:        regex,
            Modulus:      rc.Modulus,
            TargetLabel:  rc.TargetLabel,
            Replacement:  replacement,
            Action:       action,
        }
        if err := cfg.Validate(); err != nil {
            return nil, fmt.Errorf("invalid relabel config: %w", err)
        }
        return cfg, nil
    }
}

[jeschkies]

@dmitrii-bdv we are happy to accept pull requests.