prometheus.InvalidDesc.String() is formatted incorrectly

[SealOfTime]

On v1.20.5, if String() method of a prometheus.Desc returned from NewInvalidDesc called, it will cause a nil pointer derefence, which will result in an incorrectly formatted string

type Desc struct {
	//...
	variableLabels *compiledLabels
        //...
}

func NewInvalidDesc(err error) *Desc {
	return &Desc{
		err: err,
	}
}

func (d *Desc) String() string {
	//...
	vlStrings := make([]string, 0, len(d.variableLabels.names))
	for _, vl := range d.variableLabels.names {
        //...
}

NewInvalidDesc will create a Desc with variableLabels equal to nil, therefore accessing its field names will cause a nil-pointer dereference.

The result is: descriptor %!s(PANIC=String method: runtime error: invalid memory address or nil pointer dereference) is invalid: test

The consequence of that is an unpleasant error message in prometheus.Register

[dosubot]

Hey there, @SealOfTime! I'm here to assist you with any bugs, questions, or contributions while we wait for a human maintainer. Let's squash those bugs together! 🤖

To prevent a nil pointer dereference in the String() method of a prometheus.Desc when NewInvalidDesc creates a Desc with variableLabels set to nil, you can add a nil check for d.variableLabels before accessing its fields. Here's how you can modify the String() method:

func (d *Desc) String() string {
	lpStrings := make([]string, 0, len(d.constLabelPairs))
	for _, lp := range d.constLabelPairs {
		lpStrings = append(
			lpStrings,
			fmt.Sprintf("%s=%q", lp.GetName(), lp.GetValue()),
		)
	}
	
	vlStrings := []string{}
	if d.variableLabels != nil {
		vlStrings = make([]string, 0, len(d.variableLabels.names))
		for _, vl := range d.variableLabels.names {
			if fn, ok := d.variableLabels.labelConstraints[vl]; ok && fn != nil {
				vlStrings = append(vlStrings, fmt.Sprintf("c(%s)", vl))
			} else {
				vlStrings = append(vlStrings, vl)
			}
		}
	}
	
	return fmt.Sprintf(
		"Desc{fqName: %q, help: %q, constLabels: {%s}, variableLabels: {%s}}",
		d.fqName,
		d.help,
		strings.Join(lpStrings, ","),
		strings.Join(vlStrings, ","),
	)
}

This modification ensures that the code checks if d.variableLabels is nil before attempting to access its names field, thus preventing a nil pointer dereference ^[1].

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