Merge pull request #460 from prometheus/beorn7/desc

Detect invalid Descs in const metrics, but also allow empty Help strings.

prometheus/desc.go

@@ -67,7 +67,7 @@ type Desc struct {
 
 // NewDesc allocates and initializes a new Desc. Errors are recorded in the Desc
 // and will be reported on registration time. variableLabels and constLabels can
-// be nil if no such labels should be set. fqName and help must not be empty.
+// be nil if no such labels should be set. fqName must not be empty.
 //
 // variableLabels only contain the label names. Their label values are variable
 // and therefore not part of the Desc. (They are managed within the Metric.)
@@ -80,10 +80,6 @@ func NewDesc(fqName, help string, variableLabels []string, constLabels Labels) *
 		help:           help,
 		variableLabels: variableLabels,
 	}
-	if help == "" {
-		d.err = errors.New("empty help string")
-		return d
-	}
 	if !model.IsValidMetricName(model.LabelValue(fqName)) {
 		d.err = fmt.Errorf("%q is not a valid metric name", fqName)
 		return d

prometheus/examples_test.go

@@ -89,37 +89,6 @@ func ExampleGaugeFunc() {
 	// GaugeFunc 'goroutines_count' registered.
 }
 
-func ExampleCounter() {
-	pushCounter := prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "repository_pushes", // Note: No help string...
-	})
-	err := prometheus.Register(pushCounter) // ... so this will return an error.
-	if err != nil {
-		fmt.Println("Push counter couldn't be registered, no counting will happen:", err)
-		return
-	}
-
-	// Try it once more, this time with a help string.
-	pushCounter = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "repository_pushes",
-		Help: "Number of pushes to external repository.",
-	})
-	err = prometheus.Register(pushCounter)
-	if err != nil {
-		fmt.Println("Push counter couldn't be registered AGAIN, no counting will happen:", err)
-		return
-	}
-
-	pushComplete := make(chan struct{})
-	// TODO: Start a goroutine that performs repository pushes and reports
-	// each completion via the channel.
-	for range pushComplete {
-		pushCounter.Inc()
-	}
-	// Output:
-	// Push counter couldn't be registered, no counting will happen: descriptor Desc{fqName: "repository_pushes", help: "", constLabels: {}, variableLabels: []} is invalid: empty help string
-}
-
 func ExampleCounterVec() {
 	httpReqs := prometheus.NewCounterVec(
 		prometheus.CounterOpts{

prometheus/histogram.go

@@ -110,8 +110,9 @@ func ExponentialBuckets(start, factor float64, count int) []float64 {
 }
 
 // HistogramOpts bundles the options for creating a Histogram metric. It is
-// mandatory to set Name and Help to a non-empty string. All other fields are
+// mandatory to set Name to a non-empty string. All other fields are optional
-// optional and can safely be left at their zero value.
+// and can safely be left at their zero value, although it is strongly
+// encouraged to set a Help string.
 type HistogramOpts struct {
 	// Namespace, Subsystem, and Name are components of the fully-qualified
 	// name of the Histogram (created by joining these components with
@@ -122,7 +123,7 @@ type HistogramOpts struct {
 	Subsystem string
 	Name      string
 
-	// Help provides information about this Histogram. Mandatory!
+	// Help provides information about this Histogram.
 	//
 	// Metrics with the same fully-qualified name must have the same Help
 	// string.
@@ -554,7 +555,7 @@ func (h *constHistogram) Write(out *dto.Metric) error {
 // bucket.
 //
 // NewConstHistogram returns an error if the length of labelValues is not
-// consistent with the variable labels in Desc.
+// consistent with the variable labels in Desc or if Desc is invalid.
 func NewConstHistogram(
 	desc *Desc,
 	count uint64,
@@ -562,6 +563,9 @@ func NewConstHistogram(
 	buckets map[float64]uint64,
 	labelValues ...string,
 ) (Metric, error) {
+	if desc.err != nil {
+		return nil, desc.err
+	}
 	if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
 		return nil, err
 	}

prometheus/metric.go

@@ -59,8 +59,9 @@ type Metric interface {
 // implementation XXX has its own XXXOpts type, but in most cases, it is just be
 // an alias of this type (which might change when the requirement arises.)
 //
-// It is mandatory to set Name and Help to a non-empty string. All other fields
+// It is mandatory to set Name to a non-empty string. All other fields are
-// are optional and can safely be left at their zero value.
+// optional and can safely be left at their zero value, although it is strongly
+// encouraged to set a Help string.
 type Opts struct {
 	// Namespace, Subsystem, and Name are components of the fully-qualified
 	// name of the Metric (created by joining these components with
@@ -71,7 +72,7 @@ type Opts struct {
 	Subsystem string
 	Name      string
 
-	// Help provides information about this metric. Mandatory!
+	// Help provides information about this metric.
 	//
 	// Metrics with the same fully-qualified name must have the same Help
 	// string.

prometheus/summary.go

@@ -81,10 +81,10 @@ const (
 )
 
 // SummaryOpts bundles the options for creating a Summary metric. It is
-// mandatory to set Name and Help to a non-empty string. While all other fields
+// mandatory to set Name to a non-empty string. While all other fields are
-// are optional and can safely be left at their zero value, it is recommended to
+// optional and can safely be left at their zero value, it is recommended to set
-// explicitly set the Objectives field to the desired value as the default value
+// a help string and to explicitly set the Objectives field to the desired value
-// will change in the upcoming v0.10 of the library.
+// as the default value will change in the upcoming v0.10 of the library.
 type SummaryOpts struct {
 	// Namespace, Subsystem, and Name are components of the fully-qualified
 	// name of the Summary (created by joining these components with
@@ -95,7 +95,7 @@ type SummaryOpts struct {
 	Subsystem string
 	Name      string
 
-	// Help provides information about this Summary. Mandatory!
+	// Help provides information about this Summary.
 	//
 	// Metrics with the same fully-qualified name must have the same Help
 	// string.
@@ -586,7 +586,7 @@ func (s *constSummary) Write(out *dto.Metric) error {
 //     map[float64]float64{0.5: 0.23, 0.99: 0.56}
 //
 // NewConstSummary returns an error if the length of labelValues is not
-// consistent with the variable labels in Desc.
+// consistent with the variable labels in Desc or if Desc is invalid.
 func NewConstSummary(
 	desc *Desc,
 	count uint64,
@@ -594,6 +594,9 @@ func NewConstSummary(
 	quantiles map[float64]float64,
 	labelValues ...string,
 ) (Metric, error) {
+	if desc.err != nil {
+		return nil, desc.err
+	}
 	if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
 		return nil, err
 	}

prometheus/value.go

@@ -77,8 +77,12 @@ func (v *valueFunc) Write(out *dto.Metric) error {
 // operations. However, when implementing custom Collectors, it is useful as a
 // throw-away metric that is generated on the fly to send it to Prometheus in
 // the Collect method. NewConstMetric returns an error if the length of
-// labelValues is not consistent with the variable labels in Desc.
+// labelValues is not consistent with the variable labels in Desc or if Desc is
+// invalid.
 func NewConstMetric(desc *Desc, valueType ValueType, value float64, labelValues ...string) (Metric, error) {
+	if desc.err != nil {
+		return nil, desc.err
+	}
 	if err := validateLabelValues(labelValues, len(desc.variableLabels)); err != nil {
 		return nil, err
 	}