Merge pull request #45 from prometheus/beorn7/allow-collect-errors

Allow error reporting during metrics collection and simplify Register().

prometheus/collector.go

@@ -33,7 +33,9 @@ type Collector interface {
 	// duplicate descriptors. Those duplicates are simply ignored. However,
 	// two different Collectors must not send duplicate descriptors.) This
 	// method idempotently sends the same descriptors throughout the
-	// lifetime of the Collector.
+	// lifetime of the Collector. If a Collector encounters an error while
+	// executing this method, it must send an invalid descriptor (created
+	// with NewInvalidDesc) to signal the error to the registry.
 	Describe(chan<- *Desc)
 	// Collect is called by Prometheus when collecting metrics. The
 	// implementation sends each collected metric via the provided channel

prometheus/desc.go

@@ -166,6 +166,16 @@ func NewDesc(fqName, help string, variableLabels []string, constLabels Labels) *
 	return d
 }
 
+// NewInvalidDesc returns an invalid descriptor, i.e. a descriptor with the
+// provided error set. If a collector returning such a descriptor is registered,
+// registration will fail with the provided error. NewInvalidDesc can be used by
+// a Collector to signal inability to describe itself.
+func NewInvalidDesc(err error) *Desc {
+	return &Desc{
+		err: err,
+	}
+}
+
 func (d *Desc) String() string {
 	lpStrings := make([]string, 0, len(d.constLabelPairs))
 	for _, lp := range d.constLabelPairs {

prometheus/example_selfcollector_test.go

@@ -49,8 +49,9 @@ func (cm *CallbackMetric) Desc() *prometheus.Desc {
 	return cm.desc
 }
 
-func (cm *CallbackMetric) Write(m *dto.Metric) {
+func (cm *CallbackMetric) Write(m *dto.Metric) error {
 	m.Untyped = &dto.Untyped{Value: proto.Float64(cm.callback())}
+	return nil
 }
 
 func ExampleSelfCollector() {

prometheus/examples_test.go

@@ -74,7 +74,7 @@ func ExampleGaugeVec() {
 }
 
 func ExampleGaugeFunc() {
-	if _, err := prometheus.Register(prometheus.NewGaugeFunc(
+	if err := prometheus.Register(prometheus.NewGaugeFunc(
 		prometheus.GaugeOpts{
 			Subsystem: "runtime",
 			Name:      "goroutines_count",
@@ -95,7 +95,7 @@ func ExampleCounter() {
 	pushCounter := prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "repository_pushes", // Note: No help string...
 	})
-	_, err := prometheus.Register(pushCounter) // ... so this will return an error.
+	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
@@ -106,7 +106,7 @@ func ExampleCounter() {
 		Name: "repository_pushes",
 		Help: "Number of pushes to external repository.",
 	})
-	_, err = prometheus.Register(pushCounter)
+	err = prometheus.Register(pushCounter)
 	if err != nil {
 		fmt.Println("Push counter couldn't be registered AGAIN, no counting will happen:", err)
 		return
@@ -194,7 +194,7 @@ func ExampleRegister() {
 		Help:      "Total number of tasks completed.",
 	})
 	// This will register fine.
-	if _, err := prometheus.Register(taskCounter); err != nil {
+	if err := prometheus.Register(taskCounter); err != nil {
 		fmt.Println(err)
 	} else {
 		fmt.Println("taskCounter registered.")
@@ -219,7 +219,7 @@ func ExampleRegister() {
 	)
 
 	// Registering will fail because we already have a metric of that name.
-	if _, err := prometheus.Register(taskCounterVec); err != nil {
+	if err := prometheus.Register(taskCounterVec); err != nil {
 		fmt.Println("taskCounterVec not registered:", err)
 	} else {
 		fmt.Println("taskCounterVec registered.")
@@ -231,7 +231,7 @@ func ExampleRegister() {
 	}
 
 	// Try registering taskCounterVec again.
-	if _, err := prometheus.Register(taskCounterVec); err != nil {
+	if err := prometheus.Register(taskCounterVec); err != nil {
 		fmt.Println("taskCounterVec not registered:", err)
 	} else {
 		fmt.Println("taskCounterVec registered.")
@@ -253,7 +253,7 @@ func ExampleRegister() {
 		},
 		[]string{"worker_id"},
 	)
-	if _, err := prometheus.Register(taskCounterVec); err != nil {
+	if err := prometheus.Register(taskCounterVec); err != nil {
 		fmt.Println("taskCounterVec not registered:", err)
 	} else {
 		fmt.Println("taskCounterVec registered.")
@@ -299,7 +299,7 @@ func ExampleRegister() {
 		ConstLabels: prometheus.Labels{"worker_id": "42"},
 	}
 	taskCounterForWorker42 := prometheus.NewCounter(counterOpts)
-	if _, err := prometheus.Register(taskCounterForWorker42); err != nil {
+	if err := prometheus.Register(taskCounterForWorker42); err != nil {
 		fmt.Println("taskCounterVForWorker42 not registered:", err)
 	} else {
 		fmt.Println("taskCounterForWorker42 registered.")
@@ -313,7 +313,7 @@ func ExampleRegister() {
 	// counterOpts. Just change the ConstLabels.
 	counterOpts.ConstLabels = prometheus.Labels{"worker_id": "2001"}
 	taskCounterForWorker2001 := prometheus.NewCounter(counterOpts)
-	if _, err := prometheus.Register(taskCounterForWorker2001); err != nil {
+	if err := prometheus.Register(taskCounterForWorker2001); err != nil {
 		fmt.Println("taskCounterVForWorker2001 not registered:", err)
 	} else {
 		fmt.Println("taskCounterForWorker2001 registered.")

prometheus/expvar.go

@@ -82,7 +82,10 @@ func (e *ExpvarCollector) Collect(ch chan<- Metric) {
 		}
 		var v interface{}
 		labels := make([]string, len(desc.variableLabels))
-		if err := json.Unmarshal([]byte(expVar.String()), &v); err == nil {
+		if err := json.Unmarshal([]byte(expVar.String()), &v); err != nil {
+			ch <- NewInvalidMetric(desc, err)
+			continue
+		}
 		var processValue func(v interface{}, i int)
 		processValue = func(v interface{}, i int) {
 			if i >= len(labels) {
@@ -114,4 +117,3 @@ func (e *ExpvarCollector) Collect(ch chan<- Metric) {
 		processValue(v, 0)
 	}
 }
-}

prometheus/go_collector_test.go

@@ -31,7 +31,7 @@ func TestGoCollector(t *testing.T) {
 		select {
 		case metric := <-ch:
 			switch m := metric.(type) {
-			// Attention, this als catches Counter ...
+			// Attention, this also catches Counter...
 			case Gauge:
 				pb := &dto.Metric{}
 				m.Write(pb)
@@ -43,7 +43,7 @@ func TestGoCollector(t *testing.T) {
 				}
 
 				if diff := int(pb.GetGauge().GetValue()) - old; diff != 1 {
-					t.Errorf("want 1 new goroutine, got %f", diff)
+					t.Errorf("want 1 new goroutine, got %d", diff)
 				}
 
 				return

prometheus/metric.go

@@ -27,7 +27,9 @@ import (
 type Metric interface {
 	// Desc returns the descriptor for the Metric. This method idempotently
 	// returns the same descriptor throughout the lifetime of the
-	// Metric. The returned descriptor is immutable by contract.
+	// Metric. The returned descriptor is immutable by contract. A Metric
+	// unable to describe itself must return an invalid descriptor (created
+	// with NewInvalidDesc).
 	Desc() *Desc
 	// Write encodes the Metric into a "Metric" Protocol Buffer data
 	// transmission object.
@@ -46,7 +48,7 @@ type Metric interface {
 	//
 	// While populating dto.Metric, labels must be sorted lexicographically.
 	// (Implementers may find LabelPairSorter useful for that.)
-	Write(*dto.Metric)
+	Write(*dto.Metric) error
 }
 
 // Opts bundles the options for creating most Metric types. Each metric
@@ -144,3 +146,19 @@ func (s hashSorter) Swap(i, j int) {
 func (s hashSorter) Less(i, j int) bool {
 	return s[i] < s[j]
 }
+
+type invalidMetric struct {
+	desc *Desc
+	err  error
+}
+
+// NewInvalidMetric returns a metric whose Write method always returns the
+// provided error. It is useful if a Collector finds itself unable to collect
+// a metric and wishes to report an error to the registry.
+func NewInvalidMetric(desc *Desc, err error) Metric {
+	return &invalidMetric{desc, err}
+}
+
+func (m *invalidMetric) Desc() *Desc { return m.desc }
+
+func (m *invalidMetric) Write(*dto.Metric) error { return m.err }

prometheus/process_collector.go

@@ -92,10 +92,23 @@ func noopCollect(ch chan<- Metric) {}
 func (c *processCollector) procfsCollect(ch chan<- Metric) {
 	p, err := procfs.NewProc(c.pidFn())
 	if err != nil {
+		// Report collect errors for all metrics.
+		ch <- NewInvalidMetric(c.cpuTotal.Desc(), err)
+		ch <- NewInvalidMetric(c.openFDs.Desc(), err)
+		ch <- NewInvalidMetric(c.maxFDs.Desc(), err)
+		ch <- NewInvalidMetric(c.vsize.Desc(), err)
+		ch <- NewInvalidMetric(c.rss.Desc(), err)
+		ch <- NewInvalidMetric(c.startTime.Desc(), err)
 		return
 	}
 
-	if stat, err := p.NewStat(); err == nil {
+	if stat, err := p.NewStat(); err != nil {
+		// Report collect errors for metrics depending on stat.
+		ch <- NewInvalidMetric(c.vsize.Desc(), err)
+		ch <- NewInvalidMetric(c.rss.Desc(), err)
+		ch <- NewInvalidMetric(c.startTime.Desc(), err)
+		ch <- NewInvalidMetric(c.cpuTotal.Desc(), err)
+	} else {
 		c.cpuTotal.Set(stat.CPUTime())
 		ch <- c.cpuTotal
 		c.vsize.Set(float64(stat.VirtualMemory()))
@@ -103,18 +116,24 @@ func (c *processCollector) procfsCollect(ch chan<- Metric) {
 		c.rss.Set(float64(stat.ResidentMemory()))
 		ch <- c.rss
 
-		if startTime, err := stat.StartTime(); err == nil {
+		if startTime, err := stat.StartTime(); err != nil {
+			ch <- NewInvalidMetric(c.startTime.Desc(), err)
+		} else {
 			c.startTime.Set(startTime)
 			ch <- c.startTime
 		}
 	}
 
-	if fds, err := p.FileDescriptorsLen(); err == nil {
+	if fds, err := p.FileDescriptorsLen(); err != nil {
+		ch <- NewInvalidMetric(c.openFDs.Desc(), err)
+	} else {
 		c.openFDs.Set(float64(fds))
 		ch <- c.openFDs
 	}
 
-	if limits, err := p.NewLimits(); err == nil {
+	if limits, err := p.NewLimits(); err != nil {
+		ch <- NewInvalidMetric(c.maxFDs.Desc(), err)
+	} else {
 		c.maxFDs.Set(float64(limits.OpenFiles))
 		ch <- c.maxFDs
 	}

prometheus/registry.go

@@ -105,24 +105,23 @@ func UninstrumentedHandler() http.Handler {
 // returns an error if the descriptors provided by the Collector are invalid or
 // if they - in combination with descriptors of already registered Collectors -
 // do not fulfill the consistency and uniqueness criteria described in the Desc
-// documentation. If the registration is successful, the registered Collector
-// is returned.
+// documentation.
 //
 // Do not register the same Collector multiple times concurrently. (Registering
 // the same Collector twice would result in an error anyway, but on top of that,
 // it is not safe to do so concurrently.)
-func Register(m Collector) (Collector, error) {
-	return defRegistry.Register(m)
+func Register(m Collector) error {
+	_, err := defRegistry.Register(m)
+	return err
 }
 
 // MustRegister works like Register but panics where Register would have
 // returned an error.
-func MustRegister(m Collector) Collector {
-	m, err := Register(m)
+func MustRegister(m Collector) {
+	err := Register(m)
 	if err != nil {
 		panic(err)
 	}
-	return m
 }
 
 // RegisterOrGet works like Register but does not return an error if a Collector
@@ -447,7 +446,12 @@ func (r *registry) writePB(w io.Writer, writeEncoded encoder) (int, error) {
 		}
 		dtoMetric := r.getMetric()
 		defer r.giveMetric(dtoMetric)
-		metric.Write(dtoMetric)
+		if err := metric.Write(dtoMetric); err != nil {
+			// TODO: Consider different means of error reporting so
+			// that a single erroneous metric could be skipped
+			// instead of blowing up the whole collection.
+			return 0, fmt.Errorf("error collecting metric %v: %s", desc, err)
+		}
 		switch {
 		case metricFamily.Type != nil:
 			// Type already set. We are good.

prometheus/summary.go

@@ -242,7 +242,7 @@ func (s *summary) Observe(v float64) {
 	}
 }
 
-func (s *summary) Write(out *dto.Metric) {
+func (s *summary) Write(out *dto.Metric) error {
 	sum := &dto.Summary{}
 	qs := make([]*dto.Quantile, 0, len(s.objectives))
 
@@ -277,6 +277,7 @@ func (s *summary) Write(out *dto.Metric) {
 
 	out.Summary = sum
 	out.Label = s.labelPairs
+	return nil
 }
 
 func (s *summary) newStream() *quantile.Stream {

prometheus/value.go

@@ -98,12 +98,12 @@ func (v *value) Sub(val float64) {
 	v.Add(val * -1)
 }
 
-func (v *value) Write(out *dto.Metric) {
+func (v *value) Write(out *dto.Metric) error {
 	v.mtx.RLock()
 	val := v.val
 	v.mtx.RUnlock()
 
-	populateMetric(v.valType, val, v.labelPairs, out)
+	return populateMetric(v.valType, val, v.labelPairs, out)
 }
 
 // valueFunc is a generic metric for simple values retrieved on collect time
@@ -141,8 +141,8 @@ func (v *valueFunc) Desc() *Desc {
 	return v.desc
 }
 
-func (v *valueFunc) Write(out *dto.Metric) {
-	populateMetric(v.valType, v.function(), v.labelPairs, out)
+func (v *valueFunc) Write(out *dto.Metric) error {
+	return populateMetric(v.valType, v.function(), v.labelPairs, out)
 }
 
 // NewConstMetric returns a metric with one fixed value that cannot be
@@ -184,8 +184,8 @@ func (m *constMetric) Desc() *Desc {
 	return m.desc
 }
 
-func (m *constMetric) Write(out *dto.Metric) {
-	populateMetric(m.valType, m.val, m.labelPairs, out)
+func (m *constMetric) Write(out *dto.Metric) error {
+	return populateMetric(m.valType, m.val, m.labelPairs, out)
 }
 
 func populateMetric(
@@ -193,7 +193,7 @@ func populateMetric(
 	v float64,
 	labelPairs []*dto.LabelPair,
 	m *dto.Metric,
-) {
+) error {
 	m.Label = labelPairs
 	switch t {
 	case CounterValue:
@@ -203,8 +203,9 @@ func populateMetric(
 	case UntypedValue:
 		m.Untyped = &dto.Untyped{Value: proto.Float64(v)}
 	default:
-		panic(fmt.Errorf("encountered unknown type %v", t))
+		return fmt.Errorf("encountered unknown type %v", t)
 	}
+	return nil
 }
 
 func makeLabelPairs(desc *Desc, labelValues []string) []*dto.LabelPair {