Added IteratableGatherer for future zero alloc implementations.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Author: bwplotka

prometheus/promhttp/http.go

@@ -84,6 +84,16 @@ func Handler() http.Handler {
 // instrumentation. Use the InstrumentMetricHandler function to apply the same
 // kind of instrumentation as it is used by the Handler function.
 func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
+	return HandlerForIteratable(prometheus.ToIteratableGatherer(reg), opts)
+}
+
+// HandlerForIteratable returns an uninstrumented http.Handler for the provided
+// IteratableGatherer. The behavior of the Handler is defined by the provided
+// HandlerOpts. Thus, HandlerFor is useful to create http.Handlers for custom
+// Gatherers, with non-default HandlerOpts, and/or with custom (or no)
+// instrumentation. Use the InstrumentMetricHandler function to apply the same
+// kind of instrumentation as it is used by the Handler function.
+func HandlerForIteratable(reg prometheus.IteratableGatherer, opts HandlerOpts) http.Handler {
 	var (
 		inFlightSem chan struct{}
 		errCnt      = prometheus.NewCounterVec(
@@ -123,7 +133,8 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 				return
 			}
 		}
-		mfs, err := reg.Gather()
+
+		iter, err := reg.GatherIterate()
 		if err != nil {
 			if opts.ErrorLog != nil {
 				opts.ErrorLog.Println("error gathering metrics:", err)
@@ -133,7 +144,7 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 			case PanicOnError:
 				panic(err)
 			case ContinueOnError:
-				if len(mfs) == 0 {
+				if iter == nil {
 					// Still report the error if no metrics have been gathered.
 					httpError(rsp, err)
 					return
@@ -169,14 +180,22 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 
 		// handleError handles the error according to opts.ErrorHandling
 		// and returns true if we have to abort after the handling.
-		handleError := func(err error) bool {
+		handleError := func(encoding bool, err error) bool {
 			if err == nil {
 				return false
 			}
-			if opts.ErrorLog != nil {
-				opts.ErrorLog.Println("error encoding and sending metric family:", err)
+			if encoding {
+				if opts.ErrorLog != nil {
+					opts.ErrorLog.Println("error encoding and sending metric family:", err)
+				}
+				errCnt.WithLabelValues("encoding").Inc()
+			} else {
+				if opts.ErrorLog != nil {
+					opts.ErrorLog.Println("error gathering metrics:", err)
+				}
+				errCnt.WithLabelValues("gathering").Inc()
 			}
-			errCnt.WithLabelValues("encoding").Inc()
+
 			switch opts.ErrorHandling {
 			case PanicOnError:
 				panic(err)
@@ -191,14 +210,16 @@ func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
 			return false
 		}
 
-		for _, mf := range mfs {
-			if handleError(enc.Encode(mf)) {
+		for iter.Next() {
+			if handleError(true, enc.Encode(iter.At())) {
 				return
 			}
 		}
+		handleError(false, iter.Err())
+
 		if closer, ok := enc.(expfmt.Closer); ok {
 			// This in particular takes care of the final "# EOF\n" line for OpenMetrics.
-			if handleError(closer.Close()) {
+			if handleError(true, closer.Close()) {
 				return
 			}
 		}

prometheus/registry.go

@@ -161,6 +161,78 @@ type Gatherer interface {
 	Gather() ([]*dto.MetricFamily, error)
 }
 
+// IteratableGatherer is the interface for the part of a registry in charge of gathering
+// the collected metrics into a number of MetricFamilies. The IteratableGatherer interface
+// comes with the same general implication as described for the Registerer
+// interface.
+type IteratableGatherer interface {
+	// GatherIterate allows iterating over a lexicographically sorted, uniquely named
+	// dto.MetricFamily that are obtained through calls the Collect method
+	// of the registered Collectors. GatherIterate ensures that the
+	// iterator is valid until exhausting (until Next is false).
+	// As an exception to the strict consistency requirements described for
+	// metric.Desc, GatherIterate will tolerate different sets of label names for
+	// metrics of the same metric family.
+	//
+	// Even if an error occurs, GatherIterate allows to iterate over as many metrics as
+	// possible. Hence, if a non-nil error is returned, the returned
+	// MetricFamilyIterator could be nil (in case of a fatal error that
+	// prevented any meaningful metric collection) or iteratable over a number of
+	// MetricFamily protobufs, some of which might be incomplete, and some
+	// might be missing altogether. The returned error (which might be a
+	// MultiError) explains the details. Note that this is mostly useful for
+	// debugging purposes. If the gathered protobufs are to be used for
+	// exposition in actual monitoring, it is almost always better to not
+	// expose an incomplete result and instead disregard the returned
+	// MetricFamily protobufs in case the returned error is non-nil.
+	//
+	// GatherIterate allows implementations to use zero-alloc caching and
+	// other efficiency techniques.
+	//
+	// Iterated dto.MetricFamily is read only and valid only until you invoke another
+	// Next(). Note that consumer is *always* required to exhaust iterator to release all resources.
+	GatherIterate() (MetricFamilyIterator, error)
+}
+
+// MetricFamilyIterator ...
+type MetricFamilyIterator interface {
+	Next() bool
+	At() *dto.MetricFamily
+	Err() error
+}
+
+func ToIteratableGatherer(g Gatherer) IteratableGatherer {
+	return &iteratableGathererAdapter{g:g}
+}
+
+type iteratableGathererAdapter struct {
+	g Gatherer
+}
+
+func(a *iteratableGathererAdapter) GatherIterate() (MetricFamilyIterator, error) {
+	mfs, err := a.g.Gather()
+	return &mfIterator{mfs: mfs, i: -1}, err
+}
+
+type mfIterator struct {
+	mfs []*dto.MetricFamily
+	i int
+}
+
+func (m *mfIterator) Next() bool {
+	if m.i+1 >= len(m.mfs) {
+		return false
+	}
+	m.i++
+	return true
+}
+
+func (m *mfIterator) At() *dto.MetricFamily {
+	return m.mfs[m.i]
+}
+
+func (m *mfIterator) Err() error { return nil }
+
 // Register registers the provided Collector with the DefaultRegisterer.
 //
 // Register is a shortcut for DefaultRegisterer.Register(c). See there for more