Add Deregister to metrics.MultiGatherer interface (#3498)

api/metrics/label_gatherer.go

@@ -45,12 +45,14 @@ func (g *labelGatherer) Register(labelValue string, gatherer prometheus.Gatherer
 		)
 	}
 
-	g.names = append(g.names, labelValue)
-	g.gatherers = append(g.gatherers, &labeledGatherer{
-		labelName:  g.labelName,
-		labelValue: labelValue,
-		gatherer:   gatherer,
-	})
+	g.register(
+		labelValue,
+		&labeledGatherer{
+			labelName:  g.labelName,
+			labelValue: labelValue,
+			gatherer:   gatherer,
+		},
+	)
 	return nil
 }
 

api/metrics/label_gatherer_test.go

@@ -138,9 +138,13 @@ func TestLabelGatherer_Gather(t *testing.T) {
 	}
 }
 
-func TestLabelGatherer_Register(t *testing.T) {
+func TestLabelGatherer_Registration(t *testing.T) {
+	const (
+		firstName  = "first"
+		secondName = "second"
+	)
 	firstLabeledGatherer := &labeledGatherer{
-		labelValue: "first",
+		labelValue: firstName,
 		gatherer:   &testGatherer{},
 	}
 	firstLabelGatherer := func() *labelGatherer {
@@ -154,25 +158,37 @@ func TestLabelGatherer_Register(t *testing.T) {
 		}
 	}
 	secondLabeledGatherer := &labeledGatherer{
-		labelValue: "second",
+		labelValue: secondName,
 		gatherer: &testGatherer{
 			mfs: []*dto.MetricFamily{{}},
 		},
 	}
-	secondLabelGatherer := &labelGatherer{
+	secondLabelGatherer := func() *labelGatherer {
+		return &labelGatherer{
+			multiGatherer: multiGatherer{
+				names: []string{
+					firstLabeledGatherer.labelValue,
+					secondLabeledGatherer.labelValue,
+				},
+				gatherers: prometheus.Gatherers{
+					firstLabeledGatherer,
+					secondLabeledGatherer,
+				},
+			},
+		}
+	}
+	onlySecondLabeledGatherer := &labelGatherer{
 		multiGatherer: multiGatherer{
 			names: []string{
-				firstLabeledGatherer.labelValue,
 				secondLabeledGatherer.labelValue,
 			},
 			gatherers: prometheus.Gatherers{
-				firstLabeledGatherer,
 				secondLabeledGatherer,
 			},
 		},
 	}
 
-	tests := []struct {
+	registerTests := []struct {
 		name                  string
 		labelGatherer         *labelGatherer
 		labelValue            string
@@ -183,29 +199,29 @@ func TestLabelGatherer_Register(t *testing.T) {
 		{
 			name:                  "first registration",
 			labelGatherer:         &labelGatherer{},
-			labelValue:            "first",
+			labelValue:            firstName,
 			gatherer:              firstLabeledGatherer.gatherer,
 			expectedErr:           nil,
 			expectedLabelGatherer: firstLabelGatherer(),
 		},
 		{
 			name:                  "second registration",
 			labelGatherer:         firstLabelGatherer(),
-			labelValue:            "second",
+			labelValue:            secondName,
 			gatherer:              secondLabeledGatherer.gatherer,
 			expectedErr:           nil,
-			expectedLabelGatherer: secondLabelGatherer,
+			expectedLabelGatherer: secondLabelGatherer(),
 		},
 		{
 			name:                  "conflicts with previous registration",
 			labelGatherer:         firstLabelGatherer(),
-			labelValue:            "first",
+			labelValue:            firstName,
 			gatherer:              secondLabeledGatherer.gatherer,
 			expectedErr:           errDuplicateGatherer,
 			expectedLabelGatherer: firstLabelGatherer(),
 		},
 	}
-	for _, test := range tests {
+	for _, test := range registerTests {
 		t.Run(test.name, func(t *testing.T) {
 			require := require.New(t)
 
@@ -214,4 +230,64 @@ func TestLabelGatherer_Register(t *testing.T) {
 			require.Equal(test.expectedLabelGatherer, test.labelGatherer)
 		})
 	}
+
+	deregisterTests := []struct {
+		name                  string
+		labelGatherer         *labelGatherer
+		labelValue            string
+		expectedRemoved       bool
+		expectedLabelGatherer *labelGatherer
+	}{
+		{
+			name:                  "remove from nothing",
+			labelGatherer:         &labelGatherer{},
+			labelValue:            firstName,
+			expectedRemoved:       false,
+			expectedLabelGatherer: &labelGatherer{},
+		},
+		{
+			name:                  "remove unknown name",
+			labelGatherer:         firstLabelGatherer(),
+			labelValue:            secondName,
+			expectedRemoved:       false,
+			expectedLabelGatherer: firstLabelGatherer(),
+		},
+		{
+			name:            "remove first name",
+			labelGatherer:   firstLabelGatherer(),
+			labelValue:      firstName,
+			expectedRemoved: true,
+			expectedLabelGatherer: &labelGatherer{
+				multiGatherer: multiGatherer{
+					// We must populate with empty slices rather than nil slices
+					// to pass the equality check.
+					names:     []string{},
+					gatherers: prometheus.Gatherers{},
+				},
+			},
+		},
+		{
+			name:                  "remove second name",
+			labelGatherer:         secondLabelGatherer(),
+			labelValue:            secondName,
+			expectedRemoved:       true,
+			expectedLabelGatherer: firstLabelGatherer(),
+		},
+		{
+			name:                  "remove only first name",
+			labelGatherer:         secondLabelGatherer(),
+			labelValue:            firstName,
+			expectedRemoved:       true,
+			expectedLabelGatherer: onlySecondLabeledGatherer,
+		},
+	}
+	for _, test := range deregisterTests {
+		t.Run(test.name, func(t *testing.T) {
+			require := require.New(t)
+
+			removed := test.labelGatherer.Deregister(test.labelValue)
+			require.Equal(test.expectedRemoved, removed)
+			require.Equal(test.expectedLabelGatherer, test.labelGatherer)
+		})
+	}
 }

api/metrics/multi_gatherer.go

@@ -5,10 +5,13 @@ package metrics
 
 import (
 	"fmt"
+	"slices"
 	"sync"
 
 	"github.com/prometheus/client_golang/prometheus"
 
+	"github.com/ava-labs/avalanchego/utils"
+
 	dto "github.com/prometheus/client_model/go"
 )
 
@@ -20,13 +23,11 @@ type MultiGatherer interface {
 	// Register adds the outputs of [gatherer] to the results of future calls to
 	// Gather with the provided [name] added to the metrics.
 	Register(name string, gatherer prometheus.Gatherer) error
-}
 
-// Deprecated: Use NewPrefixGatherer instead.
-//
-// TODO: Remove once coreth is updated.
-func NewMultiGatherer() MultiGatherer {
-	return NewPrefixGatherer()
+	// Deregister removes the outputs of a gatherer with [name] from the results
+	// of future calls to Gather. Returns true if a gatherer with [name] was
+	// found.
+	Deregister(name string) bool
 }
 
 type multiGatherer struct {
@@ -42,6 +43,25 @@ func (g *multiGatherer) Gather() ([]*dto.MetricFamily, error) {
 	return g.gatherers.Gather()
 }
 
+func (g *multiGatherer) register(name string, gatherer prometheus.Gatherer) {
+	g.names = append(g.names, name)
+	g.gatherers = append(g.gatherers, gatherer)
+}
+
+func (g *multiGatherer) Deregister(name string) bool {
+	g.lock.Lock()
+	defer g.lock.Unlock()
+
+	index := slices.Index(g.names, name)
+	if index == -1 {
+		return false
+	}
+
+	g.names = utils.DeleteIndex(g.names, index)
+	g.gatherers = utils.DeleteIndex(g.gatherers, index)
+	return true
+}
+
 func MakeAndRegister(gatherer MultiGatherer, name string) (*prometheus.Registry, error) {
 	reg := prometheus.NewRegistry()
 	if err := gatherer.Register(name, reg); err != nil {

api/metrics/prefix_gatherer.go

@@ -45,11 +45,13 @@ func (g *prefixGatherer) Register(prefix string, gatherer prometheus.Gatherer) e
 		}
 	}
 
-	g.names = append(g.names, prefix)
-	g.gatherers = append(g.gatherers, &prefixedGatherer{
-		prefix:   prefix,
-		gatherer: gatherer,
-	})
+	g.register(
+		prefix,
+		&prefixedGatherer{
+			prefix:   prefix,
+			gatherer: gatherer,
+		},
+	)
 	return nil
 }
 

utils/slice.go

@@ -0,0 +1,18 @@
+// Copyright (C) 2019-2024, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package utils
+
+// DeleteIndex moves the last element in the slice to index [i] and shrinks the
+// size of the slice by 1.
+//
+// This is an O(1) operation that allows the removal of an element from a slice
+// when the order of the slice is not important.
+//
+// If [i] is out of bounds, this function will panic.
+func DeleteIndex[S ~[]E, E any](s S, i int) S {
+	newSize := len(s) - 1
+	s[i] = s[newSize]
+	s[newSize] = Zero[E]()
+	return s[:newSize]
+}

utils/slice_test.go

@@ -0,0 +1,50 @@
+// Copyright (C) 2019-2024, Ava Labs, Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package utils
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDeleteIndex(t *testing.T) {
+	tests := []struct {
+		name     string
+		s        []int
+		i        int
+		expected []int
+	}{
+		{
+			name:     "delete only element",
+			s:        []int{0},
+			i:        0,
+			expected: []int{},
+		},
+		{
+			name:     "delete first element",
+			s:        []int{0, 1},
+			i:        0,
+			expected: []int{1},
+		},
+		{
+			name:     "delete middle element",
+			s:        []int{0, 1, 2, 3},
+			i:        1,
+			expected: []int{0, 3, 2},
+		},
+		{
+			name:     "delete last element",
+			s:        []int{0, 1, 2, 3},
+			i:        3,
+			expected: []int{0, 1, 2},
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			s := DeleteIndex(test.s, test.i)
+			require.Equal(t, test.expected, s)
+		})
+	}
+}