Make hash collisions in the registry much less likely

go.mod

@@ -2,6 +2,7 @@ module github.com/prometheus/client_golang
 
 require (
 	github.com/beorn7/perks v1.0.1
+	github.com/cespare/xxhash/v2 v2.1.0
 	github.com/golang/protobuf v1.3.2
 	github.com/json-iterator/go v1.1.7
 	github.com/prometheus/client_model v0.0.0-20190812154241-14fe0d1b01d4

go.sum

@@ -8,6 +8,8 @@ github.com/beorn7/perks v1.0.0 h1:HWo1m869IqiPhD389kmkxeTalrjNbbJTC8LXupb+sl0=
 github.com/beorn7/perks v1.0.0/go.mod h1:KWe93zE9D1o94FZ5RNwFwVgaQK1VOXiVxmqh+CedLV8=
 github.com/beorn7/perks v1.0.1 h1:VlbKKnNfV8bJzeqoa4cOKqO6bYr3WgKZxO8Z16+hsOM=
 github.com/beorn7/perks v1.0.1/go.mod h1:G2ZrVWU2WbWT9wwq4/hrbKbnv/1ERSJQ0ibhJ6rlkpw=
+github.com/cespare/xxhash/v2 v2.1.0 h1:yTUvW7Vhb89inJ+8irsUqiWjh8iT6sQPZiQzI6ReGkA=
+github.com/cespare/xxhash/v2 v2.1.0/go.mod h1:dgIUBU3pDso/gPgZ1osOZ0iQf77oPR28Tjxl5dIMyVM=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=

prometheus/desc.go

@@ -19,6 +19,7 @@ import (
 	"sort"
 	"strings"
 
+	"github.com/cespare/xxhash/v2"
 	"github.com/golang/protobuf/proto"
 	"github.com/prometheus/common/model"
 
@@ -126,24 +127,24 @@ func NewDesc(fqName, help string, variableLabels []string, constLabels Labels) *
 		return d
 	}
 
-	vh := hashNew()
+	xxh := xxhash.New()
 	for _, val := range labelValues {
-		vh = hashAdd(vh, val)
-		vh = hashAddByte(vh, separatorByte)
+		xxh.WriteString(val)
+		xxh.Write(separatorByteSlice)
 	}
-	d.id = vh
+	d.id = xxh.Sum64()
 	// Sort labelNames so that order doesn't matter for the hash.
 	sort.Strings(labelNames)
 	// Now hash together (in this order) the help string and the sorted
 	// label names.
-	lh := hashNew()
-	lh = hashAdd(lh, help)
-	lh = hashAddByte(lh, separatorByte)
+	xxh.Reset()
+	xxh.WriteString(help)
+	xxh.Write(separatorByteSlice)
 	for _, labelName := range labelNames {
-		lh = hashAdd(lh, labelName)
-		lh = hashAddByte(lh, separatorByte)
+		xxh.WriteString(labelName)
+		xxh.Write(separatorByteSlice)
 	}
-	d.dimHash = lh
+	d.dimHash = xxh.Sum64()
 
 	d.constLabelPairs = make([]*dto.LabelPair, 0, len(constLabels))
 	for n, v := range constLabels {

prometheus/metric.go

@@ -24,6 +24,8 @@ import (
 
 const separatorByte byte = 255
 
+var separatorByteSlice = []byte{255} // For convenient use with xxhash.
+
 // A Metric models a single sample value with its meta data being exported to
 // Prometheus. Implementations of Metric in this package are Gauge, Counter,
 // Histogram, Summary, and Untyped.

prometheus/registry.go

@@ -25,6 +25,7 @@ import (
 	"sync"
 	"unicode/utf8"
 
+	"github.com/cespare/xxhash/v2"
 	"github.com/golang/protobuf/proto"
 	"github.com/prometheus/common/expfmt"
 
@@ -266,7 +267,7 @@ func (r *Registry) Register(c Collector) error {
 		descChan           = make(chan *Desc, capDescChan)
 		newDescIDs         = map[uint64]struct{}{}
 		newDimHashesByName = map[string]uint64{}
-		collectorID        uint64 // Just a sum of all desc IDs.
+		collectorID        uint64 // All desc IDs XOR'd together.
 		duplicateDescErr   error
 	)
 	go func() {
@@ -293,12 +294,12 @@ func (r *Registry) Register(c Collector) error {
 		if _, exists := r.descIDs[desc.id]; exists {
 			duplicateDescErr = fmt.Errorf("descriptor %s already exists with the same fully-qualified name and const label values", desc)
 		}
-		// If it is not a duplicate desc in this collector, add it to
+		// If it is not a duplicate desc in this collector, XOR it to
 		// the collectorID.  (We allow duplicate descs within the same
 		// collector, but their existence must be a no-op.)
 		if _, exists := newDescIDs[desc.id]; !exists {
 			newDescIDs[desc.id] = struct{}{}
-			collectorID += desc.id
+			collectorID ^= desc.id
 		}
 
 		// Are all the label names and the help string consistent with
@@ -875,9 +876,9 @@ func checkMetricConsistency(
 	}
 
 	// Is the metric unique (i.e. no other metric with the same name and the same labels)?
-	h := hashNew()
-	h = hashAdd(h, name)
-	h = hashAddByte(h, separatorByte)
+	h := xxhash.New()
+	h.WriteString(name)
+	h.Write(separatorByteSlice)
 	// Make sure label pairs are sorted. We depend on it for the consistency
 	// check.
 	if !sort.IsSorted(labelPairSorter(dtoMetric.Label)) {
@@ -888,18 +889,19 @@ func checkMetricConsistency(
 		dtoMetric.Label = copiedLabels
 	}
 	for _, lp := range dtoMetric.Label {
-		h = hashAdd(h, lp.GetName())
-		h = hashAddByte(h, separatorByte)
-		h = hashAdd(h, lp.GetValue())
-		h = hashAddByte(h, separatorByte)
+		h.WriteString(lp.GetName())
+		h.Write(separatorByteSlice)
+		h.WriteString(lp.GetValue())
+		h.Write(separatorByteSlice)
 	}
-	if _, exists := metricHashes[h]; exists {
+	hSum := h.Sum64()
+	if _, exists := metricHashes[hSum]; exists {
 		return fmt.Errorf(
 			"collected metric %q { %s} was collected before with the same name and label values",
 			name, dtoMetric,
 		)
 	}
-	metricHashes[h] = struct{}{}
+	metricHashes[hSum] = struct{}{}
 	return nil
 }
 

prometheus/registry_test.go

@@ -1070,3 +1070,90 @@ test_summary_count{name="foo"} 2
 		)
 	}
 }
+
+// collidingCollector is a collection of prometheus.Collectors,
+// and is itself a prometheus.Collector.
+type collidingCollector struct {
+	i    int
+	name string
+
+	a, b, c, d prometheus.Collector
+}
+
+// Describe satisifies part of the prometheus.Collector interface.
+func (m *collidingCollector) Describe(desc chan<- *prometheus.Desc) {
+	m.a.Describe(desc)
+	m.b.Describe(desc)
+	m.c.Describe(desc)
+	m.d.Describe(desc)
+}
+
+// Collect satisifies part of the prometheus.Collector interface.
+func (m *collidingCollector) Collect(metric chan<- prometheus.Metric) {
+	m.a.Collect(metric)
+	m.b.Collect(metric)
+	m.c.Collect(metric)
+	m.d.Collect(metric)
+}
+
+// TestAlreadyRegistered will fail with the old, weaker hash function.  It is
+// taken from https://play.golang.org/p/HpV7YE6LI_4 , authored by @awilliams.
+func TestAlreadyRegisteredCollision(t *testing.T) {
+
+	reg := prometheus.NewRegistry()
+
+	for i := 0; i < 10000; i++ {
+		// A collector should be considered unique if its name and const
+		// label values are unique.
+
+		name := fmt.Sprintf("test-collector-%010d", i)
+
+		collector := collidingCollector{
+			i:    i,
+			name: name,
+
+			a: prometheus.NewCounter(prometheus.CounterOpts{
+				Name: "my_collector_a",
+				ConstLabels: prometheus.Labels{
+					"name": name,
+					"type": "test",
+				},
+			}),
+			b: prometheus.NewCounter(prometheus.CounterOpts{
+				Name: "my_collector_b",
+				ConstLabels: prometheus.Labels{
+					"name": name,
+					"type": "test",
+				},
+			}),
+			c: prometheus.NewCounter(prometheus.CounterOpts{
+				Name: "my_collector_c",
+				ConstLabels: prometheus.Labels{
+					"name": name,
+					"type": "test",
+				},
+			}),
+			d: prometheus.NewCounter(prometheus.CounterOpts{
+				Name: "my_collector_d",
+				ConstLabels: prometheus.Labels{
+					"name": name,
+					"type": "test",
+				},
+			}),
+		}
+
+		// Register should not fail, since each collector has a unique
+		// set of sub-collectors, determined by their names and const label values.
+		if err := reg.Register(&collector); err != nil {
+			alreadyRegErr, ok := err.(prometheus.AlreadyRegisteredError)
+			if !ok {
+				t.Fatal(err)
+			}
+
+			previous := alreadyRegErr.ExistingCollector.(*collidingCollector)
+			current := alreadyRegErr.NewCollector.(*collidingCollector)
+
+			t.Errorf("Unexpected registration error: %q\nprevious collector: %s (i=%d)\ncurrent collector %s (i=%d)", alreadyRegErr, previous.name, previous.i, current.name, current.i)
+		}
+	}
+}