Log warning when a trace attribute/event/link is discarded due to limits

CHANGELOG.md

@@ -18,6 +18,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Log a warning to the OpenTelemetry internal logger when a `Record` in `go.opentelemetry.io/otel/sdk/log` drops an attribute due to a limit being reached. (#5376)
 - Identify the `Tracer` returned from the global `TracerProvider` in `go.opentelemetry.io/otel/global` with its schema URL. (#5426)
 - Identify the `Meter` returned from the global `MeterProvider` in `go.opentelemetry.io/otel/global` with its schema URL. (#5426)
+- Log a warning to the OpenTelemetry internal logger when a `Span` in `go.opentelemetry.io/otel/sdk/trace` drops an attribute, event, or link due to a limit being reached. (#5434)
 
 ## [1.27.0/0.49.0/0.3.0] 2024-05-21
 

sdk/trace/evictedqueue.go

@@ -3,23 +3,38 @@
 
 package trace // import "go.opentelemetry.io/otel/sdk/trace"
 
+import (
+	"fmt"
+	"slices"
+	"sync"
+
+	"go.opentelemetry.io/otel/internal/global"
+)
+
 // evictedQueue is a FIFO queue with a configurable capacity.
-type evictedQueue struct {
-	queue        []interface{}
+type evictedQueue[T any] struct {
+	queue        []T
 	capacity     int
 	droppedCount int
+	logDropped   func()
 }
 
-func newEvictedQueue(capacity int) evictedQueue {
+func newEvictedQueue[T any](capacity int) evictedQueue[T] {
+	var tVal T
+	msg := fmt.Sprintf("limit reached: dropping trace %T", tVal)
 	// Do not pre-allocate queue, do this lazily.
-	return evictedQueue{capacity: capacity}
+	return evictedQueue[T]{
+		capacity:   capacity,
+		logDropped: sync.OnceFunc(func() { global.Warn(msg) }),
+	}
 }
 
 // add adds value to the evictedQueue eq. If eq is at capacity, the oldest
 // queued value will be discarded and the drop count incremented.
-func (eq *evictedQueue) add(value interface{}) {
+func (eq *evictedQueue[T]) add(value T) {
 	if eq.capacity == 0 {
 		eq.droppedCount++
+		eq.logDropped()
 		return
 	}
 
@@ -28,6 +43,12 @@ func (eq *evictedQueue) add(value interface{}) {
 		copy(eq.queue[:eq.capacity-1], eq.queue[1:])
 		eq.queue = eq.queue[:eq.capacity-1]
 		eq.droppedCount++
+		eq.logDropped()
 	}
 	eq.queue = append(eq.queue, value)
 }
+
+// copy returns a copy of the evictedQueue.
+func (eq *evictedQueue[T]) copy() []T {
+	return slices.Clone(eq.queue)
+}

sdk/trace/evictedqueue_test.go

@@ -6,34 +6,47 @@ package trace
 import (
 	"reflect"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func init() {
 }
 
 func TestAdd(t *testing.T) {
-	q := newEvictedQueue(3)
+	q := newEvictedQueue[string](3)
 	q.add("value1")
 	q.add("value2")
 	if wantLen, gotLen := 2, len(q.queue); wantLen != gotLen {
 		t.Errorf("got queue length %d want %d", gotLen, wantLen)
 	}
 }
 
-func (eq *evictedQueue) queueToArray() []string {
-	arr := make([]string, 0)
-	for _, value := range eq.queue {
-		arr = append(arr, value.(string))
-	}
-	return arr
+func TestCopy(t *testing.T) {
+	q := newEvictedQueue[string](3)
+	q.add("value1")
+	cp := q.copy()
+
+	q.add("value2")
+	assert.Equal(t, []string{"value1"}, cp, "queue update modified copy")
+
+	cp[0] = "value0"
+	assert.Equal(t, "value1", q.queue[0], "copy update modified queue")
 }
 
 func TestDropCount(t *testing.T) {
-	q := newEvictedQueue(3)
+	q := newEvictedQueue[string](3)
+	var called bool
+	q.logDropped = func() { called = true }
+
 	q.add("value1")
+	assert.False(t, called, `"value1" logged as dropped`)
 	q.add("value2")
+	assert.False(t, called, `"value2" logged as dropped`)
 	q.add("value3")
+	assert.False(t, called, `"value3" logged as dropped`)
 	q.add("value1")
+	assert.True(t, called, `"value2" not logged as dropped`)
 	q.add("value4")
 	if wantLen, gotLen := 3, len(q.queue); wantLen != gotLen {
 		t.Errorf("got queue length %d want %d", gotLen, wantLen)
@@ -42,7 +55,7 @@ func TestDropCount(t *testing.T) {
 		t.Errorf("got drop count %d want %d", gotDropCount, wantDropCount)
 	}
 	wantArr := []string{"value3", "value1", "value4"}
-	gotArr := q.queueToArray()
+	gotArr := q.copy()
 
 	if wantLen, gotLen := len(wantArr), len(gotArr); gotLen != wantLen {
 		t.Errorf("got array len %d want %d", gotLen, wantLen)

sdk/trace/span.go

@@ -17,6 +17,7 @@ import (
 
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/internal/global"
 	"go.opentelemetry.io/otel/sdk/instrumentation"
 	"go.opentelemetry.io/otel/sdk/internal"
 	"go.opentelemetry.io/otel/sdk/resource"
@@ -137,12 +138,13 @@ type recordingSpan struct {
 	// ReadOnlySpan exported when the span ends.
 	attributes        []attribute.KeyValue
 	droppedAttributes int
+	logDropAttrsOnce  sync.Once
 
 	// events are stored in FIFO queue capped by configured limit.
-	events evictedQueue
+	events evictedQueue[Event]
 
 	// links are stored in FIFO queue capped by configured limit.
-	links evictedQueue
+	links evictedQueue[Link]
 
 	// executionTracerTaskEnd ends the execution tracer span.
 	executionTracerTaskEnd func()
@@ -219,7 +221,7 @@ func (s *recordingSpan) SetAttributes(attributes ...attribute.KeyValue) {
 	limit := s.tracer.provider.spanLimits.AttributeCountLimit
 	if limit == 0 {
 		// No attributes allowed.
-		s.droppedAttributes += len(attributes)
+		s.addDroppedAttr(len(attributes))
 		return
 	}
 
@@ -236,14 +238,30 @@ func (s *recordingSpan) SetAttributes(attributes ...attribute.KeyValue) {
 	for _, a := range attributes {
 		if !a.Valid() {
 			// Drop all invalid attributes.
-			s.droppedAttributes++
+			s.addDroppedAttr(1)
 			continue
 		}
 		a = truncateAttr(s.tracer.provider.spanLimits.AttributeValueLengthLimit, a)
 		s.attributes = append(s.attributes, a)
 	}
 }
 
+// Declared as a var so tests can override.
+var logDropAttrs = func() {
+	global.Warn("limit reached: dropping trace Span attributes")
+}
+
+// addDroppedAttr adds incr to the count of dropped attributes.
+//
+// The first, and only the first, time this method is called a warning will be
+// logged.
+//
+// This method assumes s.mu.Lock is held by the caller.
+func (s *recordingSpan) addDroppedAttr(incr int) {
+	s.droppedAttributes += incr
+	s.logDropAttrsOnce.Do(logDropAttrs)
+}
+
 // addOverCapAttrs adds the attributes attrs to the span s while
 // de-duplicating the attributes of s and attrs and dropping attributes that
 // exceed the limit.
@@ -273,7 +291,7 @@ func (s *recordingSpan) addOverCapAttrs(limit int, attrs []attribute.KeyValue) {
 	for _, a := range attrs {
 		if !a.Valid() {
 			// Drop all invalid attributes.
-			s.droppedAttributes++
+			s.addDroppedAttr(1)
 			continue
 		}
 
@@ -286,7 +304,7 @@ func (s *recordingSpan) addOverCapAttrs(limit int, attrs []attribute.KeyValue) {
 		if len(s.attributes) >= limit {
 			// Do not just drop all of the remaining attributes, make sure
 			// updates are checked and performed.
-			s.droppedAttributes++
+			s.addDroppedAttr(1)
 		} else {
 			a = truncateAttr(s.tracer.provider.spanLimits.AttributeValueLengthLimit, a)
 			s.attributes = append(s.attributes, a)
@@ -585,7 +603,7 @@ func (s *recordingSpan) Links() []Link {
 	if len(s.links.queue) == 0 {
 		return []Link{}
 	}
-	return s.interfaceArrayToLinksArray()
+	return s.links.copy()
 }
 
 // Events returns the events of this span.
@@ -595,7 +613,7 @@ func (s *recordingSpan) Events() []Event {
 	if len(s.events.queue) == 0 {
 		return []Event{}
 	}
-	return s.interfaceArrayToEventArray()
+	return s.events.copy()
 }
 
 // Status returns the status of this span.
@@ -717,32 +735,16 @@ func (s *recordingSpan) snapshot() ReadOnlySpan {
 	}
 	sd.droppedAttributeCount = s.droppedAttributes
 	if len(s.events.queue) > 0 {
-		sd.events = s.interfaceArrayToEventArray()
+		sd.events = s.events.copy()
 		sd.droppedEventCount = s.events.droppedCount
 	}
 	if len(s.links.queue) > 0 {
-		sd.links = s.interfaceArrayToLinksArray()
+		sd.links = s.links.copy()
 		sd.droppedLinkCount = s.links.droppedCount
 	}
 	return &sd
 }
 
-func (s *recordingSpan) interfaceArrayToLinksArray() []Link {
-	linkArr := make([]Link, 0)
-	for _, value := range s.links.queue {
-		linkArr = append(linkArr, value.(Link))
-	}
-	return linkArr
-}
-
-func (s *recordingSpan) interfaceArrayToEventArray() []Event {
-	eventArr := make([]Event, 0)
-	for _, value := range s.events.queue {
-		eventArr = append(eventArr, value.(Event))
-	}
-	return eventArr
-}
-
 func (s *recordingSpan) addChild() {
 	if !s.IsRecording() {
 		return

sdk/trace/span_test.go

@@ -234,6 +234,22 @@ func TestTruncateAttr(t *testing.T) {
 	}
 }
 
+func TestLogDropAttrs(t *testing.T) {
+	orig := logDropAttrs
+	t.Cleanup(func() { logDropAttrs = orig })
+
+	var called bool
+	logDropAttrs = func() { called = true }
+
+	s := &recordingSpan{}
+	s.addDroppedAttr(1)
+	assert.True(t, called, "logDropAttrs not called")
+
+	called = false
+	s.addDroppedAttr(1)
+	assert.False(t, called, "logDropAttrs called multiple times for same Span")
+}
+
 func BenchmarkRecordingSpanSetAttributes(b *testing.B) {
 	var attrs []attribute.KeyValue
 	for i := 0; i < 100; i++ {

sdk/trace/tracer.go

@@ -132,8 +132,8 @@ func (tr *tracer) newRecordingSpan(psc, sc trace.SpanContext, name string, sr Sa
 		spanKind:    trace.ValidateSpanKind(config.SpanKind()),
 		name:        name,
 		startTime:   startTime,
-		events:      newEvictedQueue(tr.provider.spanLimits.EventCountLimit),
-		links:       newEvictedQueue(tr.provider.spanLimits.LinkCountLimit),
+		events:      newEvictedQueue[Event](tr.provider.spanLimits.EventCountLimit),
+		links:       newEvictedQueue[Link](tr.provider.spanLimits.LinkCountLimit),
 		tracer:      tr,
 	}