fix: handle FixedSizeReservoir size=0 without panic (#8295)

Fixes #8232

- Added guard for size=0 in FixedSizeReservoir
- Prevents panic during Offer/update
- Added test to ensure no panic occurs

---------

Co-authored-by: David Ashpole <dashpole@google.com>

CHANGELOG.md

@@ -64,6 +64,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Fixed
 
+- Fix `FixedSizeReservoir` in `go.opentelemetry.io/otel/sdk/metric/exemplar` to safely handle zero size.
+  A capacity check in the constructor initializes the reservoir safely and skips initialization for zero-cap; early returns in `Offer()` and `Collect()` ensure no-op behavior. (#8295)
 - Limit OTLP request size to 32 MiB by default in `go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc`.
   The limit applies before compression, oversized requests are treated as non-retryable errors, and the limit can be configured with the new `WithMaxRequestSize` option. (#8157)
 - Limit OTLP request size to 32 MiB by default in `go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp`.

sdk/metric/exemplar/fixed_size_reservoir.go

@@ -54,7 +54,9 @@ func newFixedSizeReservoir(s *storage) *FixedSizeReservoir {
 	r := &FixedSizeReservoir{
 		storage: s,
 	}
-	r.reset()
+	if cap(r.measurements) > 0 {
+		r.reset()
+	}
 	return r
 }
 
@@ -86,6 +88,10 @@ func (*FixedSizeReservoir) randomFloat64() float64 {
 // parameters are the value and dropped (filtered) attributes of the
 // measurement respectively.
 func (r *FixedSizeReservoir) Offer(ctx context.Context, t time.Time, n Value, a []attribute.KeyValue) {
+	if cap(r.measurements) == 0 {
+		return
+	}
+
 	// The following algorithm is "Algorithm L" from Li, Kim-Hung (4 December
 	// 1994). "Reservoir-Sampling Algorithms of Time Complexity
 	// O(n(1+log(N/n)))". ACM Transactions on Mathematical Software. 20 (4):
@@ -194,6 +200,10 @@ func (r *FixedSizeReservoir) advance() {
 //
 // The Reservoir state is preserved after this call.
 func (r *FixedSizeReservoir) Collect(dest *[]Exemplar) {
+	if cap(r.measurements) == 0 {
+		*dest = (*dest)[:0]
+		return
+	}
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	r.storage.Collect(dest)

sdk/metric/exemplar/fixed_size_reservoir_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestNewFixedSizeReservoir(t *testing.T) {
@@ -22,6 +23,19 @@ func TestNewFixedSizeReservoir(t *testing.T) {
 	}))
 }
 
+func TestNewFixedSizeReservoirZeroSize(t *testing.T) {
+	r := NewFixedSizeReservoir(0)
+	require.NotNil(t, r)
+
+	// Offer should be a no-op and not panic.
+	r.Offer(t.Context(), staticTime, NewValue(float64(10)), nil)
+
+	// Collect should leave dest empty.
+	dest := []Exemplar{{}} // pre-filled sentinel
+	r.Collect(&dest)
+	assert.Empty(t, dest)
+}
+
 func TestNewFixedSizeReservoirSamplingCorrectness(t *testing.T) {
 	intensity := 0.1
 	sampleSize := 1000