Disable "taking" from reservoir if there is no capacity, regardless of borrow value (#3684)

* do not borrow from reservoir if there is no reservoir capacity

* add and edit tests

* add changelog

Author: jj22ee

CHANGELOG.md

@@ -13,6 +13,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - AWS SDK add `rpc.system` attribute in `go.opentelemetry.io/contrib/instrumentation/github.com/aws/aws-sdk-go-v2/otelaws`. (#3582, #3617)
 - Add the new `go.opentelemetry.io/contrib/instrgen` package to provide auto-generated source code instrumentation. (#3068, #3108)
 
+### Fixed
+
+- Prevent taking from reservoir in AWS XRay Remote Sampler when there is zero capacity in `go.opentelemetry.io/contrib/samplers/aws/xray`. (#3684)
+
 ## [1.16.0-rc.2/0.41.0-rc.2/0.10.0-rc.2] - 2023-03-23
 
 ### Added

samplers/aws/xray/internal/reservoir.go

@@ -59,6 +59,10 @@ func (r *reservoir) take(now time.Time, borrowed bool, itemCost float64) bool {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 
+	if r.capacity == 0 {
+		return false
+	}
+
 	if r.lastTick.IsZero() {
 		r.lastTick = now
 

samplers/aws/xray/internal/reservoir_test.go

@@ -161,6 +161,30 @@ func TestConsumeFromReservoir(t *testing.T) {
 	assert.Equal(t, r.quotaBalance, 7.0)
 }
 
+func TestZeroCapacityFailBorrow(t *testing.T) {
+	clock := &mockClock{
+		nowTime: 1500000000,
+	}
+
+	r := &reservoir{
+		quota:    0,
+		capacity: 0,
+	}
+
+	// start with no quota balance
+	assert.Equal(t, r.quotaBalance, 0.0)
+	// attempt to borrow from reservoir, and should fail since there is no capacity
+	assert.False(t, r.take(clock.now(), true, 1.0))
+
+	// increase the clock by 5
+	clock.nowTime = 1500000005
+
+	// validate there is still no quota balance
+	assert.Equal(t, r.quotaBalance, 0.0)
+	// again, attempt to borrow from reservoir, and should fail since there is no capacity
+	assert.False(t, r.take(clock.now(), true, 1.0))
+}
+
 func TestResetQuotaUsageRotation(t *testing.T) {
 	clock := &mockClock{
 		nowTime: 1500000000,

samplers/aws/xray/internal/rule_test.go

@@ -162,6 +162,7 @@ func TestConsumeFromReservoirSample(t *testing.T) {
 			RuleName: "r1",
 		},
 		reservoir: &reservoir{
+			capacity:  10,
 			quota:     10,
 			expiresAt: time.Unix(1500000060, 0),
 		},