Backport of Zero value check for api gateway limits into release/2.0.x (#23556)

backport of commit 744998d

Co-authored-by: LakshmiNarayananDesikan <lakshminarayanan.desikan@hashicorp.com>

Author: hc-github-team-consul-core

agent/xds/clusters.go

@@ -1097,13 +1097,10 @@ func mergedAPIGatewayUpstreamConfig(configMap map[string]interface{}, limits *st
 
 func mergeAPIGatewayUpstreamLimits(existing, incoming *structs.UpstreamLimits) *structs.UpstreamLimits {
 	if existing == nil {
-		if incoming == nil {
-			return nil
-		}
-		return incoming.Clone()
+		return normalizeAPIGatewayUpstreamLimits(incoming)
 	}
 	if incoming == nil {
-		return existing.Clone()
+		return normalizeAPIGatewayUpstreamLimits(existing)
 	}
 
 	return &structs.UpstreamLimits{
@@ -1115,13 +1112,10 @@ func mergeAPIGatewayUpstreamLimits(existing, incoming *structs.UpstreamLimits) *
 
 func mergeAPIGatewayLimitValue(existing, incoming *int) *int {
 	if existing == nil {
-		if incoming == nil {
-			return nil
-		}
-		return intPointerCopy(incoming)
+		return positiveIntPointerCopy(incoming)
 	}
 	if incoming == nil {
-		return intPointerCopy(existing)
+		return positiveIntPointerCopy(existing)
 	}
 
 	e := *existing
@@ -1141,8 +1135,7 @@ func mergeAPIGatewayLimitValue(existing, incoming *int) *int {
 		return intPointerCopy(incoming)
 	}
 
-	zero := 0
-	return &zero
+	return nil
 }
 
 func intPointerCopy(v *int) *int {
@@ -1153,6 +1146,25 @@ func intPointerCopy(v *int) *int {
 	return &v2
 }
 
+func positiveIntPointerCopy(v *int) *int {
+	if v == nil || *v <= 0 {
+		return nil
+	}
+	return intPointerCopy(v)
+}
+
+func normalizeAPIGatewayUpstreamLimits(limits *structs.UpstreamLimits) *structs.UpstreamLimits {
+	if limits == nil {
+		return nil
+	}
+
+	return &structs.UpstreamLimits{
+		MaxConnections:        positiveIntPointerCopy(limits.MaxConnections),
+		MaxPendingRequests:    positiveIntPointerCopy(limits.MaxPendingRequests),
+		MaxConcurrentRequests: positiveIntPointerCopy(limits.MaxConcurrentRequests),
+	}
+}
+
 func (s *ResourceGenerator) configIngressUpstreamCluster(c *envoy_cluster_v3.Cluster, cfgSnap *proxycfg.ConfigSnapshot, listenerKey proxycfg.IngressListenerKey, u *structs.Upstream) {
 	var threshold *envoy_cluster_v3.CircuitBreakers_Thresholds
 	setThresholdLimit := func(limitType string, limit int) {
@@ -2351,17 +2363,25 @@ func makeThresholdsIfNeeded(limits *structs.UpstreamLimits) []*envoy_cluster_v3.
 	}
 
 	threshold := &envoy_cluster_v3.CircuitBreakers_Thresholds{}
+	hasLimit := false
 
 	// Likewise, make sure to not set any threshold values on the zero-value in
 	// order to rely on Envoy defaults
 	if limits.MaxConnections != nil {
 		threshold.MaxConnections = response.MakeUint32Value(*limits.MaxConnections)
+		hasLimit = true
 	}
 	if limits.MaxPendingRequests != nil {
 		threshold.MaxPendingRequests = response.MakeUint32Value(*limits.MaxPendingRequests)
+		hasLimit = true
 	}
 	if limits.MaxConcurrentRequests != nil {
 		threshold.MaxRequests = response.MakeUint32Value(*limits.MaxConcurrentRequests)
+		hasLimit = true
+	}
+
+	if !hasLimit {
+		return nil
 	}
 
 	return []*envoy_cluster_v3.CircuitBreakers_Thresholds{threshold}

agent/xds/clusters_test.go

@@ -963,6 +963,43 @@ func TestMergeAPIGatewayUpstreamLimits(t *testing.T) {
 	require.Equal(t, 50, *merged.MaxConcurrentRequests)
 }
 
+func TestMergeAPIGatewayUpstreamLimits_ZeroValuesOmitted(t *testing.T) {
+	t.Parallel()
+
+	merged := mergeAPIGatewayUpstreamLimits(
+		&structs.UpstreamLimits{
+			MaxConnections:        intPointer(0),
+			MaxPendingRequests:    intPointer(4),
+			MaxConcurrentRequests: intPointer(0),
+		},
+		&structs.UpstreamLimits{
+			MaxPendingRequests:    intPointer(3),
+			MaxConcurrentRequests: intPointer(2),
+		},
+	)
+
+	require.NotNil(t, merged)
+	require.Nil(t, merged.MaxConnections)
+	require.NotNil(t, merged.MaxPendingRequests)
+	require.Equal(t, 3, *merged.MaxPendingRequests)
+	require.NotNil(t, merged.MaxConcurrentRequests)
+	require.Equal(t, 2, *merged.MaxConcurrentRequests)
+
+	merged = mergeAPIGatewayUpstreamLimits(
+		&structs.UpstreamLimits{MaxConnections: intPointer(0)},
+		nil,
+	)
+	require.NotNil(t, merged)
+	require.Nil(t, merged.MaxConnections)
+
+	merged = mergeAPIGatewayUpstreamLimits(
+		nil,
+		&structs.UpstreamLimits{MaxConnections: intPointer(0)},
+	)
+	require.NotNil(t, merged)
+	require.Nil(t, merged.MaxConnections)
+}
+
 func TestMergedAPIGatewayUpstreamConfig(t *testing.T) {
 	t.Parallel()