Add 503 status code when there are zero upstream endpoints

Author: bjee19

internal/mode/static/nginx/agent/agent.go

@@ -179,6 +179,16 @@ func buildStreamUpstreamServers(upstream dataplane.Upstream) *pb.UpdateStreamSer
 }
 
 func buildUpstreamServers(upstream dataplane.Upstream) []*structpb.Struct {
+	if len(upstream.Endpoints) == 0 {
+		return []*structpb.Struct{
+			{
+				Fields: map[string]*structpb.Value{
+					"server": structpb.NewStringValue("unix:/var/run/nginx/nginx-503-server.sock"),
+				},
+			},
+		}
+	}
+
 	servers := make([]*structpb.Struct, 0, len(upstream.Endpoints))
 
 	for _, endpoint := range upstream.Endpoints {

internal/mode/static/nginx/agent/agent_test.go

@@ -179,6 +179,10 @@ func TestUpdateUpstreamServers(t *testing.T) {
 								},
 							},
 						},
+						{
+							Name:      "empty-upstream",
+							Endpoints: []resolver.Endpoint{},
+						},
 					},
 					StreamUpstreams: []dataplane.Upstream{
 						{
@@ -212,6 +216,20 @@ func TestUpdateUpstreamServers(t *testing.T) {
 							},
 						},
 					},
+					{
+						Action: &pb.NGINXPlusAction_UpdateHttpUpstreamServers{
+							UpdateHttpUpstreamServers: &pb.UpdateHTTPUpstreamServers{
+								HttpUpstreamName: "empty-upstream",
+								Servers: []*structpb.Struct{
+									{
+										Fields: map[string]*structpb.Value{
+											"server": structpb.NewStringValue("unix:/var/run/nginx/nginx-503-server.sock"),
+										},
+									},
+								},
+							},
+						},
+					},
 					{
 						Action: &pb.NGINXPlusAction_UpdateStreamServers{
 							UpdateStreamServers: &pb.UpdateStreamServers{
@@ -234,13 +252,14 @@ func TestUpdateUpstreamServers(t *testing.T) {
 				g.Expect(fakeBroadcaster.SendCallCount()).To(Equal(0))
 			} else if test.buildUpstreams {
 				g.Expect(deployment.GetNGINXPlusActions()).To(Equal(expActions))
-				g.Expect(fakeBroadcaster.SendCallCount()).To(Equal(2))
+				g.Expect(fakeBroadcaster.SendCallCount()).To(Equal(3))
 			}
 
 			if test.expErr {
 				expErr := errors.Join(
 					fmt.Errorf("couldn't update upstream via the API: %w", testErr),
 					fmt.Errorf("couldn't update upstream via the API: %w", testErr),
+					fmt.Errorf("couldn't update upstream via the API: %w", testErr),
 				)
 
 				g.Expect(deployment.GetLatestUpstreamError()).To(Equal(expErr))