Merge remote-tracking branch 'origin/3.1.x'

spring-cloud-loadbalancer/src/main/java/org/springframework/cloud/loadbalancer/core/RoundRobinLoadBalancer.java

@@ -100,6 +100,12 @@ public class RoundRobinLoadBalancer implements ReactorServiceInstanceLoadBalance
 			return new EmptyResponse();
 		}
 
+		// Do not move position when there is only 1 instance, especially some suppliers
+		// have already filtered instances
+		if (instances.size() == 1) {
+			return new DefaultResponse(instances.get(0));
+		}
+
 		// Ignore the sign bit, this allows pos to loop sequentially from 0 to
 		// Integer.MAX_VALUE
 		int pos = this.position.incrementAndGet() & Integer.MAX_VALUE;

spring-cloud-loadbalancer/src/test/java/org/springframework/cloud/loadbalancer/core/RoundRobinLoadBalancerTests.java

@@ -17,11 +17,13 @@
 package org.springframework.cloud.loadbalancer.core;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Flux;
 
+import org.springframework.cloud.client.DefaultServiceInstance;
 import org.springframework.cloud.client.ServiceInstance;
 import org.springframework.cloud.loadbalancer.support.SimpleObjectProvider;
 
@@ -52,6 +54,20 @@ class RoundRobinLoadBalancerTests {
 		assertOrderEnforced(MAX_VALUE);
 	}
 
+	@Test
+	void shouldNotMovePositionIfOnlyOneInstance() {
+		ServiceInstanceListSupplier supplier = mock(ServiceInstanceListSupplier.class);
+		when(supplier.get(any())).thenReturn(Flux.just(Collections.singletonList(new DefaultServiceInstance())));
+		RoundRobinLoadBalancer loadBalancer = new RoundRobinLoadBalancer(new SimpleObjectProvider<>(supplier),
+				"shouldNotMovePositionIfOnlyOneInstance", 0);
+
+		loadBalancer.choose().block();
+		assertThat(loadBalancer.position).hasValue(0);
+
+		loadBalancer.choose().block();
+		assertThat(loadBalancer.position).hasValue(0);
+	}
+
 	@SuppressWarnings("all")
 	void assertOrderEnforced(int seed) {
 		List<ServiceInstance> instances = new ArrayList<>();