Provides configuration for Netty's http client pool leasing strategy (#3575)

docs/modules/ROOT/partials/_configprops.adoc

@@ -90,6 +90,7 @@
 |spring.cloud.gateway.httpclient.pool.metrics | `+++false+++` | Enables channel pools metrics to be collected and registered in Micrometer. Disabled by default.
 |spring.cloud.gateway.httpclient.pool.name | `+++proxy+++` | The channel pool map name, defaults to proxy.
 |spring.cloud.gateway.httpclient.pool.type | `+++elastic+++` | Type of pool for HttpClient to use, defaults to ELASTIC.
+|spring.cloud.gateway.httpclient.pool.leasingStrategy | `+++fifo+++` | Configures the leasing strategy for the pool, defaults to FIFO.
 |spring.cloud.gateway.httpclient.proxy.host |  | Hostname for proxy configuration of Netty HttpClient.
 |spring.cloud.gateway.httpclient.proxy.non-proxy-hosts-pattern |  | Regular expression (Java) for a configured list of hosts. that should be reached directly, bypassing the proxy
 |spring.cloud.gateway.httpclient.proxy.password |  | Password for proxy configuration of Netty HttpClient.

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/config/HttpClientFactory.java

@@ -33,6 +33,7 @@ import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.StringUtils;
 
+import static org.springframework.cloud.gateway.config.HttpClientProperties.Pool.LeasingStrategy.FIFO;
 import static org.springframework.cloud.gateway.config.HttpClientProperties.Pool.PoolType.DISABLED;
 import static org.springframework.cloud.gateway.config.HttpClientProperties.Pool.PoolType.FIXED;
 
@@ -189,6 +190,16 @@ public class HttpClientFactory extends AbstractFactoryBean<HttpClient> {
 			}
 			builder.evictInBackground(pool.getEvictionInterval());
 			builder.metrics(pool.isMetrics());
+
+			// Define the pool leasing strategy
+			if (pool.getLeasingStrategy() == FIFO) {
+				builder.fifo();
+			}
+			else {
+				// LIFO
+				builder.lifo();
+			}
+
 			connectionProvider = builder.build();
 		}
 		return connectionProvider;

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/config/HttpClientProperties.java

@@ -208,6 +208,11 @@ public class HttpClientProperties {
 		 */
 		private boolean metrics = false;
 
+		/**
+		 * Configures the leasing strategy for the pool, defaults to FIFO which is Netty's default.
+		 */
+		private LeasingStrategy leasingStrategy = LeasingStrategy.FIFO;
+
 		public PoolType getType() {
 			return type;
 		}
@@ -272,11 +277,20 @@ public class HttpClientProperties {
 			this.metrics = metrics;
 		}
 
+		public LeasingStrategy getLeasingStrategy() {
+			return leasingStrategy;
+		}
+
+		public void setLeasingStrategy(LeasingStrategy leasingStrategy) {
+			this.leasingStrategy = leasingStrategy;
+		}
+
 		@Override
 		public String toString() {
 			return "Pool{" + "type=" + type + ", name='" + name + '\'' + ", maxConnections=" + maxConnections
 					+ ", acquireTimeout=" + acquireTimeout + ", maxIdleTime=" + maxIdleTime + ", maxLifeTime="
-					+ maxLifeTime + ", evictionInterval=" + evictionInterval + ", metrics=" + metrics + '}';
+					+ maxLifeTime + ", evictionInterval=" + evictionInterval + ", metrics=" + metrics
+					+ ", leasingStrategy=" + leasingStrategy + '}';
 		}
 
 		public enum PoolType {
@@ -298,6 +312,20 @@ public class HttpClientProperties {
 
 		}
 
+		public enum LeasingStrategy {
+
+			/**
+			 * FIFO leasing strategy.
+			 */
+			FIFO,
+
+			/**
+			 * LIFO leasing strategy.
+			 */
+			LIFO
+
+		}
+
 	}
 
 	public static class Proxy {

spring-cloud-gateway-server/src/test/java/org/springframework/cloud/gateway/config/GatewayAutoConfigurationTests.java

@@ -52,6 +52,7 @@ import org.springframework.boot.test.context.runner.ReactiveWebApplicationContex
 import org.springframework.cloud.gateway.actuate.GatewayControllerEndpoint;
 import org.springframework.cloud.gateway.actuate.GatewayLegacyControllerEndpoint;
 import org.springframework.cloud.gateway.config.GatewayAutoConfigurationTests.CustomHttpClientFactory.CustomSslConfigurer;
+import org.springframework.cloud.gateway.config.HttpClientProperties.Pool.LeasingStrategy;
 import org.springframework.cloud.gateway.filter.factory.TokenRelayGatewayFilterFactory;
 import org.springframework.cloud.gateway.filter.headers.GRPCRequestHeadersFilter;
 import org.springframework.cloud.gateway.filter.headers.GRPCResponseHeadersFilter;
@@ -119,6 +120,7 @@ public class GatewayAutoConfigurationTests {
 					"spring.cloud.gateway.httpclient.pool.eviction-interval=10s",
 					"spring.cloud.gateway.httpclient.pool.type=fixed",
 					"spring.cloud.gateway.httpclient.pool.metrics=true",
+					"spring.cloud.gateway.httpclient.pool.leasing-strategy=lifo",
 					"spring.cloud.gateway.httpclient.compression=true", "spring.cloud.gateway.httpclient.wiretap=true",
 					// greater than integer max value
 					"spring.cloud.gateway.httpclient.max-initial-line-length=2147483647",
@@ -133,6 +135,7 @@ public class GatewayAutoConfigurationTests {
 				assertThat(properties.isCompression()).isEqualTo(true);
 				assertThat(properties.getPool().getEvictionInterval()).hasSeconds(10);
 				assertThat(properties.getPool().isMetrics()).isEqualTo(true);
+				assertThat(properties.getPool().getLeasingStrategy()).isEqualTo(LeasingStrategy.LIFO);
 
 				assertThat(httpClient.configuration().isAcceptGzip()).isTrue();
 				assertThat(httpClient.configuration().loggingHandler()).isNotNull();