Switch to Reactor 2020.0.0 snapshots

See gh-25373

build.gradle

@@ -25,7 +25,7 @@ configure(allprojects) { project ->
 		imports {
 			mavenBom "com.fasterxml.jackson:jackson-bom:2.11.0"
 			mavenBom "io.netty:netty-bom:4.1.50.Final"
-			mavenBom "io.projectreactor:reactor-bom:2020.0.0-M1"
+			mavenBom "io.projectreactor:reactor-bom:2020.0.0-SNAPSHOT"
 			mavenBom "io.r2dbc:r2dbc-bom:Arabba-SR5"
 			mavenBom "io.rsocket:rsocket-bom:1.0.1"
 			mavenBom "org.eclipse.jetty:jetty-bom:9.4.30.v20200611"
@@ -285,7 +285,7 @@ configure(allprojects) { project ->
 		repositories {
 			mavenCentral()
 			maven { url "https://repo.spring.io/libs-spring-framework-build" }
-			maven { url "https://repo.spring.io/milestone" } // Reactor
+			maven { url "https://repo.spring.io/snapshot" } // Reactor
 		}
 	}
 	configurations.all {

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpResponse.java

@@ -58,7 +58,7 @@ class ReactorClientHttpResponse implements ClientHttpResponse {
 
 	private final NettyDataBufferFactory bufferFactory;
 
-	// 0 - not subscribed, 1 - subscribed, 2 - cancelled, 3 - cancelled via connector (before subscribe)
+	// 0 - not subscribed, 1 - subscribed, 2 - cancelled via connector (before subscribe)
 	private final AtomicInteger state = new AtomicInteger(0);
 
 	private final String logPrefix;
@@ -100,20 +100,11 @@ class ReactorClientHttpResponse implements ClientHttpResponse {
 					if (this.state.compareAndSet(0, 1)) {
 						return;
 					}
-					// https://github.com/reactor/reactor-netty/issues/503
-					// FluxReceive rejects multiple subscribers, but not after a cancel().
-					// Subsequent subscribers after cancel() will not be rejected, but will hang instead.
-					// So we need to reject once in cancelled state.
 					if (this.state.get() == 2) {
-						throw new IllegalStateException(
-								"The client response body can only be consumed once.");
-					}
-					else if (this.state.get() == 3) {
 						throw new IllegalStateException(
 								"The client response body has been released already due to cancellation.");
 					}
 				})
-				.doOnCancel(() -> this.state.compareAndSet(1, 2))
 				.map(byteBuf -> {
 					byteBuf.retain();
 					return this.bufferFactory.wrap(byteBuf);
@@ -159,7 +150,7 @@ class ReactorClientHttpResponse implements ClientHttpResponse {
 	 * reading was delayed for some reason.
 	 */
 	void releaseAfterCancel(HttpMethod method) {
-		if (mayHaveBody(method) && this.state.compareAndSet(0, 3)) {
+		if (mayHaveBody(method) && this.state.compareAndSet(0, 2)) {
 			if (logger.isDebugEnabled()) {
 				logger.debug(this.logPrefix + "Releasing body, not yet subscribed.");
 			}