Fall back on default server response status code

Update the ServerHttpRespnose contract to indicate that server specific
sub-classes should fall back on the default status, if a status code
has not been set explicitly.

Issue: SPR-17368

spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java

@@ -106,7 +106,7 @@ public abstract class AbstractServerHttpResponse implements ServerHttpResponse {
 	@Override
 	@Nullable
 	public HttpStatus getStatusCode() {
-		return (this.statusCode != null ? HttpStatus.resolve(this.statusCode) : null);
+		return this.statusCode != null ? HttpStatus.resolve(this.statusCode) : null;
 	}
 
 	/**

spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java

@@ -31,6 +31,7 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.NettyDataBufferFactory;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseCookie;
 import org.springframework.http.ZeroCopyHttpOutputMessage;
 import org.springframework.util.Assert;
@@ -60,12 +61,23 @@ class ReactorServerHttpResponse extends AbstractServerHttpResponse implements Ze
 		return (T) this.response;
 	}
 
+	@Override
+	@SuppressWarnings("ConstantConditions")
+	public HttpStatus getStatusCode() {
+		HttpStatus httpStatus = super.getStatusCode();
+		if (httpStatus == null) {
+			HttpResponseStatus status = this.response.status();
+			httpStatus = status != null ? HttpStatus.resolve(status.code()) : null;
+		}
+		return httpStatus;
+	}
+
 
 	@Override
 	protected void applyStatusCode() {
 		Integer statusCode = getStatusCodeValue();
 		if (statusCode != null) {
-			this.response.status(HttpResponseStatus.valueOf(statusCode));
+			this.response.status(statusCode);
 		}
 	}
 

spring-web/src/main/java/org/springframework/http/server/reactive/ServerHttpResponse.java

 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,13 +34,17 @@ public interface ServerHttpResponse extends ReactiveHttpOutputMessage {
 	/**
 	 * Set the HTTP status code of the response.
 	 * @param status the HTTP status as an {@link HttpStatus} enum value
-	 * @return {@code false} if the status code has not been set because the HTTP response
-	 * is already committed, {@code true} if it has been set correctly.
+	 * @return {@code false} if the status code has not been set because the
+	 * HTTP response is already committed, {@code true} if successfully set.
 	 */
 	boolean setStatusCode(@Nullable HttpStatus status);
 
 	/**
-	 * Return the HTTP status code or {@code null} if not set.
+	 * Return the status code set via {@link #setStatusCode}, or if the status
+	 * has not been set, return the default status code from the underlying
+	 * server response. The return value may be {@code null} if the status code
+	 * value is outside the {@link HttpStatus} enum range, or if the underlying
+	 * server response does not have a default value.
 	 */
 	@Nullable
 	HttpStatus getStatusCode();

spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpResponse.java

@@ -34,6 +34,7 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseCookie;
 import org.springframework.lang.Nullable;
@@ -96,6 +97,12 @@ class ServletServerHttpResponse extends AbstractListenerServerHttpResponse {
 		return (T) this.response;
 	}
 
+	@Override
+	public HttpStatus getStatusCode() {
+		HttpStatus httpStatus = super.getStatusCode();
+		return httpStatus != null ? httpStatus : HttpStatus.resolve(this.response.getStatus());
+	}
+
 	@Override
 	protected void applyStatusCode() {
 		Integer statusCode = getStatusCodeValue();

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java

@@ -35,6 +35,7 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseCookie;
 import org.springframework.http.ZeroCopyHttpOutputMessage;
 import org.springframework.lang.Nullable;
@@ -80,6 +81,12 @@ class UndertowServerHttpResponse extends AbstractListenerServerHttpResponse impl
 		return (T) this.exchange;
 	}
 
+	@Override
+	public HttpStatus getStatusCode() {
+		HttpStatus httpStatus = super.getStatusCode();
+		return httpStatus != null ? httpStatus : HttpStatus.resolve(this.exchange.getStatusCode());
+	}
+
 
 	@Override
 	protected void applyStatusCode() {