Short circuit caching request body if already done.

Fixes gh-2969

Author: spencergibb

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/support/ServerWebExchangeUtils.java

@@ -361,6 +361,14 @@ public static <T> Mono<T> cacheRequestBody(ServerWebExchange exchange,
 	 */
 	private static <T> Mono<T> cacheRequestBody(ServerWebExchange exchange, boolean cacheDecoratedRequest,
 			Function<ServerHttpRequest, Mono<T>> function) {
+		// don't cache if body is already cached
+		Object cachedDataBuffer = exchange.getAttribute(CACHED_REQUEST_BODY_ATTR);
+		if (cachedDataBuffer instanceof DataBuffer) {
+			if (log.isTraceEnabled()) {
+				log.trace("body already in exchange attribute, short circuiting");
+			}
+			return Mono.just(exchange.getRequest()).flatMap(function);
+		}
 		ServerHttpResponse response = exchange.getResponse();
 		DataBufferFactory factory = response.bufferFactory();
 		// Join all the DataBuffers so we have a single DataBuffer for the body