Schedule blocking ResourceHttpMessageWriter operations on bounded elastic

This commit schedules blocking I/O operations on the bounded elastic
scheduler, which includes retrieving the content length and writing
the resource (region).

Closes gh-30928

spring-web/src/main/java/org/springframework/http/codec/ResourceHttpMessageWriter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -20,12 +20,12 @@ import java.io.File;
 import java.io.IOException;
 import java.util.List;
 import java.util.Map;
-import java.util.Optional;
 
 import org.apache.commons.logging.Log;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.Hints;
@@ -116,39 +116,61 @@ public class ResourceHttpMessageWriter implements HttpMessageWriter<Resource> {
 	private Mono<Void> writeResource(Resource resource, ResolvableType type, @Nullable MediaType mediaType,
 			ReactiveHttpOutputMessage message, Map<String, Object> hints) {
 
-		addHeaders(message, resource, mediaType, hints);
-
-		return zeroCopy(resource, null, message, hints)
-				.orElseGet(() -> {
-					Mono<Resource> input = Mono.just(resource);
-					DataBufferFactory factory = message.bufferFactory();
-					Flux<DataBuffer> body = this.encoder.encode(input, factory, type, message.getHeaders().getContentType(), hints);
-					if (logger.isDebugEnabled()) {
-						body = body.doOnNext(buffer -> Hints.touchDataBuffer(buffer, hints, logger));
+		return addDefaultHeaders(message, resource, mediaType, hints)
+				.then(Mono.defer(() -> {
+					Mono<Void> result = zeroCopy(resource, null, message, hints);
+					if (result != null) {
+						return result;
+					}
+					else {
+						Mono<Resource> input = Mono.just(resource);
+						DataBufferFactory factory = message.bufferFactory();
+						Flux<DataBuffer> body = this.encoder.encode(input, factory, type, message.getHeaders().getContentType(), hints)
+								.subscribeOn(Schedulers.boundedElastic());
+						if (logger.isDebugEnabled()) {
+							body = body.doOnNext(buffer -> Hints.touchDataBuffer(buffer, hints, logger));
+						}
+						return message.writeWith(body);
 					}
-					return message.writeWith(body);
-				});
+				}));
 	}
 
 	/**
 	 * Adds the default headers for the given resource to the given message.
 	 * @since 6.0
+	 * @deprecated since 6.1, in favor of {@link #addDefaultHeaders(ReactiveHttpOutputMessage, Resource, MediaType, Map)},
+	 * for removal = 6.2
 	 */
+	@Deprecated(since = "6.1", forRemoval = true)
 	public void addHeaders(ReactiveHttpOutputMessage message, Resource resource, @Nullable MediaType contentType, Map<String, Object> hints) {
-		HttpHeaders headers = message.getHeaders();
-		MediaType resourceMediaType = getResourceMediaType(contentType, resource, hints);
-		headers.setContentType(resourceMediaType);
-
-		if (headers.getContentLength() < 0) {
-			long length = lengthOf(resource);
-			if (length != -1) {
-				headers.setContentLength(length);
+		addDefaultHeaders(message, resource, contentType, hints).block();
+	}
+
+	/**
+	 * Adds the default headers for the given resource to the given message.
+	 * @since 6.1
+	 */
+	public Mono<Void> addDefaultHeaders(ReactiveHttpOutputMessage message, Resource resource, @Nullable MediaType contentType, Map<String, Object> hints) {
+		return Mono.defer(() -> {
+			HttpHeaders headers = message.getHeaders();
+			MediaType resourceMediaType = getResourceMediaType(contentType, resource, hints);
+			headers.setContentType(resourceMediaType);
+			if (message instanceof ServerHttpResponse) {
+				// server side
+				headers.set(HttpHeaders.ACCEPT_RANGES, "bytes");
 			}
-		}
-		if (message instanceof ServerHttpResponse) {
-			// server side
-			headers.set(HttpHeaders.ACCEPT_RANGES, "bytes");
-		}
+
+			if (headers.getContentLength() < 0) {
+				return lengthOf(resource)
+						.flatMap(contentLength -> {
+							headers.setContentLength(contentLength);
+							return Mono.empty();
+						});
+			}
+			else {
+				return Mono.empty();
+			}
+		});
 	}
 
 	private static MediaType getResourceMediaType(
@@ -164,19 +186,21 @@ public class ResourceHttpMessageWriter implements HttpMessageWriter<Resource> {
 		return mediaType;
 	}
 
-	private static long lengthOf(Resource resource) {
+	private static Mono<Long> lengthOf(Resource resource) {
 		// Don't consume InputStream...
 		if (InputStreamResource.class != resource.getClass()) {
-			try {
-				return resource.contentLength();
-			}
-			catch (IOException ignored) {
-			}
+			return Mono.fromCallable(resource::contentLength)
+					.filter(length -> length != -1)
+					.onErrorResume(IOException.class, t -> Mono.empty())
+					.subscribeOn(Schedulers.boundedElastic());
+		}
+		else {
+			return Mono.empty();
 		}
-		return -1;
 	}
 
-	private static Optional<Mono<Void>> zeroCopy(Resource resource, @Nullable ResourceRegion region,
+	@Nullable
+	private static Mono<Void> zeroCopy(Resource resource, @Nullable ResourceRegion region,
 			ReactiveHttpOutputMessage message, Map<String, Object> hints) {
 
 		if (message instanceof ZeroCopyHttpOutputMessage zeroCopyHttpOutputMessage && resource.isFile()) {
@@ -188,13 +212,13 @@ public class ResourceHttpMessageWriter implements HttpMessageWriter<Resource> {
 					String formatted = region != null ? "region " + pos + "-" + (count) + " of " : "";
 					logger.debug(Hints.getLogPrefix(hints) + "Zero-copy " + formatted + "[" + resource + "]");
 				}
-				return Optional.of(zeroCopyHttpOutputMessage.writeWith(file, pos, count));
+				return zeroCopyHttpOutputMessage.writeWith(file, pos, count);
 			}
 			catch (IOException ex) {
 				// should not happen
 			}
 		}
-		return Optional.empty();
+		return null;
 	}
 
 
@@ -227,15 +251,16 @@ public class ResourceHttpMessageWriter implements HttpMessageWriter<Resource> {
 			if (regions.size() == 1){
 				ResourceRegion region = regions.get(0);
 				headers.setContentType(resourceMediaType);
-				long contentLength = lengthOf(resource);
-				if (contentLength != -1) {
-					long start = region.getPosition();
-					long end = start + region.getCount() - 1;
-					end = Math.min(end, contentLength - 1);
-					headers.add("Content-Range", "bytes " + start + '-' + end + '/' + contentLength);
-					headers.setContentLength(end - start + 1);
-				}
-				return writeSingleRegion(region, response, hints);
+				return lengthOf(resource)
+						.flatMap(contentLength -> {
+							long start = region.getPosition();
+							long end = start + region.getCount() - 1;
+							end = Math.min(end, contentLength - 1);
+							headers.add("Content-Range", "bytes " + start + '-' + end + '/' + contentLength);
+							headers.setContentLength(end - start + 1);
+							return Mono.empty();
+						})
+						.then(writeSingleRegion(region, response, hints));
 			}
 			else {
 				String boundary = MimeTypeUtils.generateMultipartBoundaryString();
@@ -250,19 +275,23 @@ public class ResourceHttpMessageWriter implements HttpMessageWriter<Resource> {
 	private Mono<Void> writeSingleRegion(ResourceRegion region, ReactiveHttpOutputMessage message,
 			Map<String, Object> hints) {
 
-		return zeroCopy(region.getResource(), region, message, hints)
-				.orElseGet(() -> {
-					Publisher<? extends ResourceRegion> input = Mono.just(region);
-					MediaType mediaType = message.getHeaders().getContentType();
-					return encodeAndWriteRegions(input, mediaType, message, hints);
-				});
+		Mono<Void> result = zeroCopy(region.getResource(), region, message, hints);
+		if (result != null) {
+			return result;
+		}
+		else {
+			Publisher<? extends ResourceRegion> input = Mono.just(region);
+			MediaType mediaType = message.getHeaders().getContentType();
+			return encodeAndWriteRegions(input, mediaType, message, hints);
+		}
 	}
 
 	private Mono<Void> encodeAndWriteRegions(Publisher<? extends ResourceRegion> publisher,
 			@Nullable MediaType mediaType, ReactiveHttpOutputMessage message, Map<String, Object> hints) {
 
-		Flux<DataBuffer> body = this.regionEncoder.encode(
-				publisher, message.bufferFactory(), REGION_TYPE, mediaType, hints);
+		Flux<DataBuffer> body = this.regionEncoder
+				.encode(publisher, message.bufferFactory(), REGION_TYPE, mediaType,hints)
+				.subscribeOn(Schedulers.boundedElastic());
 
 		return message.writeWith(body);
 	}

spring-webflux/src/main/java/org/springframework/web/reactive/resource/ResourceWebHandler.java

@@ -437,9 +437,9 @@ public class ResourceWebHandler implements WebHandler, InitializingBean {
 						ResourceHttpMessageWriter writer = getResourceHttpMessageWriter();
 						Assert.state(writer != null, "No ResourceHttpMessageWriter");
 						if (HttpMethod.HEAD == httpMethod) {
-							writer.addHeaders(exchange.getResponse(), resource, mediaType,
-									Hints.from(Hints.LOG_PREFIX_HINT, exchange.getLogPrefix()));
-							return exchange.getResponse().setComplete();
+							return writer.addDefaultHeaders(exchange.getResponse(), resource, mediaType,
+											Hints.from(Hints.LOG_PREFIX_HINT, exchange.getLogPrefix()))
+									.then(exchange.getResponse().setComplete());
 						}
 						else {
 							return writer.write(Mono.just(resource),