Ensure DefaultPartHttpMessageReader temp directories do not collide

This commit makes sure that the DefaultPartHttpMessageReader uses a
random temporary directory to store uploaded files, so that two
instances do not collide.

See gh-26931

spring-web/src/test/java/org/springframework/http/codec/multipart/FileStorageTests.java

@@ -0,0 +1,82 @@
+/*
+ * Copyright 2002-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http.codec.multipart;
+
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
+import reactor.test.StepVerifier;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class FileStorageTests {
+
+	@Test
+	void fromPath() throws IOException {
+		Path path = Files.createTempFile("spring", "test");
+		FileStorage storage = FileStorage.fromPath(path);
+
+		Mono<Path> directory = storage.directory();
+		StepVerifier.create(directory)
+				.expectNext(path)
+				.verifyComplete();
+	}
+
+	@Test
+	void tempDirectory() {
+		FileStorage storage = FileStorage.tempDirectory(Schedulers::boundedElastic);
+
+		Mono<Path> directory = storage.directory();
+		StepVerifier.create(directory)
+				.consumeNextWith(path -> {
+					assertThat(path).exists();
+					StepVerifier.create(directory)
+							.expectNext(path)
+							.verifyComplete();
+				})
+				.verifyComplete();
+	}
+
+	@Test
+	void tempDirectoryDeleted() {
+		FileStorage storage = FileStorage.tempDirectory(Schedulers::boundedElastic);
+
+		Mono<Path> directory = storage.directory();
+		StepVerifier.create(directory)
+				.consumeNextWith(path1 -> {
+					try {
+						Files.delete(path1);
+						StepVerifier.create(directory)
+								.consumeNextWith(path2 -> assertThat(path2).isNotEqualTo(path1))
+								.verifyComplete();
+					}
+					catch (IOException ex) {
+						throw new UncheckedIOException(ex);
+					}
+				})
+				.verifyComplete();
+	}
+
+}