Let @TempDir fail fast if createTempDirectory does not return a directory

documentation/src/docs/asciidoc/release-notes/release-notes-5.11.0-RC1.adoc

@@ -60,7 +60,9 @@ repository on GitHub.
   `assertInstanceOf` methods introduced in `5.8` version.
 * New generators in `DynamicTest` that take a `Stream`/`Iterator` of `Named<Executable>`
   along with a convenient `NamedExecutable` interface that can simplify writing dynamic
-  tests, in particular in recent version of Java that support records.
+  tests, in particular in recent versions of Java that support records.
+* `@TempDir` now fails fast in case `TempDirFactory::createTempDirectory` returns
+  `null`, a file, or a symbolic link to a file.
 
 
 [[release-notes-5.11.0-RC1-junit-vintage]]

documentation/src/test/java/example/TempDirectoryDemo.java

@@ -137,7 +137,7 @@ static class JimfsTempDirFactory implements TempDirFactory {
 			@Override
 			public Path createTempDirectory(AnnotatedElementContext elementContext, ExtensionContext extensionContext)
 					throws IOException {
-				return Files.createTempDirectory(fileSystem.getPath("/"), "junit");
+				return Files.createTempDirectory(fileSystem.getPath("/"), "junit-");
 			}
 
 			@Override

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/extension/TempDirectory.java

@@ -284,14 +284,19 @@ static class CloseablePath implements CloseableResource {
 		private final CleanupMode cleanupMode;
 		private final ExtensionContext extensionContext;
 
-		CloseablePath(TempDirFactory factory, CleanupMode cleanupMode, AnnotatedElementContext elementContext,
+		private CloseablePath(TempDirFactory factory, CleanupMode cleanupMode, AnnotatedElementContext elementContext,
 				ExtensionContext extensionContext) throws Exception {
-			this.dir = factory.createTempDirectory(elementContext, extensionContext);
+			this.dir = validateTempDirectory(factory.createTempDirectory(elementContext, extensionContext));
 			this.factory = factory;
 			this.cleanupMode = cleanupMode;
 			this.extensionContext = extensionContext;
 		}
 
+		private static Path validateTempDirectory(Path dir) {
+			Preconditions.condition(dir != null && Files.isDirectory(dir), "temp directory must be a directory");
+			return dir;
+		}
+
 		Path get() {
 			return dir;
 		}

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/extension/CloseablePathTests.java

@@ -0,0 +1,236 @@
+/*
+ * Copyright 2015-2024 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.jupiter.engine.extension;
+
+import static java.nio.file.Files.createDirectory;
+import static java.nio.file.Files.createFile;
+import static java.nio.file.Files.createSymbolicLink;
+import static java.nio.file.Files.createTempDirectory;
+import static java.nio.file.Files.delete;
+import static java.nio.file.Files.deleteIfExists;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.junit.jupiter.api.io.CleanupMode.ALWAYS;
+import static org.junit.jupiter.api.io.CleanupMode.DEFAULT;
+import static org.junit.jupiter.api.io.CleanupMode.NEVER;
+import static org.junit.jupiter.api.io.CleanupMode.ON_SUCCESS;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.util.Optional;
+
+import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.AnnotatedElementContext;
+import org.junit.jupiter.api.extension.ExtensionConfigurationException;
+import org.junit.jupiter.api.extension.ExtensionContext;
+import org.junit.jupiter.api.extension.ExtensionContext.Namespace;
+import org.junit.jupiter.api.io.CleanupMode;
+import org.junit.jupiter.api.io.TempDir;
+import org.junit.jupiter.api.io.TempDirFactory;
+import org.junit.jupiter.engine.AbstractJupiterTestEngineTests;
+import org.junit.jupiter.engine.execution.NamespaceAwareStore;
+import org.junit.platform.commons.PreconditionViolationException;
+import org.junit.platform.engine.support.store.NamespacedHierarchicalStore;
+
+/**
+ * Integration tests for the creation and cleanup of the {@link TempDirectory}.
+ *
+ * @since 5.9
+ */
+@DisplayName("Temporary directory")
+class CloseablePathTests extends AbstractJupiterTestEngineTests {
+
+	private final AnnotatedElementContext elementContext = mock();
+	private final ExtensionContext extensionContext = mock();
+
+	private TempDirectory.CloseablePath closeablePath;
+
+	@BeforeEach
+	void setUpExtensionContext() {
+		var store = new NamespaceAwareStore(new NamespacedHierarchicalStore<>(null), Namespace.GLOBAL);
+		when(extensionContext.getStore(any())).thenReturn(store);
+	}
+
+	/**
+	 * Integration tests for the creation of the {@link TempDirectory} based on the different result
+	 * that {@link TempDirFactory#createTempDirectory(AnnotatedElementContext, ExtensionContext)} may provide.
+	 *
+	 * @since 5.11
+	 *
+	 * @see TempDirFactory
+	 */
+	@Nested
+	@DisplayName("creation")
+	class Creation {
+
+		private Path root;
+
+		@BeforeEach
+		void setUpRootFolder() throws IOException {
+			root = createTempDirectory("root");
+		}
+
+		@AfterEach
+		void cleanupRoot() throws IOException {
+			delete(root);
+		}
+
+		@Test
+		@DisplayName("succeeds if the factory returns a directory")
+		void factoryReturnsDirectory() throws Exception {
+			TempDirFactory factory = (elementContext, extensionContext) -> createDirectory(root.resolve("directory"));
+
+			closeablePath = TempDirectory.createTempDir(factory, DEFAULT, elementContext, extensionContext);
+			assertThat(closeablePath.get()).isDirectory();
+
+			delete(closeablePath.get());
+		}
+
+		@Test
+		@DisplayName("succeeds if the factory returns a symbolic link to a directory")
+		void factoryReturnsSymbolicLinkToDirectory() throws Exception {
+			Path directory = createDirectory(root.resolve("directory"));
+			TempDirFactory factory = (elementContext,
+					extensionContext) -> createSymbolicLink(root.resolve("symbolicLink"), directory);
+
+			closeablePath = TempDirectory.createTempDir(factory, DEFAULT, elementContext, extensionContext);
+			assertThat(closeablePath.get()).isDirectory();
+
+			delete(closeablePath.get());
+			delete(directory);
+		}
+
+		@Test
+		@DisplayName("fails if the factory returns null")
+		void factoryReturnsNull() {
+			TempDirFactory factory = (elementContext, extensionContext) -> null;
+
+			assertThatExtensionConfigurationExceptionIsThrownBy(
+				() -> TempDirectory.createTempDir(factory, DEFAULT, elementContext, extensionContext));
+		}
+
+		@Test
+		@DisplayName("fails if the factory returns a file")
+		void factoryReturnsFile() throws IOException {
+			Path file = createFile(root.resolve("file"));
+			TempDirFactory factory = (elementContext, extensionContext) -> file;
+
+			assertThatExtensionConfigurationExceptionIsThrownBy(
+				() -> TempDirectory.createTempDir(factory, DEFAULT, elementContext, extensionContext));
+
+			delete(file);
+		}
+
+		@Test
+		@DisplayName("fails if the factory returns a symbolic link to a file")
+		void factoryReturnsSymbolicLinkToFile() throws IOException {
+			Path file = createFile(root.resolve("file"));
+			Path symbolicLink = createSymbolicLink(root.resolve("symbolicLink"), file);
+			TempDirFactory factory = (elementContext, extensionContext) -> symbolicLink;
+
+			assertThatExtensionConfigurationExceptionIsThrownBy(
+				() -> TempDirectory.createTempDir(factory, DEFAULT, elementContext, extensionContext));
+
+			delete(symbolicLink);
+			delete(file);
+		}
+
+		private static void assertThatExtensionConfigurationExceptionIsThrownBy(ThrowingCallable callable) {
+			assertThatExceptionOfType(ExtensionConfigurationException.class)//
+					.isThrownBy(callable)//
+					.withMessage("Failed to create default temp directory")//
+					.withCauseInstanceOf(PreconditionViolationException.class)//
+					.havingCause().withMessage("temp directory must be a directory");
+		}
+
+	}
+
+	/**
+	 * Integration tests for cleanup of the {@link TempDirectory} when the {@link CleanupMode} is
+	 * set to {@link CleanupMode#ALWAYS}, {@link CleanupMode#NEVER}, or {@link CleanupMode#ON_SUCCESS}.
+	 *
+	 * @since 5.9
+	 *
+	 * @see TempDir
+	 * @see CleanupMode
+	 */
+	@Nested
+	@DisplayName("cleanup")
+	class Cleanup {
+
+		private final TempDirFactory factory = spy(TempDirFactory.Standard.INSTANCE);
+
+		@AfterEach
+		void cleanupTempDirectory() throws IOException {
+			deleteIfExists(closeablePath.get());
+		}
+
+		@Test
+		@DisplayName("is done for a cleanup mode of ALWAYS")
+		void always() throws IOException {
+			closeablePath = TempDirectory.createTempDir(factory, ALWAYS, elementContext, extensionContext);
+			assertThat(closeablePath.get()).isDirectory();
+
+			closeablePath.close();
+			assertThat(closeablePath.get()).doesNotExist();
+			verify(factory).close();
+		}
+
+		@Test
+		@DisplayName("is not done for a cleanup mode of NEVER")
+		void never() throws IOException {
+			closeablePath = TempDirectory.createTempDir(factory, NEVER, elementContext, extensionContext);
+			assertThat(closeablePath.get()).isDirectory();
+
+			closeablePath.close();
+			assertThat(closeablePath.get()).exists();
+			verify(factory).close();
+		}
+
+		@Test
+		@DisplayName("is not done for a cleanup mode of ON_SUCCESS, if there is an exception")
+		void onSuccessWithException() throws IOException {
+			when(extensionContext.getExecutionException()).thenReturn(Optional.of(new Exception()));
+
+			closeablePath = TempDirectory.createTempDir(factory, ON_SUCCESS, elementContext, extensionContext);
+			assertThat(closeablePath.get()).isDirectory();
+
+			closeablePath.close();
+			assertThat(closeablePath.get()).exists();
+			verify(factory).close();
+		}
+
+		@Test
+		@DisplayName("is done for a cleanup mode of ON_SUCCESS, if there is no exception")
+		void onSuccessWithNoException() throws IOException {
+			when(extensionContext.getExecutionException()).thenReturn(Optional.empty());
+
+			closeablePath = TempDirectory.createTempDir(factory, ON_SUCCESS, elementContext, extensionContext);
+			assertThat(closeablePath.get()).isDirectory();
+
+			closeablePath.close();
+			assertThat(closeablePath.get()).doesNotExist();
+			verify(factory).close();
+		}
+
+	}
+
+}