Make FileSystemWatcherTests more robust

Previously, several tests in FileSystemWatcherTests assumed that all
of the changes detected by the watcher would be grouped into a single
change set. This assumption breaks down when a test runs slowly (due
to CPU or IO contention, for example), and making changes to the file
system takes long then the watcher's polling interval. When this
happens, the changes will be split across two (or more).

This commit attempts to make the tests more robust. The tests now
tolerate multiple changes sets by combining them and asserting that
across the n change sets, only the expected changes were detected.

Closes gh-25901

spring-boot-project/spring-boot-devtools/src/test/java/org/springframework/boot/devtools/filewatch/FileSystemWatcherTests.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -23,10 +23,10 @@ import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashSet;
-import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
 import java.util.UUID;
+import java.util.stream.Collectors;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -44,6 +44,7 @@ import static org.mockito.Mockito.mock;
  * Tests for {@link FileSystemWatcher}.
  *
  * @author Phillip Webb
+ * @author Andy Wilkinson
  */
 class FileSystemWatcherTests {
 
@@ -120,9 +121,8 @@ class FileSystemWatcherTests {
 		File directory = startWithNewDirectory();
 		File file = touch(new File(directory, "test.txt"));
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
 		ChangedFile expected = new ChangedFile(directory, file, Type.ADD);
-		assertThat(changedFiles.getFiles()).contains(expected);
+		assertThat(getAllFileChanges()).containsExactly(expected);
 	}
 
 	@Test
@@ -130,9 +130,8 @@ class FileSystemWatcherTests {
 		File directory = startWithNewDirectory();
 		File file = touch(new File(new File(directory, "sub"), "text.txt"));
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
 		ChangedFile expected = new ChangedFile(directory, file, Type.ADD);
-		assertThat(changedFiles.getFiles()).contains(expected);
+		assertThat(getAllFileChanges()).containsExactly(expected);
 	}
 
 	@Test
@@ -144,9 +143,8 @@ class FileSystemWatcherTests {
 		directory.mkdirs();
 		File file = touch(new File(directory, "text.txt"));
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
 		ChangedFile expected = new ChangedFile(directory, file, Type.ADD);
-		assertThat(changedFiles.getFiles()).contains(expected);
+		assertThat(getAllFileChanges()).containsExactly(expected);
 	}
 
 	@Test
@@ -171,8 +169,7 @@ class FileSystemWatcherTests {
 			Thread.sleep(10);
 		}
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
-		assertThat(changedFiles.getFiles()).hasSize(100);
+		assertThat(getAllFileChanges()).hasSize(100);
 	}
 
 	@Test
@@ -184,9 +181,8 @@ class FileSystemWatcherTests {
 		this.watcher.start();
 		File file = touch(new File(directory, "test2.txt"));
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
 		ChangedFile expected = new ChangedFile(directory, file, Type.ADD);
-		assertThat(changedFiles.getFiles()).contains(expected);
+		assertThat(getAllFileChanges()).contains(expected);
 	}
 
 	@Test
@@ -201,7 +197,7 @@ class FileSystemWatcherTests {
 		File file1 = touch(new File(directory1, "test.txt"));
 		File file2 = touch(new File(directory2, "test.txt"));
 		this.watcher.stopAfter(1);
-		Set<ChangedFiles> change = getSingleOnChange();
+		Set<ChangedFiles> change = this.changes.stream().flatMap(Set<ChangedFiles>::stream).collect(Collectors.toSet());
 		assertThat(change.size()).isEqualTo(2);
 		for (ChangedFiles changedFiles : change) {
 			if (changedFiles.getSourceDirectory().equals(directory1)) {
@@ -219,16 +215,16 @@ class FileSystemWatcherTests {
 	void multipleListeners() throws Exception {
 		File directory = new File(this.tempDir, UUID.randomUUID().toString());
 		directory.mkdir();
-		final Set<ChangedFiles> listener2Changes = new LinkedHashSet<>();
+		final List<Set<ChangedFiles>> listener2Changes = new ArrayList<>();
 		this.watcher.addSourceDirectory(directory);
-		this.watcher.addListener(listener2Changes::addAll);
+		this.watcher.addListener(listener2Changes::add);
 		this.watcher.start();
 		File file = touch(new File(directory, "test.txt"));
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
 		ChangedFile expected = new ChangedFile(directory, file, Type.ADD);
-		assertThat(changedFiles.getFiles()).contains(expected);
-		assertThat(listener2Changes).isEqualTo(this.changes.get(0));
+		Set<ChangedFile> changeSet = getAllFileChanges();
+		assertThat(changeSet).contains(expected);
+		assertThat(getAllFileChanges(listener2Changes)).isEqualTo(changeSet);
 	}
 
 	@Test
@@ -243,8 +239,7 @@ class FileSystemWatcherTests {
 		delete.delete();
 		File add = touch(new File(directory, "add.txt"));
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
-		Set<ChangedFile> actual = changedFiles.getFiles();
+		Set<ChangedFile> actual = getAllFileChanges();
 		Set<ChangedFile> expected = new HashSet<>();
 		expected.add(new ChangedFile(directory, modify, Type.MODIFY));
 		expected.add(new ChangedFile(directory, delete, Type.DELETE));
@@ -266,8 +261,7 @@ class FileSystemWatcherTests {
 		assertThat(this.changes).isEmpty();
 		FileCopyUtils.copy("abc".getBytes(), trigger);
 		this.watcher.stopAfter(1);
-		ChangedFiles changedFiles = getSingleChangedFiles();
-		Set<ChangedFile> actual = changedFiles.getFiles();
+		Set<ChangedFile> actual = getAllFileChanges();
 		Set<ChangedFile> expected = new HashSet<>();
 		expected.add(new ChangedFile(directory, file, Type.MODIFY));
 		assertThat(actual).isEqualTo(expected);
@@ -286,15 +280,13 @@ class FileSystemWatcherTests {
 		return directory;
 	}
 
-	private ChangedFiles getSingleChangedFiles() {
-		Set<ChangedFiles> singleChange = getSingleOnChange();
-		assertThat(singleChange).hasSize(1);
-		return singleChange.iterator().next();
+	private Set<ChangedFile> getAllFileChanges() {
+		return getAllFileChanges(this.changes);
 	}
 
-	private Set<ChangedFiles> getSingleOnChange() {
-		assertThat(this.changes).hasSize(1);
-		return this.changes.get(0);
+	private Set<ChangedFile> getAllFileChanges(List<Set<ChangedFiles>> changes) {
+		return changes.stream().flatMap(Set<ChangedFiles>::stream)
+				.flatMap((changedFiles) -> changedFiles.getFiles().stream()).collect(Collectors.toSet());
 	}
 
 	private File touch(File file) throws IOException {