Add a test for Gradle incremental compilation with AutoValue.

value/src/it/functional/pom.xml

@@ -90,6 +90,12 @@
       <artifactId>compile-testing</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>dev.gradleplugins</groupId>
+      <artifactId>gradle-test-kit</artifactId>
+      <version>6.8.3</version>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.mockito</groupId>
       <artifactId>mockito-core</artifactId>
@@ -194,6 +200,9 @@
         <version>2.22.2</version>
         <configuration>
           <argLine>${test.jvm.flags}</argLine>
+          <systemPropertyVariables>
+            <autoValueVersion>${project.version}</autoValueVersion>
+          </systemPropertyVariables>
         </configuration>
       </plugin>
     </plugins>

value/src/it/functional/src/test/java/com/google/auto/value/CompileWithEclipseTest.java

@@ -66,7 +66,8 @@ public static void setSourceRoot() {
   public @Rule TemporaryFolder tmp = new TemporaryFolder();
 
   private static final ImmutableSet<String> IGNORED_TEST_FILES =
-      ImmutableSet.of("AutoValueNotEclipseTest.java", "CompileWithEclipseTest.java");
+      ImmutableSet.of(
+          "AutoValueNotEclipseTest.java", "CompileWithEclipseTest.java", "GradleTest.java");
 
   private static final Predicate<File> JAVA_FILE =
       f -> f.getName().endsWith(".java") && !IGNORED_TEST_FILES.contains(f.getName());

value/src/it/functional/src/test/java/com/google/auto/value/GradleTest.java

@@ -0,0 +1,176 @@
+/*
+ * Copyright 2021 Google LLC
+ *
+ * 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
+ *
+ * http://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 com.google.auto.value;
+
+import static com.google.common.truth.Truth.assertThat;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.collect.ImmutableList;
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Optional;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import java.util.stream.Stream;
+import org.gradle.testkit.runner.BuildResult;
+import org.gradle.testkit.runner.GradleRunner;
+import org.gradle.util.GradleVersion;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class GradleTest {
+  @Rule public TemporaryFolder fakeProject = new TemporaryFolder();
+
+  private static final String BUILD_GRADLE_TEXT =
+      String.join("\n",
+          "plugins {",
+          "  id 'java-library'",
+          "}",
+          "repositories {",
+          "  maven { url = uri('${localRepository}') }",
+          "}",
+          "dependencies {",
+          "  compileOnlyApi      'com.google.auto.value:auto-value-annotations:${autoValueVersion}'",
+          "  annotationProcessor 'com.google.auto.value:auto-value:${autoValueVersion}'",
+          "}");
+
+  private static final String FOO_TEXT =
+      String.join("\n",
+          "package com.example;",
+          "",
+          "import com.google.auto.value.AutoValue;",
+          "",
+          "@AutoValue",
+          "abstract class Foo {",
+          "  abstract String bar();",
+          "",
+          "  static Foo of(String bar) {",
+          "    return new AutoValue_Foo(bar);",
+          "  }",
+          "}");
+
+  private static final Optional<File> GRADLE_INSTALLATION = getGradleInstallation();
+
+  @Test
+  public void basic() throws IOException {
+    String autoValueVersion = System.getProperty("autoValueVersion");
+    assertThat(autoValueVersion).isNotNull();
+    String localRepository = System.getProperty("localRepository");
+    assertThat(localRepository).isNotNull();
+
+    // Set up the fake Gradle project.
+    String buildGradleText = expandSystemProperties(BUILD_GRADLE_TEXT);
+    writeFile(fakeProject.newFile("build.gradle").toPath(), buildGradleText);
+    Path srcDir = fakeProject.newFolder("src", "main", "java", "com", "example").toPath();
+    writeFile(srcDir.resolve("Foo.java"), FOO_TEXT);
+
+    // Build it the first time.
+    BuildResult result1 = buildFakeProject();
+    assertThat(result1.getOutput()).contains(
+        "Full recompilation is required because no incremental change information is available");
+    Path output = fakeProject.getRoot().toPath()
+        .resolve("build/classes/java/main/com/example/AutoValue_Foo.class");
+    assertThat(Files.exists(output)).isTrue();
+
+    // Add a source file to the project.
+    String barText = FOO_TEXT.replace("Foo", "Bar");
+    Path barFile = srcDir.resolve("Bar.java");
+    writeFile(barFile, barText);
+
+    // Build it a second time.
+    BuildResult result2 = buildFakeProject();
+    assertThat(result2.getOutput()).doesNotContain("Full recompilation is required");
+
+    // Remove the second source file and build a third time. If incremental annotation processing
+    // is not working, this will produce a message like this:
+    //   Full recompilation is required because com.google.auto.value.processor.AutoValueProcessor
+    //   is not incremental
+    Files.delete(barFile);
+    BuildResult result3 = buildFakeProject();
+    assertThat(result3.getOutput()).doesNotContain("Full recompilation is required");
+  }
+
+  private BuildResult buildFakeProject() throws IOException {
+    GradleRunner runner = GradleRunner.create()
+        .withProjectDir(fakeProject.getRoot())
+        .withArguments("--info", "compileJava");
+    if (GRADLE_INSTALLATION.isPresent()) {
+      runner.withGradleInstallation(GRADLE_INSTALLATION.get());
+    } else {
+      runner.withGradleVersion(GradleVersion.current().getVersion());
+    }
+    return runner.build();
+  }
+
+  private static Optional<File> getGradleInstallation() {
+    String gradleHome = System.getenv("GRADLE_HOME");
+    if (gradleHome != null) {
+      File gradleHomeFile = new File(gradleHome);
+      if (gradleHomeFile.isDirectory()) {
+        return Optional.of(new File(gradleHome));
+      }
+    }
+    try {
+      Path gradleExecutable = Paths.get("/usr/bin/gradle");
+      Path gradleLink =
+          gradleExecutable.resolveSibling(Files.readSymbolicLink(gradleExecutable));
+      if (!gradleLink.endsWith("bin/gradle")) {
+        return Optional.empty();
+      }
+      Path installationPath = gradleLink.getParent().getParent();
+      if (!Files.isDirectory(installationPath)) {
+        return Optional.empty();
+      }
+      Optional<Path> coreJar;
+      Pattern corePattern = Pattern.compile("gradle-core-([0-9]+)\\..*\\.jar");
+      try (Stream<Path> files = Files.walk(installationPath.resolve("lib"))) {
+        coreJar =
+            files.filter(p -> {
+              Matcher matcher = corePattern.matcher(p.getFileName().toString());
+              if (matcher.matches()) {
+                int version = Integer.parseInt(matcher.group(1));
+                if (version >= 5) {
+                  return true;
+                }
+              }
+              return false;
+            }).findFirst();
+      }
+      return coreJar.map(unused -> installationPath.toFile());
+    } catch (IOException e) {
+      return Optional.empty();
+    }
+  }
+
+  private static String expandSystemProperties(String s) {
+    for (String name : System.getProperties().stringPropertyNames()) {
+      String value = System.getProperty(name);
+      s = s.replace("${" + name + "}", value);
+    }
+    return s;
+  }
+
+  private static void writeFile(Path file, String text) throws IOException {
+    Files.write(file, ImmutableList.of(text), UTF_8);
+  }
+}