Fix prepare optimizations task not being cacheable (#898)

The task which prepares optimizations makes use of a properties file
as an input (the configuration of the optimizer). Unfortunately, this
properties file, generated by the config writer task, contains a
timestamp, which breaks cacheability.

This commit fixes the problem by making sure that comments from
the writer task are removed.

Fixes #888

aot-plugin/src/main/java/io/micronaut/gradle/aot/MicronautAOTConfigWriterTask.java

@@ -45,12 +45,13 @@
 import org.gradle.api.tasks.PathSensitivity;
 import org.gradle.api.tasks.TaskAction;
 
+import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.FileInputStream;
-import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
-import java.io.OutputStream;
+import java.io.PrintWriter;
+import java.nio.file.Files;
 import java.util.List;
 import java.util.Properties;
 
@@ -60,6 +61,8 @@
  */
 public abstract class MicronautAOTConfigWriterTask extends DefaultTask {
 
+    public static final String GENERATED_BY_GRADLE_COMMENT = "Generated by Gradle";
+
     @InputFile
     @PathSensitive(PathSensitivity.NONE)
     @Optional
@@ -140,8 +143,22 @@ void writeConfigFile() {
         }
         File outputFile = getOutputFile().getAsFile().get();
         if (outputFile.getParentFile().isDirectory() || outputFile.getParentFile().mkdirs()) {
-            try (OutputStream out = new FileOutputStream(outputFile)) {
-                props.store(out, "Generated by Gradle");
+            var baos = new ByteArrayOutputStream();
+            try {
+                props.store(baos, GENERATED_BY_GRADLE_COMMENT);
+            } catch (IOException e) {
+                throw new GradleException("Unable to write output file: " + outputFile, e);
+            }
+            String content = baos.toString();
+            try (var writer = new PrintWriter(Files.newBufferedWriter(outputFile.toPath()))) {
+                content.lines()
+                    .filter(line -> {
+                        if (line.startsWith("#") && !line.contains(GENERATED_BY_GRADLE_COMMENT)) {
+                            return false;
+                        }
+                        return true;
+                    })
+                    .forEach(writer::println);
             } catch (IOException e) {
                 throw new GradleException("Unable to write output file: " + outputFile, e);
             }

functional-tests/src/test/groovy/io/micronaut/gradle/aot/BasicMicronautAOTSpec.groovy

@@ -245,6 +245,25 @@ class BasicMicronautAOTSpec extends AbstractAOTPluginSpec {
 
     }
 
+    @Issue("https://github.com/micronaut-projects/micronaut-gradle-plugin/issues/888")
+    def "prepare optimizations task is cacheable"() {
+        withSample("aot/basic-app")
+        withPlugins(Plugins.MINIMAL_APPLICATION)
+        file("gradle.properties") << "org.gradle.caching=true"
+
+        when:
+        def result = build "prepareJitOptimizations"
+
+        then:
+        result.task(":prepareJitOptimizations").outcome == TaskOutcome.SUCCESS
+
+        when:
+        result = build "clean", "prepareJitOptimizations"
+
+        then:
+        result.task(":prepareJitOptimizations").outcome == TaskOutcome.FROM_CACHE
+    }
+
     private List<GString> calculatePossiblePackages(File outputDir) {
         def list = new ArrayList()
         outputDir.eachDirRecurse { list.add(subpath(it, outputDir)) }
@@ -256,4 +275,4 @@ class BasicMicronautAOTSpec extends AbstractAOTPluginSpec {
         path.substring(basePath.getAbsolutePath().length() + 1, path.size()).replaceAll("\\\\", "/")
     }
 
-}
+}