Kapt: fix sequential build with kapt when Kotlin source file was modified

Changelog.md

@@ -24,6 +24,9 @@
 - Drop line breaks between operator arguments (except '+', "-", "&&" and "||")
 - Add non-null assertions on call site for non-null parameters
 
+### Tools. Android
+- Fixed sequential build with kapt and stubs enabled when Kotlin source file was modified and no Java source files were modified
+
 ### IDE
 - Debugger can distinguish nested inline arguments
 - Add kotlinClassName() and kotlinFunctionName() macros for use in live templates

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/AnnotationProcessingManager.kt

@@ -24,6 +24,7 @@ import org.gradle.api.tasks.compile.JavaCompile
 import org.jetbrains.kotlin.gradle.plugin.*
 import java.io.File
 import java.io.IOException
+import java.util.*
 import java.util.zip.ZipFile
 
 fun Project.initKapt(
@@ -125,6 +126,7 @@ public class AnnotationProcessingManager(
         private val androidVariant: Any? = null) {
 
     private val project = task.project
+    private val random = Random()
 
     private companion object {
         val JAVA_FQNAME_PATTERN = "^([\\p{L}_$][\\p{L}\\p{N}_$]*\\.)*[\\p{L}_$][\\p{L}\\p{N}_$]*$".toRegex()
@@ -184,10 +186,16 @@ public class AnnotationProcessingManager(
         if (!javaHackPackageDir.exists()) javaHackPackageDir.mkdirs()
 
         val javaHackClFile = File(javaHackPackageDir, "Cl.java")
-        if (!javaHackClFile.exists()) {
-            javaHackClFile.writeText("package __gen.annotation; class Cl { @__gen.KotlinAptAnnotation boolean v; }")
-            project.logger.kotlinDebug("kapt: Java file stub generated: $javaHackClFile")
-        }
+        val previouslyExisted = javaHackClFile.exists()
+        val comment = System.currentTimeMillis().toString() + "-" + random.nextInt()
+
+        javaHackClFile.writeText(
+                "// $comment\n" +
+                        "package __gen.annotation;\n" +
+                        "class Cl { @__gen.KotlinAptAnnotation boolean v; }")
+
+        project.logger.kotlinDebug("kapt: Java file stub generated: $javaHackClFile " +
+                "(previously existed: $previouslyExisted)")
 
         if (!javaTask.source.contains(javaHackClFile)) {
             javaTask.source(javaAptSourceDir)

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/KotlinGradlePluginIT.kt

@@ -209,6 +209,26 @@ class KotlinGradleIT: BaseGradleIT() {
         }
     }
 
+    @Test
+    fun testKaptStubsIncrementalBuild() {
+        val project = Project("kaptStubs", "1.12")
+
+        project.build("build") {
+            assertSuccessful()
+
+            // Modify the Kotlin source file somehow
+            val someJavaFile = fileInWorkingDir("src/main/java/test.kt")
+            someJavaFile.appendText(" ")
+        }
+
+        project.build("build") {
+            assertSuccessful()
+            assertContains(":compileKotlin")
+            assertContains(":compileJava")
+            assertNotContains(":compileJava UP-TO-DATE")
+        }
+    }
+
     @Test
     fun testKaptArguments() {
         Project("kaptArguments", "1.12").build("build") {
@@ -248,5 +268,4 @@ class KotlinGradleIT: BaseGradleIT() {
             assertFileExists("build/classes/main/example/TestClassCustomized.class")
         }
     }
-
 }