Implementing workaround for not being able to set java task classpath statically and therefore making it unable to detect changes in kotlin classes

KT-8487

libraries/tools/kotlin-gradle-plugin-core/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -62,6 +62,8 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments>() : AbstractCo
     var kotlinOptions: T = createBlankArgs()
     var kotlinDestinationDir: File? = destinationDir
     var compilerCalled: Boolean = false
+    // TODO: consider more reliable approach (see usage)
+    var anyClassesCompiled: Boolean = false
 
     private val loggerInstance = Logging.getLogger(this.javaClass)
     override fun getLogger() = loggerInstance
@@ -361,6 +363,8 @@ open class KotlinCompile() : AbstractKotlinCompile<K2JVMCompilerArguments>() {
         fun outputRelativePath(f: File) = f.toRelativeString(outputDir)
 
 
+        anyClassesCompiled = false
+
         // TODO: decide what to do if no files are considered dirty - rebuild or skip the module
         var (sourcesToCompile, isIncrementalDecided) = calculateSourcesToCompile()
 
@@ -450,6 +454,9 @@ open class KotlinCompile() : AbstractKotlinCompile<K2JVMCompilerArguments>() {
         lookupStorage.flush(false)
         lookupStorage.close()
         caches.values.forEach { it.flush(false); it.close() }
+        if (allGeneratedFiles.isNotEmpty()) {
+            anyClassesCompiled = true
+        }
     }
 
     private data class CompileChangedResults(val exitCode: ExitCode, val generatedFiles: List<GeneratedFile<TargetId>>)

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

@@ -133,6 +133,20 @@ class Kotlin2JvmSourceSetProcessor(
         val javaTask = project.tasks.findByName(sourceSet.compileJavaTaskName) as AbstractCompile?
 
         if (javaTask != null) {
+            // Since we cannot update classpath statically, java not able to detect changes in the classpath after kotlin compiler.
+            // Therefore this (probably inefficient since java cannot decide "uptodateness" by the list of changed class files, but told
+            // explicitly being out of date whenever any kotlin files are compiled
+            kotlinTask.property("anyClassesCompiled")?.let {
+                kotlinTask.setProperty("anyClassesCompiled", false)
+                javaTask.outputs.upToDateWhen { task ->
+                    val kotlinClassesCompiled = kotlinTask.property("anyClassesCompiled") as? Boolean ?: false
+                    if (kotlinClassesCompiled) {
+                        logger.info("Marking $task out of date, because kotlin classes are changed")
+                    }
+                    !kotlinClassesCompiled
+                }
+            }
+
             javaTask.dependsOn(kotlinTaskName)
             javaTask.doFirst {
                 javaTask.classpath += project.files(kotlinDestinationDir)