Fix logging in workers

Setting `forkMode` to `ForkMode.NEVER` switches
`isolationMode` to `IsolationMode.CLASSLOADER`.
Classloader isolation mode prevented passing task's
logger to the unit of work.
Also it might resulted in multiple daemon instances starting
in parallel.

This change removes setting `forMode` (it was unnecessary).
Also a logger is used for the fallback message instead of printing it
to stderr. An assertion is added to tests, ensuring that
the fallback does not happen.

    #KT-29394 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/NewMultiplatformIT.kt

@@ -328,35 +328,6 @@ class NewMultiplatformIT : BaseGradleIT() {
         }
     }
 
-    @Test
-    fun testParallelTasks() {
-        parallelTasksImpl(isParallel = true)
-    }
-
-    @Test
-    fun testNoParallelTasks() {
-        parallelTasksImpl(isParallel = false)
-    }
-
-    private fun parallelTasksImpl(isParallel: Boolean) = with(Project("new-mpp-parallel", gradleVersion)) {
-        val options = defaultBuildOptions().copy(parallelTasksInProject = isParallel)
-        build("assemble", options = options) {
-            assertSuccessful()
-            val tasks = arrayOf(":compileKotlinMetadata", ":compileKotlinJvm", ":compileKotlinJs")
-            if (isParallel) {
-                assertTasksSubmittedWork(*tasks)
-            } else {
-                assertTasksDidNotSubmitWork(*tasks)
-            }
-            val expectedKotlinOutputFiles = listOf(
-                kotlinClassesDir(sourceSet = "metadata/main") + "common/A.kotlin_metadata",
-                kotlinClassesDir(sourceSet = "jvm/main") + "common/A.class",
-                kotlinClassesDir(sourceSet = "js/main") + "new-mpp-parallel.js"
-            )
-            expectedKotlinOutputFiles.forEach { assertFileExists(it) }
-        }
-    }
-
     @Test
     fun testLibWithTests() = doTestLibWithTests(Project("new-mpp-lib-with-tests", gradleVersion))
 

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/WorkersIT.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.gradle
+
+import org.junit.Test
+
+class WorkersIT : BaseGradleIT() {
+    @Test
+    fun testParallelTasks() {
+        parallelTasksImpl(isParallel = true)
+    }
+
+    @Test
+    fun testNoParallelTasks() {
+        parallelTasksImpl(isParallel = false)
+    }
+
+    private fun parallelTasksImpl(isParallel: Boolean) =
+        with(Project("new-mpp-parallel", GradleVersionRequired.AtLeast("4.7"))) {
+            val options = defaultBuildOptions().copy(parallelTasksInProject = isParallel, withDaemon = false)
+            val traceLoading = "-Dorg.jetbrains.kotlin.compilerRunner.GradleKotlinCompilerWork.trace.loading=true"
+            build("assemble", traceLoading, options = options) {
+                assertSuccessful()
+                val tasks = arrayOf(":compileKotlinMetadata", ":compileKotlinJvm", ":compileKotlinJs")
+                if (isParallel) {
+                    assertTasksSubmittedWork(*tasks)
+                } else {
+                    assertTasksDidNotSubmitWork(*tasks)
+                }
+                val expectedKotlinOutputFiles = listOf(
+                    kotlinClassesDir(sourceSet = "metadata/main") + "common/A.kotlin_metadata",
+                    kotlinClassesDir(sourceSet = "jvm/main") + "common/A.class",
+                    kotlinClassesDir(sourceSet = "js/main") + "new-mpp-parallel.js"
+                )
+                expectedKotlinOutputFiles.forEach { assertFileExists(it) }
+                assertSubstringCount("Loaded GradleKotlinCompilerWork", 1)
+                assertCompiledKotlinSources(project.relativize(project.allKotlinFiles))
+                assertNotContains("Falling back to sl4j logger")
+            }
+        }
+}

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

@@ -20,7 +20,6 @@ internal class GradleCompilerRunnerWithWorkers(
         // todo: write tests with Workers enabled;
         workersExecutor.submit(GradleKotlinCompilerWork::class.java) { config ->
             config.isolationMode = IsolationMode.NONE
-            config.forkMode = ForkMode.NEVER
             config.params(workArgs)
         }
     }

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

@@ -66,6 +66,15 @@ internal class GradleKotlinCompilerWork @Inject constructor(
      */
     config: GradleKotlinCompilerWorkArguments
 ) : Runnable {
+
+    companion object {
+        init {
+            if (System.getProperty("org.jetbrains.kotlin.compilerRunner.GradleKotlinCompilerWork.trace.loading") == "true") {
+                System.out.println("Loaded GradleKotlinCompilerWork")
+            }
+        }
+    }
+
     private val projectRootFile = config.projectFiles.projectRootFile
     private val clientIsAliveFlagFile = config.projectFiles.clientIsAliveFlagFile
     private val sessionFlagFile = config.projectFiles.sessionFlagFile
@@ -80,10 +89,16 @@ internal class GradleKotlinCompilerWork @Inject constructor(
     private val taskPath = config.taskPath
 
     private val log: KotlinLogger =
-        TaskLoggers.get(taskPath)?.let { GradleKotlinLogger(it) }
+        TaskLoggers.get(taskPath)?.let { GradleKotlinLogger(it).apply { debug("Using '$taskPath' logger") } }
             ?: run {
-                System.err.println("Could not get logger for '$taskPath'. Falling back to sl4j logger")
-                SL4JKotlinLogger(LoggerFactory.getLogger("GradleKotlinCompilerWork"))
+                val logger = LoggerFactory.getLogger("GradleKotlinCompilerWork")
+                val kotlinLogger = if (logger is org.gradle.api.logging.Logger) {
+                    GradleKotlinLogger(logger)
+                } else SL4JKotlinLogger(logger)
+
+                kotlinLogger.apply {
+                    debug("Could not get logger for '$taskPath'. Falling back to sl4j logger")
+                }
             }
 
     private val isIncremental: Boolean