Implemented resolving the compiler jar using the class loader

Added resolving the compiler jar using the class loader urls as the
first step with the fallback to the original resolution method. Also
helps if the compiler is not present in the classpath dependencies.

Issues: #KT-16580 Fixed

Changed to getting plugin version from the applied plugin instead of
the plugin JAR name.

Removed unnecessary working with version of AbstractKotlinPlugin.

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

@@ -59,4 +59,13 @@ class KotlinGradlePluginMultiVersionIT : BaseMultiGradleVersionIT() {
             assertNotContains("None of the classes needs to be compiled!")
         }
     }
+
+    @Test fun testApplyPluginFromBuildSrc() {
+        val project = Project("kotlinProjectWithBuildSrc", gradleVersion)
+        project.setupWorkingDir()
+        File(project.projectDir, "buildSrc/build.gradle").modify { it.replace("\$kotlin_version", KOTLIN_VERSION) }
+        project.build("build") {
+            assertSuccessful()
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinProjectWithBuildSrc/build.gradle

@@ -0,0 +1 @@
+apply plugin: 'kotlin'

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinProjectWithBuildSrc/buildSrc/build.gradle

@@ -0,0 +1,9 @@
+repositories {
+    mavenLocal()
+}
+
+apply plugin: 'java'
+
+dependencies {
+    runtime "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+}

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinProjectWithBuildSrc/src/main/kotlin/Hello.kt

@@ -0,0 +1 @@
+fun hello() = "Hello"

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

@@ -33,7 +33,7 @@ import javax.inject.Inject
 
 abstract class KotlinBasePluginWrapper(protected val fileResolver: FileResolver): Plugin<Project> {
     private val log = Logging.getLogger(this.javaClass)
-    protected val kotlinPluginVersion = loadKotlinVersionFromResource(log)
+    val kotlinPluginVersion = loadKotlinVersionFromResource(log)
 
     override fun apply(project: Project) {
         // TODO: consider only set if if daemon or parallel compilation are enabled, though this way it should be safe too

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

@@ -20,7 +20,10 @@ import org.gradle.api.Project
 import org.gradle.api.artifacts.Configuration
 import org.gradle.api.artifacts.ResolvedDependency
 import org.gradle.api.initialization.dsl.ScriptHandler
+import org.jetbrains.kotlin.gradle.plugin.KotlinBasePluginWrapper
+import org.jetbrains.kotlin.gradle.plugin.KotlinPlugin
 import java.io.File
+import java.net.URLClassLoader
 import java.net.URLDecoder
 import java.nio.charset.Charset
 import java.util.zip.ZipFile
@@ -55,9 +58,23 @@ private fun findJarByClass(klass: Class<*>): File? {
 }
 
 private fun findKotlinCompilerJar(project: Project, compilerClassName: String): File? {
-    val filesToCheck = findPotentialCompilerJars(project)
+    val pluginVersion = pluginVersionFromAppliedPlugin(project)
+
+    val filesToCheck = sequenceOf(pluginVersion?.let(::getCompilerFromClassLoader)) +
+                       Sequence { findPotentialCompilerJars(project).iterator() } //call the body only when queried
     val entryToFind = compilerClassName.replace(".", "/") + ".class"
-    return filesToCheck.firstOrNull { it.hasEntry(entryToFind) }
+    return filesToCheck.filterNotNull().firstOrNull { it.hasEntry(entryToFind) }
+}
+
+private fun pluginVersionFromAppliedPlugin(project: Project): String? =
+        project.plugins.filterIsInstance<KotlinBasePluginWrapper>().firstOrNull()?.kotlinPluginVersion
+
+private fun getCompilerFromClassLoader(pluginVersion: String): File? {
+    val urlClassLoader = KotlinPlugin::class.java.classLoader as? URLClassLoader ?: return null
+    return urlClassLoader.urLs
+            .firstOrNull { it.toString().endsWith("kotlin-compiler-embeddable-$pluginVersion.jar") }
+            ?.let { File(it.toURI()) }
+            ?.takeIf(File::exists)
 }
 
 private fun findPotentialCompilerJars(project: Project): Iterable<File> {