[KAPT] Cache annotation processors' classloaders

Adds experimental feature to cache annotation processors' classloaders
It should increase speed of KAPT for consecutive runs
Works for gradle only (it stores cl in gradle daemon)
#KT-28901 Fixed

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

@@ -258,7 +258,8 @@ abstract class BaseGradleIT {
         val verbose: Boolean,
         val useWorkers: Boolean,
         val incrementalKapt: Boolean = false,
-        val includeCompileClasspath: Boolean = true
+        val includeCompileClasspath: Boolean = true,
+        val classLoadersCacheSize: Int? = null
     )
 
     open inner class Project(
@@ -913,6 +914,9 @@ Finished executing task ':$taskName'|
                 add("-Pkapt.use.worker.api=${kaptOptions.useWorkers}")
                 add("-Pkapt.incremental.apt=${kaptOptions.incrementalKapt}")
                 add("-Pkapt.include.compile.classpath=${kaptOptions.includeCompileClasspath}")
+                kaptOptions.classLoadersCacheSize?.also { cacheSize ->
+                    add("-Pkapt.classloaders.cache.size=$cacheSize")
+                }
             }
 
             options.parallelTasksInProject?.let {

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kapt3IT.kt

@@ -21,11 +21,14 @@ import org.jetbrains.kotlin.gradle.tasks.USING_JVM_INCREMENTAL_COMPILATION_MESSA
 import org.jetbrains.kotlin.gradle.util.*
 import org.junit.Assert
 import org.junit.Assume
+import org.junit.Ignore
 import org.junit.Test
 import java.io.File
+import java.util.regex.Pattern
 import java.util.zip.ZipFile
 import kotlin.test.assertNotNull
 import kotlin.test.assertNull
+import kotlin.test.assertTrue
 
 abstract class Kapt3BaseIT : BaseGradleIT() {
     companion object {
@@ -43,7 +46,7 @@ abstract class Kapt3BaseIT : BaseGradleIT() {
     }
 }
 
-class Kapt3WorkersIT : Kapt3IT() {
+open class Kapt3WorkersIT : Kapt3IT() {
     override fun kaptOptions(): KaptOptions =
         super.kaptOptions().copy(useWorkers = true)
 
@@ -53,7 +56,9 @@ class Kapt3WorkersIT : Kapt3IT() {
             Project("javacIsLoadedOnce", directoryPrefix = "kapt2")
         project.build("build") {
             assertSuccessful()
-            assertSubstringCount("Loaded com.sun.tools.javac.util.Context from", 1)
+
+            val loadsCount = Pattern.quote("Loaded com.sun.tools.javac.util.Context from").toRegex().findAll(output).count()
+            assertTrue(loadsCount <= 1, "javac is loaded more than once")
         }
     }
 
@@ -95,9 +100,65 @@ class Kapt3WorkersIT : Kapt3IT() {
     }
 }
 
+class Kapt3ClassLoadersCacheIT : Kapt3WorkersIT() {
+    override fun kaptOptions(): KaptOptions =
+        super.kaptOptions().copy(classLoadersCacheSize = 10, includeCompileClasspath = false)
+
+    @Ignore
+    override fun testDisableDiscoveryInCompileClasspath() {
+        //classloaders cache is incompatible with AP discovery in classpath
+    }
+
+    @Test
+    override fun testAnnotationProcessorAsFqName() {
+        val project = Project("annotationProcessorAsFqName", directoryPrefix = "kapt2").also { it.setupWorkingDir() }
+
+        //classloaders caching is not compatible with includeCompileClasspath
+        project.projectDir.getFileByName("build.gradle").modify {
+            it.addBeforeSubstring(
+                "kapt \"org.jetbrains.kotlin:annotation-processor-example:\$kotlin_version\"\n",
+                "implementation \"org.jetbrains.kotlin:annotation-processor-example"
+            )
+        }
+
+        project.build("build") {
+            assertSuccessful()
+            assertKaptSuccessful()
+            assertTasksExecuted(":compileKotlin", ":compileJava")
+            assertFileExists("build/generated/source/kapt/main/example/TestClassGenerated.java")
+            assertFileExists(kotlinClassesDir() + "example/TestClass.class")
+            assertFileExists(javaClassesDir() + "example/TestClassGenerated.class")
+        }
+    }
+
+    @Test
+    fun testAnnotationProcessorClassIsLoadedOnce() {
+        val project = Project("javacIsLoadedOnce", directoryPrefix = "kapt2")
+
+        fun CompiledProject.classLoadingCount() =
+            Pattern.quote("Loaded example.ExampleAnnotationProcessor from").toRegex().findAll(output).count()
+
+        project.build("build") {
+            assertSuccessful()
+            assertTasksExecuted(":module1:kaptKotlin", ":module2:kaptKotlin")
+            assertTrue(classLoadingCount() <= 1, "AP class is loaded more than once")
+        }
+
+        project.projectDir.getFilesByNames("Module1Class.kt", "Module2Class.kt").forEach {
+            it.appendText("\n fun touch() = null")
+        }
+
+        project.build("build") {
+            assertSuccessful()
+            assertTasksExecuted(":module1:kaptKotlin", ":module2:kaptKotlin")
+            assertTrue(classLoadingCount() == 0, "AP class shouldn't be loaded on the second build")
+        }
+    }
+}
+
 open class Kapt3IT : Kapt3BaseIT() {
     @Test
-    fun testAnnotationProcessorAsFqName() {
+    open fun testAnnotationProcessorAsFqName() {
         val project = Project("annotationProcessorAsFqName", directoryPrefix = "kapt2")
 
         project.build("build") {
@@ -487,7 +548,7 @@ open class Kapt3IT : Kapt3BaseIT() {
     }
 
     @Test
-    fun testDisableDiscoveryInCompileClasspath() = with(Project("kaptAvoidance", directoryPrefix = "kapt2")) {
+    open fun testDisableDiscoveryInCompileClasspath() = with(Project("kaptAvoidance", directoryPrefix = "kapt2")) {
         setupWorkingDir()
         val buildGradle = projectDir.resolve("app/build.gradle")
         buildGradle.modify {
@@ -510,6 +571,12 @@ open class Kapt3IT : Kapt3BaseIT() {
 
     @Test
     fun testKaptAvoidance() = with(Project("kaptAvoidance", directoryPrefix = "kapt2")) {
+        setupWorkingDir()
+
+        projectDir.resolve("app/build.gradle").modify {
+            "$it\n\nkapt.includeCompileClasspath = true"
+        }
+
         build("assemble") {
             assertSuccessful()
             assertTasksExecuted(
@@ -538,7 +605,10 @@ open class Kapt3IT : Kapt3BaseIT() {
 
         // enable discovery
         projectDir.resolve("app/build.gradle").modify {
-            "$it\n\nkapt.includeCompileClasspath = false"
+            it.replace(
+                "kapt.includeCompileClasspath = true",
+                "kapt.includeCompileClasspath = false"
+            )
         }
         build("assemble") {
             assertSuccessful()
@@ -710,7 +780,7 @@ open class Kapt3IT : Kapt3BaseIT() {
             assertSuccessful()
             assertKaptSuccessful()
             assertTasksExecuted(":compileKotlin", ":compileJava")
-           
+
         }
     }
 }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kapt2/mpp-kapt-presence/build.gradle

@@ -3,9 +3,12 @@ buildscript {
       'kotlin': [
           'stdlib': [
               'common': "org.jetbrains.kotlin:kotlin-stdlib-common:$kotlin_version",
-              'jdk': "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version",
-          ],
+              'jdk': "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
+          ]
       ],
+      'other' : [
+          'ap_example': "org.jetbrains.kotlin:annotation-processor-example:$kotlin_version"
+      ]
   ]
 
   repositories {

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kapt2/mpp-kapt-presence/dac/jdk/build.gradle

@@ -7,4 +7,5 @@ targetCompatibility = JavaVersion.VERSION_1_8
 dependencies {
   expectedBy project(':dac')
   implementation deps.kotlin.stdlib.jdk
+  kapt deps.other.ap_example
 }