Remove a workaround for JDK 6

Author: dkhalanskyjb

kotlinx-coroutines-core/jvm/src/Executors.kt

@@ -117,13 +117,12 @@ private class DispatcherExecutor(@JvmField val dispatcher: CoroutineDispatcher)
 
 internal class ExecutorCoroutineDispatcherImpl(override val executor: Executor) : ExecutorCoroutineDispatcher(), Delay {
 
-    /*
-     * Attempts to reflectively (to be Java 6 compatible) invoke
-     * ScheduledThreadPoolExecutor.setRemoveOnCancelPolicy in order to cleanup
-     * internal scheduler queue on cancellation.
-     */
     init {
-        removeFutureOnCancel(executor)
+        /* Attempt to invoke ScheduledThreadPoolExecutor.setRemoveOnCancelPolicy in order to clean up
+         * the internal scheduler queue on cancellation. */
+        if (executor is ScheduledThreadPoolExecutor) {
+            executor.removeOnCancelPolicy = true
+        }
     }
 
     override fun dispatch(context: CoroutineContext, block: Runnable) {

kotlinx-coroutines-core/jvm/src/internal/Concurrent.kt

@@ -1,8 +1,6 @@
 package kotlinx.coroutines.internal
 
-import java.lang.reflect.*
 import java.util.*
-import java.util.concurrent.*
 import kotlin.concurrent.withLock as withLockJvm
 
 @Suppress("ACTUAL_WITHOUT_EXPECT")
@@ -22,20 +20,3 @@ internal actual annotation class BenignDataRace()
 @Suppress("NOTHING_TO_INLINE") // So that R8 can completely remove ConcurrentKt class
 internal actual inline fun <E> identitySet(expectedSize: Int): MutableSet<E> =
     Collections.newSetFromMap(IdentityHashMap(expectedSize))
-
-private val REMOVE_FUTURE_ON_CANCEL: Method? = try {
-    ScheduledThreadPoolExecutor::class.java.getMethod("setRemoveOnCancelPolicy", Boolean::class.java)
-} catch (e: Throwable) {
-    null
-}
-
-@Suppress("NAME_SHADOWING")
-internal fun removeFutureOnCancel(executor: Executor): Boolean {
-    try {
-        val executor = executor as? ScheduledThreadPoolExecutor ?: return false
-        (REMOVE_FUTURE_ON_CANCEL ?: return false).invoke(executor, true)
-        return true
-    } catch (e: Throwable) {
-        return false // failed to setRemoveOnCancelPolicy, assume it does not removes future on cancel
-    }
-}

kotlinx-coroutines-core/jvm/test/RejectedExecutionTest.kt

@@ -87,7 +87,6 @@ class RejectedExecutionTest : TestBase() {
 
     @Test
     fun testRejectOnDelay() = runTest {
-        if (!removeFutureOnCancel(executor)) return@runTest // Skip this test on old JDKs
         expect(1)
         executor.acceptTasks = 1 // accept one task
         assertFailsWith<CancellationException> {
@@ -109,7 +108,6 @@ class RejectedExecutionTest : TestBase() {
 
     @Test
     fun testRejectWithTimeout() = runTest {
-        if (!removeFutureOnCancel(executor)) return@runTest // Skip this test on old JDKs
         expect(1)
         executor.acceptTasks = 1 // accept one task
         assertFailsWith<CancellationException> {