Restore an Android compatibility trick (removed in #4196)

Author: dkhalanskyjb

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

@@ -117,12 +117,13 @@ 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 {
-        /* Attempt to invoke ScheduledThreadPoolExecutor.setRemoveOnCancelPolicy in order to clean up
-         * the internal scheduler queue on cancellation. */
-        if (executor is ScheduledThreadPoolExecutor) {
-            executor.removeOnCancelPolicy = true
-        }
+        removeFutureOnCancel(executor)
     }
 
     override fun dispatch(context: CoroutineContext, block: Runnable) {

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

@@ -1,6 +1,9 @@
 package kotlinx.coroutines.internal
 
+import java.lang.reflect.Method
 import java.util.*
+import java.util.concurrent.Executor
+import java.util.concurrent.ScheduledThreadPoolExecutor
 import kotlin.concurrent.withLock as withLockJvm
 
 internal actual typealias ReentrantLock = java.util.concurrent.locks.ReentrantLock
@@ -18,3 +21,20 @@ 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 (_: 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 (_: Throwable) {
+        return false // failed to setRemoveOnCancelPolicy, assume it does not remove the future on cancel
+    }
+}