Fix limitedParallelism implementation on K/N

kotlinx-coroutines-core/common/src/internal/LimitedDispatcher.kt

@@ -23,6 +23,9 @@ internal class LimitedDispatcher(
 
     private val queue = LockFreeTaskQueue<Runnable>(singleConsumer = false)
 
+    // A separate object that we can synchronize on for K/N
+    private val workerAllocationLock = SynchronizedObject()
+
     @ExperimentalCoroutinesApi
     override fun limitedParallelism(parallelism: Int): CoroutineDispatcher {
         parallelism.checkParallelism()
@@ -50,8 +53,7 @@ internal class LimitedDispatcher(
                 continue
             }
 
-            @Suppress("CAST_NEVER_SUCCEEDS")
-            synchronized(this as SynchronizedObject) {
+            synchronized(workerAllocationLock) {
                 --runningWorkers
                 if (queue.size == 0) return
                 ++runningWorkers
@@ -87,8 +89,7 @@ internal class LimitedDispatcher(
     }
 
     private fun tryAllocateWorker(): Boolean {
-        @Suppress("CAST_NEVER_SUCCEEDS")
-        synchronized(this as SynchronizedObject) {
+        synchronized(workerAllocationLock) {
             if (runningWorkers >= parallelism) return false
             ++runningWorkers
             return true

kotlinx-coroutines-core/common/test/LimitedParallelismSharedTest.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2016-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+
+import kotlin.test.*
+
+class LimitedParallelismSharedTest : TestBase() {
+    @Test
+    fun testTaskFairness() = runTest {
+        val view = Dispatchers.Default.limitedParallelism(1)
+        val view2 = Dispatchers.Default.limitedParallelism(1)
+        val j1 = launch(view) {
+            while (true) {
+                yield()
+            }
+        }
+        val j2 = launch(view2) { j1.cancel() }
+        joinAll(j1, j2)
+    }
+
+    @Test
+    fun testParallelismSpec() {
+        assertFailsWith<IllegalArgumentException> { Dispatchers.Default.limitedParallelism(0) }
+        assertFailsWith<IllegalArgumentException> { Dispatchers.Default.limitedParallelism(-1) }
+        assertFailsWith<IllegalArgumentException> { Dispatchers.Default.limitedParallelism(Int.MIN_VALUE) }
+        Dispatchers.Default.limitedParallelism(Int.MAX_VALUE)
+    }
+}

kotlinx-coroutines-core/concurrent/test/LimitedParallelismSharedStressTest.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2016-2022 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+import kotlinx.atomicfu.*
+import kotlinx.coroutines.*
+import kotlinx.coroutines.exceptions.*
+import kotlin.test.*
+
+class LimitedParallelismSharedStressTest : TestBase() {
+
+    private val targetParallelism = 4
+    private val iterations = 100_000
+    private val parallelism = atomic(0)
+
+    private fun checkParallelism() {
+        val value = parallelism.incrementAndGet()
+        randomWait()
+        assertTrue { value <= targetParallelism }
+        parallelism.decrementAndGet()
+    }
+
+    @Test
+    fun testLimitedExecutor() = runMtTest {
+        val executor = newFixedThreadPoolContext(targetParallelism, "test")
+        val view = executor.limitedParallelism(targetParallelism)
+        doStress {
+            repeat(iterations) {
+                launch(view) {
+                    checkParallelism()
+                }
+            }
+        }
+        executor.close()
+    }
+
+    private suspend inline fun doStress(crossinline block: suspend CoroutineScope.() -> Unit) {
+        repeat(stressTestMultiplier) {
+            coroutineScope {
+                block()
+            }
+        }
+    }
+}

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

@@ -11,14 +11,6 @@ import kotlin.test.*
 
 class LimitedParallelismTest : TestBase() {
 
-    @Test
-    fun testParallelismSpec() {
-        assertFailsWith<IllegalArgumentException> { Dispatchers.Default.limitedParallelism(0) }
-        assertFailsWith<IllegalArgumentException> { Dispatchers.Default.limitedParallelism(-1) }
-        assertFailsWith<IllegalArgumentException> { Dispatchers.Default.limitedParallelism(Int.MIN_VALUE) }
-        Dispatchers.Default.limitedParallelism(Int.MAX_VALUE)
-    }
-
     @Test
     fun testTaskFairness() = runTest {
         val executor = newSingleThreadContext("test")