Fix MutexCancellationStressTest flakiness (#3724)

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

@@ -8,7 +8,11 @@ import kotlinx.coroutines.internal.*
 import kotlinx.coroutines.selects.*
 import kotlinx.coroutines.sync.*
 import org.junit.*
+import org.junit.Test
 import java.util.concurrent.*
+import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicInteger
+import kotlin.test.*
 
 class MutexCancellationStressTest : TestBase() {
     @Test
@@ -18,13 +22,16 @@ class MutexCancellationStressTest : TestBase() {
         val mutexOwners = Array(mutexJobNumber) { "$it" }
         val dispatcher = Executors.newFixedThreadPool(mutexJobNumber + 2).asCoroutineDispatcher()
         var counter = 0
-        val counterLocal = Array(mutexJobNumber) { LocalAtomicInt(0) }
-        val completed = LocalAtomicInt(0)
+        val counterLocal = Array(mutexJobNumber) { AtomicInteger(0) }
+        val completed = AtomicBoolean(false)
         val mutexJobLauncher: (jobNumber: Int) -> Job = { jobId ->
             val coroutineName = "MutexJob-$jobId"
-            launch(dispatcher + CoroutineName(coroutineName)) {
-                while (completed.value == 0) {
+            // ATOMIC to always have a chance to proceed
+            launch(dispatcher + CoroutineName(coroutineName), CoroutineStart.ATOMIC) {
+                while (!completed.get()) {
+                    // Stress out holdsLock
                     mutex.holdsLock(mutexOwners[(jobId + 1) % mutexJobNumber])
+                    // Stress out lock-like primitives
                     if (mutex.tryLock(mutexOwners[jobId])) {
                         counterLocal[jobId].incrementAndGet()
                         counter++
@@ -47,30 +54,32 @@ class MutexCancellationStressTest : TestBase() {
         val mutexJobs = (0 until mutexJobNumber).map { mutexJobLauncher(it) }.toMutableList()
         val checkProgressJob = launch(dispatcher + CoroutineName("checkProgressJob")) {
             var lastCounterLocalSnapshot = (0 until mutexJobNumber).map { 0 }
-            while (completed.value == 0) {
-                delay(1000)
+            while (!completed.get()) {
+                delay(500)
+                // If we've caught the completion after delay, then there is a chance no progress were made whatsoever, bail out
+                if (completed.get()) return@launch
                 val c = counterLocal.map { it.value }
                 for (i in 0 until mutexJobNumber) {
-                    assert(c[i] > lastCounterLocalSnapshot[i]) { "No progress in MutexJob-$i" }
+                    assert(c[i] > lastCounterLocalSnapshot[i]) { "No progress in MutexJob-$i, last observed state: ${c[i]}" }
                 }
                 lastCounterLocalSnapshot = c
             }
         }
         val cancellationJob = launch(dispatcher + CoroutineName("cancellationJob")) {
             var cancellingJobId = 0
-            while (completed.value == 0) {
+            while (!completed.get()) {
                 val jobToCancel = mutexJobs.removeFirst()
                 jobToCancel.cancelAndJoin()
                 mutexJobs += mutexJobLauncher(cancellingJobId)
                 cancellingJobId = (cancellingJobId + 1) % mutexJobNumber
             }
         }
         delay(2000L * stressTestMultiplier)
-        completed.value = 1
+        completed.set(true)
         cancellationJob.join()
         mutexJobs.forEach { it.join() }
         checkProgressJob.join()
-        check(counter == counterLocal.sumOf { it.value })
+        assertEquals(counter, counterLocal.sumOf { it.value })
         dispatcher.close()
     }
 }