Disallow reconfiguring the executor in newFixedThreadPoolContext (#4364)

Author: dkhalanskyjb

Diff for: kotlinx-coroutines-core/jvm/src/ThreadPoolDispatcher.kt

@@ -14,5 +14,5 @@ public actual fun newFixedThreadPoolContext(nThreads: Int, name: String): Execut
         t.isDaemon = true
         t
     }
-    return executor.asCoroutineDispatcher()
+    return Executors.unconfigurableExecutorService(executor).asCoroutineDispatcher()
 }

Diff for: kotlinx-coroutines-core/jvm/test/MultithreadedDispatchersJvmTest.kt

@@ -0,0 +1,30 @@
+package kotlinx.coroutines
+
+import kotlinx.coroutines.internal.LocalAtomicInt
+import kotlinx.coroutines.testing.*
+import java.util.concurrent.ScheduledThreadPoolExecutor
+import kotlin.coroutines.EmptyCoroutineContext
+import kotlin.test.*
+
+class MultithreadedDispatchersJvmTest: TestBase() {
+    /** Tests that the executor created in [newFixedThreadPoolContext] can not leak and be reconfigured. */
+    @Test
+    fun testExecutorReconfiguration() {
+        newFixedThreadPoolContext(1, "test").apply {
+            (executor as? ScheduledThreadPoolExecutor)?.corePoolSize = 2
+        }.use { ctx ->
+            val atomicInt = LocalAtomicInt(0)
+            repeat(100) {
+                ctx.dispatch(EmptyCoroutineContext, Runnable {
+                    val entered = atomicInt.incrementAndGet()
+                    Thread.yield() // allow other tasks to run
+                    try {
+                        check(entered == 1) { "Expected only one thread to be used, observed $entered" }
+                    } finally {
+                        atomicInt.decrementAndGet()
+                    }
+                })
+            }
+        }
+    }
+}

Diff for: kotlinx-coroutines-core/jvm/test/knit/ClosedAfterGuideTestExecutor.kt

@@ -7,9 +7,6 @@ import kotlin.coroutines.*
 
 internal fun newSingleThreadContext(name: String): ExecutorCoroutineDispatcher = ClosedAfterGuideTestDispatcher(1, name)
 
-internal fun newFixedThreadPoolContext(nThreads: Int, name: String): ExecutorCoroutineDispatcher =
-    ClosedAfterGuideTestDispatcher(nThreads, name)
-
 private class ClosedAfterGuideTestDispatcher(
     private val nThreads: Int,
     private val name: String

Diff for: test-utils/jvm/src/FieldWalker.kt

@@ -127,6 +127,9 @@ object FieldWalker {
             element is AtomicLongFieldUpdater<*> -> {
                 /* filter it out here to suppress its subclasses too */
             }
+            element is ExecutorService && type.name == "java.util.concurrent.Executors\$DelegatedExecutorService" -> {
+                /* can't access anything in the executor */
+            }
             // All the other classes are reflectively scanned
             else -> fields(type, statics).forEach { field ->
                 push(field.get(element), visited, stack) { Ref.FieldRef(element, field.name) }