Introduce first version of Dispatchers.IO for K/N (Kotlin#3576)

* Emulate expect declaration refinement via extension property as the only way to do it in a backwards-compatible manner: in the current model, it is impossible to have common 'expect' Dispatchers declaration, then refined 'concurrent' Dispatchers declaration with 'expect val IO' and then JVM declaration with JVM-specific members. Current solutions seems to be the less intrusive one

Fixes Kotlin#3205

README.md

@@ -212,7 +212,7 @@ See [Contributing Guidelines](CONTRIBUTING.md).
 [MainScope()]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-main-scope.html
 [SupervisorJob()]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-supervisor-job.html
 [CoroutineExceptionHandler]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-exception-handler/index.html
-[Dispatchers.IO]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-dispatchers/-i-o.html
+[Dispatchers.IO]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-i-o.html
 [asCoroutineDispatcher]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/as-coroutine-dispatcher.html
 [Promise.await]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/await.html
 [promise]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/promise.html

kotlinx-coroutines-core/api/kotlinx-coroutines-core.api

@@ -302,6 +302,7 @@ public final class kotlinx/coroutines/Dispatchers {
 
 public final class kotlinx/coroutines/DispatchersKt {
  public static final field IO_PARALLELISM_PROPERTY_NAME Ljava/lang/String;
+ public static final synthetic fun getIO (Lkotlinx/coroutines/Dispatchers;)Lkotlinx/coroutines/CoroutineDispatcher;
 }
 
 public abstract interface class kotlinx/coroutines/DisposableHandle {

kotlinx-coroutines-core/common/src/Dispatchers.common.kt

@@ -12,7 +12,7 @@ import kotlin.coroutines.*
 public expect object Dispatchers {
  /**
  * The default [CoroutineDispatcher] that is used by all standard builders like
- * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc
+ * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc.
  * if neither a dispatcher nor any other [ContinuationInterceptor] is specified in their context.
  *
  * It is backed by a shared pool of threads on JVM and Native. By default, the maximum number of threads used

kotlinx-coroutines-core/concurrent/src/Dispatchers.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2016-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+
+/**
+ * The [CoroutineDispatcher] that is designed for offloading blocking IO tasks to a shared pool of threads.
+ * Additional threads in this pool are created on demand.
+ * Default IO pool size is `64`; on JVM it can be configured using JVM-specific mechanisms,
+ * please refer to `Dispatchers.IO` documentation on JVM platform.
+ *
+ * ### Elasticity for limited parallelism
+ *
+ * `Dispatchers.IO` has a unique property of elasticity: its views
+ * obtained with [CoroutineDispatcher.limitedParallelism] are
+ * not restricted by the `Dispatchers.IO` parallelism. Conceptually, there is
+ * a dispatcher backed by an unlimited pool of threads, and both `Dispatchers.IO`
+ * and views of `Dispatchers.IO` are actually views of that dispatcher. In practice
+ * this means that, despite not abiding by `Dispatchers.IO`'s parallelism
+ * restrictions, its views share threads and resources with it.
+ *
+ * In the following example
+ * ```
+ * // 100 threads for MySQL connection
+ * val myMysqlDbDispatcher = Dispatchers.IO.limitedParallelism(100)
+ * // 60 threads for MongoDB connection
+ * val myMongoDbDispatcher = Dispatchers.IO.limitedParallelism(60)
+ * ```
+ * the system may have up to `64 + 100 + 60` threads dedicated to blocking tasks during peak loads,
+ * but during its steady state there is only a small number of threads shared
+ * among `Dispatchers.IO`, `myMysqlDbDispatcher` and `myMongoDbDispatcher`
+ */
+@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
+public expect val Dispatchers.IO: CoroutineDispatcher
+
+

kotlinx-coroutines-core/concurrent/test/DefaultDispatcherConcurrencyTest.kt → kotlinx-coroutines-core/concurrent/test/DefaultDispatchersConcurrencyTest.kt

@@ -6,3 +6,7 @@ package kotlinx.coroutines
 class DefaultDispatcherConcurrencyTest : AbstractDispatcherConcurrencyTest() {
  override val dispatcher: CoroutineDispatcher = Dispatchers.Default
 }
+
+class IoDispatcherConcurrencyTest : AbstractDispatcherConcurrencyTest() {
+ override val dispatcher: CoroutineDispatcher = Dispatchers.IO
+}

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

@@ -1,9 +1,7 @@
 /*
- * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2016-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
-@file:Suppress("unused")
-
 package kotlinx.coroutines
 
 import kotlinx.coroutines.internal.*
@@ -97,3 +95,13 @@ public actual object Dispatchers {
  DefaultScheduler.shutdown()
  }
 }
+
+/**
+ * `actual` counterpart of the corresponding `expect` declaration.
+ * Should never be used directly from JVM sources, all accesses
+ * to `Dispatchers.IO` should be resolved to the corresponding member of [Dispatchers] object.
+ * @suppress
+ */
+@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
+@Deprecated(message = "Should not be used directly", level = DeprecationLevel.HIDDEN)
+public actual val Dispatchers.IO: CoroutineDispatcher get() = Dispatchers.IO

kotlinx-coroutines-core/native/src/Dispatchers.kt

@@ -4,6 +4,9 @@
 
 package kotlinx.coroutines
 
+import kotlinx.coroutines.internal.*
+import kotlin.coroutines.*
+
 
 public actual object Dispatchers {
  public actual val Default: CoroutineDispatcher = createDefaultDispatcher()
@@ -19,6 +22,35 @@ public actual object Dispatchers {
  internal fun injectMain(dispatcher: MainCoroutineDispatcher) {
  injectedMainDispatcher = dispatcher
  }
+
+ internal val IO: CoroutineDispatcher = DefaultIoScheduler
 }
 
+internal object DefaultIoScheduler : CoroutineDispatcher() {
+ // 2048 is an arbitrary KMP-friendly constant
+ private val unlimitedPool = newFixedThreadPoolContext(2048, "Dispatchers.IO")
+ private val io = unlimitedPool.limitedParallelism(64) // Default JVM size
+
+ @ExperimentalCoroutinesApi
+ override fun limitedParallelism(parallelism: Int): CoroutineDispatcher {
+ // See documentation to Dispatchers.IO for the rationale
+ return unlimitedPool.limitedParallelism(parallelism)
+ }
+
+ override fun dispatch(context: CoroutineContext, block: Runnable) {
+ io.dispatch(context, block)
+ }
+
+ @InternalCoroutinesApi
+ override fun dispatchYield(context: CoroutineContext, block: Runnable) {
+ io.dispatchYield(context, block)
+ }
+
+ override fun toString(): String = "Dispatchers.IO"
+}
+
+
+@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
+public actual val Dispatchers.IO: CoroutineDispatcher get() = IO
+
 internal expect fun createMainDispatcher(default: CoroutineDispatcher): MainCoroutineDispatcher