Minor improvements of debugging experience (#2093)

* Recognizable toString implementation in default dispatchers
* Fast-path for disabled debug probes

Author: qwwdfsad

docs/flow.md

@@ -697,7 +697,7 @@ This code produces the following exception:
 ```text
 Exception in thread "main" java.lang.IllegalStateException: Flow invariant is violated:
 		Flow was collected in [CoroutineId(1), "coroutine#1":BlockingCoroutine{Active}@5511c7f8, BlockingEventLoop@2eac3323],
-		but emission happened in [CoroutineId(1), "coroutine#1":DispatchedCoroutine{Active}@2dae0000, DefaultDispatcher].
+		but emission happened in [CoroutineId(1), "coroutine#1":DispatchedCoroutine{Active}@2dae0000, Dispatchers.Default].
 		Please refer to 'flow' documentation or use 'flowOn' instead
 	at ...
 ``` 

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

@@ -26,7 +26,7 @@ internal object Unconfined : CoroutineDispatcher() {
             "isDispatchNeeded and dispatch calls.")
     }
 
-    override fun toString(): String = "Unconfined"
+    override fun toString(): String = "Dispatchers.Unconfined"
 }
 
 /**

kotlinx-coroutines-core/jvm/src/debug/internal/DebugProbesImpl.kt

@@ -272,6 +272,7 @@ internal object DebugProbesImpl {
     internal fun probeCoroutineSuspended(frame: Continuation<*>) = updateState(frame, SUSPENDED)
 
     private fun updateState(frame: Continuation<*>, state: String) {
+        if (!isInstalled) return
         // KT-29997 is here only since 1.3.30
         if (state == RUNNING && KotlinVersion.CURRENT.isAtLeast(1, 3, 30)) {
             val stackFrame = frame as? CoroutineStackFrame ?: return

kotlinx-coroutines-core/jvm/src/scheduling/Dispatcher.kt

@@ -14,13 +14,18 @@ import kotlin.coroutines.*
  * Default instance of coroutine dispatcher.
  */
 internal object DefaultScheduler : ExperimentalCoroutineDispatcher() {
-    val IO = blocking(systemProp(IO_PARALLELISM_PROPERTY_NAME, 64.coerceAtLeast(AVAILABLE_PROCESSORS)))
+    val IO: CoroutineDispatcher = LimitingDispatcher(
+        this,
+        systemProp(IO_PARALLELISM_PROPERTY_NAME, 64.coerceAtLeast(AVAILABLE_PROCESSORS)),
+        "Dispatchers.IO",
+        TASK_PROBABLY_BLOCKING
+    )
 
     override fun close() {
-        throw UnsupportedOperationException("$DEFAULT_SCHEDULER_NAME cannot be closed")
+        throw UnsupportedOperationException("$DEFAULT_DISPATCHER_NAME cannot be closed")
     }
 
-    override fun toString(): String = DEFAULT_SCHEDULER_NAME
+    override fun toString(): String = DEFAULT_DISPATCHER_NAME
 
     @InternalCoroutinesApi
     @Suppress("UNUSED")
@@ -85,7 +90,7 @@ public open class ExperimentalCoroutineDispatcher(
      */
     public fun blocking(parallelism: Int = BLOCKING_DEFAULT_PARALLELISM): CoroutineDispatcher {
         require(parallelism > 0) { "Expected positive parallelism level, but have $parallelism" }
-        return LimitingDispatcher(this, parallelism, TASK_PROBABLY_BLOCKING)
+        return LimitingDispatcher(this, parallelism, null, TASK_PROBABLY_BLOCKING)
     }
 
     /**
@@ -98,7 +103,7 @@ public open class ExperimentalCoroutineDispatcher(
     public fun limited(parallelism: Int): CoroutineDispatcher {
         require(parallelism > 0) { "Expected positive parallelism level, but have $parallelism" }
         require(parallelism <= corePoolSize) { "Expected parallelism level lesser than core pool size ($corePoolSize), but have $parallelism" }
-        return LimitingDispatcher(this, parallelism, TASK_NON_BLOCKING)
+        return LimitingDispatcher(this, parallelism, null, TASK_NON_BLOCKING)
     }
 
     internal fun dispatchWithContext(block: Runnable, context: TaskContext, tailDispatch: Boolean) {
@@ -130,8 +135,9 @@ public open class ExperimentalCoroutineDispatcher(
 }
 
 private class LimitingDispatcher(
-    val dispatcher: ExperimentalCoroutineDispatcher,
-    val parallelism: Int,
+    private val dispatcher: ExperimentalCoroutineDispatcher,
+    private val parallelism: Int,
+    private val name: String?,
     override val taskMode: Int
 ) : ExecutorCoroutineDispatcher(), TaskContext, Executor {
 
@@ -190,7 +196,7 @@ private class LimitingDispatcher(
     }
 
     override fun toString(): String {
-        return "${super.toString()}[dispatcher = $dispatcher]"
+        return name ?: "${super.toString()}[dispatcher = $dispatcher]"
     }
 
     /**

kotlinx-coroutines-core/jvm/src/scheduling/Tasks.kt

@@ -11,6 +11,9 @@ import java.util.concurrent.*
 
 // TODO most of these fields will be moved to 'object ExperimentalDispatcher'
 
+// User-visible name
+internal const val DEFAULT_DISPATCHER_NAME = "Dispatchers.Default"
+// Internal debuggability name + thread name prefixes
 internal const val DEFAULT_SCHEDULER_NAME = "DefaultDispatcher"
 
 // 100us as default

kotlinx-coroutines-core/jvm/test/guide/test/FlowGuideTest.kt

@@ -151,7 +151,7 @@ class FlowGuideTest {
         test("ExampleFlow14") { kotlinx.coroutines.guide.exampleFlow14.main() }.verifyExceptions(
             "Exception in thread \"main\" java.lang.IllegalStateException: Flow invariant is violated:",
             "\t\tFlow was collected in [CoroutineId(1), \"coroutine#1\":BlockingCoroutine{Active}@5511c7f8, BlockingEventLoop@2eac3323],",
-            "\t\tbut emission happened in [CoroutineId(1), \"coroutine#1\":DispatchedCoroutine{Active}@2dae0000, DefaultDispatcher].",
+            "\t\tbut emission happened in [CoroutineId(1), \"coroutine#1\":DispatchedCoroutine{Active}@2dae0000, Dispatchers.Default].",
             "\t\tPlease refer to 'flow' documentation or use 'flowOn' instead",
             "\tat ..."
         )