Improve docs for CoroutineExceptionHandler

Fixes #1746

Author: elizarov

docs/exception-handling.md

@@ -78,9 +78,10 @@ Caught ArithmeticException
 
 ### CoroutineExceptionHandler
 
-But what if one does not want to print all exceptions to the console?
-[CoroutineExceptionHandler] context element is used as generic `catch` block of coroutine where custom logging or exception handling may take place.
-It is similar to using [`Thread.uncaughtExceptionHandler`](https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#setUncaughtExceptionHandler(java.lang.Thread.UncaughtExceptionHandler)).
+What if one does not want to print all exceptions to the console?
+[CoroutineExceptionHandler] context element on a _root_ coroutine can be used as generic `catch` block for
+this root coroutine and all its children where custom logging or exception handling may take place.
+It is similar to [`Thread.uncaughtExceptionHandler`](https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#setUncaughtExceptionHandler(java.lang.Thread.UncaughtExceptionHandler)).
 
 On JVM it is possible to redefine global exception handler for all coroutines by registering [CoroutineExceptionHandler] via
 [`ServiceLoader`](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html).
@@ -89,8 +90,12 @@ Global exception handler is similar to
 which is used when no more specific handlers are registered.
 On Android, `uncaughtExceptionPreHandler` is installed as a global coroutine exception handler.
 
-[CoroutineExceptionHandler] is invoked only on exceptions which are not expected to be handled by the user, 
-so registering it in [async] builder and the like of it has no effect.
+`CoroutineExceptionHandler` is invoked only on **uncaught** exceptions — exceptions that were not handled in any other way.
+In particular, all _children_ coroutines (coroutines created in the context of another [Job]) delegate handling of
+their exceptions to their parent coroutine, which also delegates to the parent, and so on until the root,
+so the `CoroutineExceptionHandler` installed in their context is never used. 
+In addition to that, [async] builder always catches all exceptions and represents them in the resulting [Deferred] object, 
+so its `CoroutineExceptionHandler` has no effect either.
 
 <div class="sample" markdown="1" theme="idea" data-min-compiler-version="1.3">
 
@@ -102,10 +107,10 @@ fun main() = runBlocking {
     val handler = CoroutineExceptionHandler { _, exception -> 
         println("Caught $exception") 
     }
-    val job = GlobalScope.launch(handler) {
+    val job = GlobalScope.launch(handler) { // root coroutine, running in GlobalScope
         throw AssertionError()
     }
-    val deferred = GlobalScope.async(handler) {
+    val deferred = GlobalScope.async(handler) { // also root, but async instead of launch
         throw ArithmeticException() // Nothing will be printed, relying on user to call deferred.await()
     }
     joinAll(job, deferred)
@@ -501,6 +506,8 @@ Scope is completed
 [Deferred.await]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-deferred/await.html
 [GlobalScope]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-global-scope/index.html
 [CoroutineExceptionHandler]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-exception-handler/index.html
+[Job]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html
+[Deferred]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-deferred/index.html
 [Job.cancel]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/cancel.html
 [runBlocking]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/run-blocking.html
 [SupervisorJob()]: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-supervisor-job.html

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

@@ -52,11 +52,32 @@ public inline fun CoroutineExceptionHandler(crossinline handler: (CoroutineConte
     }
 
 /**
- * An optional element in the coroutine context to handle uncaught exceptions.
+ * An optional element in the coroutine context to handle **uncaught** exceptions.
  *
- * Normally, uncaught exceptions can only result from coroutines created using the [launch][CoroutineScope.launch] builder.
+ * Normally, uncaught exceptions can only result from _root_ coroutines created using the [launch][CoroutineScope.launch] builder.
+ * All _children_ coroutines (coroutines created in the context of another [Job]) delegate handling of their
+ * exceptions to their parent coroutine, which also delegates to the parent, and so on until the root,
+ * so the `CoroutineExceptionHandler` installed in their context is never used.
  * A coroutine that was created using [async][CoroutineScope.async] always catches all its exceptions and represents them
- * in the resulting [Deferred] object.
+ * in the resulting [Deferred] object, so it cannot result in uncaught exceptions either.
+ *
+ * ### Handling coroutine exceptions
+ *
+ * `CoroutineExceptionHandler` is a last-resort mechanism for global "catch all" behavior.
+ * If you need to handle exception in a specific part of the code, it is recommended to use `try`/`catch` around
+ * the corresponding code inside your coroutine, like this:
+ *
+ * ```
+ * scope.launch { // launch child coroutine in a scope
+ *     try {
+ *          // do something
+ *     } catch (e: Throwable) {
+ *          // handle exception
+ *     }
+ * }
+ * ```
+ *
+ * ### Implementation details
  *
  * By default, when no handler is installed, uncaught exception are handled in the following way:
  * * If exception is [CancellationException] then it is ignored

kotlinx-coroutines-core/jvm/test/guide/example-exceptions-02.kt

@@ -11,10 +11,10 @@ fun main() = runBlocking {
     val handler = CoroutineExceptionHandler { _, exception -> 
         println("Caught $exception") 
     }
-    val job = GlobalScope.launch(handler) {
+    val job = GlobalScope.launch(handler) { // root coroutine, running in GlobalScope
         throw AssertionError()
     }
-    val deferred = GlobalScope.async(handler) {
+    val deferred = GlobalScope.async(handler) { // also root, but async instead of launch
         throw ArithmeticException() // Nothing will be printed, relying on user to call deferred.await()
     }
     joinAll(job, deferred)