Materialize an exception for Job onCancelling handlers

Fixes #436

Author: qwwdfsad

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

@@ -10,7 +10,6 @@ import kotlinx.coroutines.experimental.internalAnnotations.*
 import kotlinx.coroutines.experimental.intrinsics.*
 import kotlinx.coroutines.experimental.selects.*
 import kotlin.coroutines.experimental.*
-import kotlin.coroutines.experimental.intrinsics.*
 
 /**
  * A concrete implementation of [Job]. It is optionally a child to a parent job.
@@ -524,7 +523,8 @@ internal open class JobSupport constructor(active: Boolean) : Job, SelectClause0
         if (!_state.compareAndSet(expect, Finishing(list, cancelled, false))) return false
         onFinishingInternal(cancelled)
         onCancellationInternal(cancelled)
-        notifyCancellation(list, cause)
+        // Materialize cause
+        notifyCancellation(list, cancelled.cause)
         return true
     }
 

common/kotlinx-coroutines-core-common/test/JobTest.kt

@@ -157,4 +157,22 @@ class JobTest : TestBase() {
         parent.join()
         finish(6)
     }
-}
+
+    @Test
+    fun testOnCancellingHandler() = runTest {
+        val job = launch(coroutineContext) {
+            expect(2)
+            delay(Long.MAX_VALUE)
+        }
+
+        job.invokeOnCompletion(onCancelling = true) {
+            assertNotNull(it)
+            expect(3)
+        }
+
+        expect(1)
+        yield()
+        job.cancelAndJoin()
+        finish(4)
+    }
+}