Do not report already handled exception in select builder

Fixes #1433

Author: qwwdfsad

kotlinx-coroutines-core/common/src/selects/Select.kt

@@ -311,10 +311,17 @@ internal class SelectBuilderImpl<in R>(
     internal fun handleBuilderException(e: Throwable) {
         if (trySelect(null)) {
             resumeWithException(e)
-        } else {
-            // Cannot handle this exception -- builder was already resumed with a different exception,
-            // so treat it as "unhandled exception"
-            handleCoroutineException(context, e)
+        } else if (e !is CancellationException) {
+            /*
+             * Cannot handle this exception -- builder was already resumed with a different exception,
+             * so treat it as "unhandled exception". But only if  it is not the completion reason
+             *  and it's not the cancellation. Otherwise, in the face of structured concurrency
+             * the same exception will be reported to theglobal exception handler.
+             */
+            val result = getResult()
+            if (result !is CompletedExceptionally || unwrap(result.cause) !== unwrap(e)) {
+                handleCoroutineException(context, e)
+            }
         }
     }
 

kotlinx-coroutines-core/jvm/test/flow/CombineStressTest.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines.flow
+
+import kotlinx.coroutines.*
+import org.junit.*
+
+class CombineStressTest : TestBase() {
+
+    @Test
+    public fun testCancellation() = runTest {
+        withContext(Dispatchers.Default + CoroutineExceptionHandler { _, _ -> expectUnreached() }) {
+            flow {
+                expect(1)
+                repeat(1_000 * stressTestMultiplier) {
+                    emit(it)
+                }
+            }.flatMapLatest {
+                combine(flowOf(it), flowOf(it)) { arr -> arr[0] }
+            }.collect()
+            finish(2)
+            reset()
+        }
+    }
+
+    @Test
+    public fun testFailure() = runTest {
+        val innerIterations = 100 * stressTestMultiplierSqrt
+        val outerIterations = 10 * stressTestMultiplierSqrt
+        withContext(Dispatchers.Default + CoroutineExceptionHandler { _, _ -> expectUnreached() }) {
+            repeat(outerIterations) {
+                try {
+                    flow {
+                        expect(1)
+                        repeat(innerIterations) {
+                            emit(it)
+                        }
+                    }.flatMapLatest {
+                        combine(flowOf(it), flowOf(it)) { arr -> arr[0] }
+                    }.onEach {
+                        if (it >= innerIterations / 2) throw TestException()
+                    }.collect()
+                } catch (e: TestException) {
+                    expect(2)
+                }
+                finish(3)
+                reset()
+            }
+        }
+    }
+}