Fixed spurious exception during select.onJoin clause registration

* Removed #1130 workaround from debounce
* Fixed race in debounce

Fixes #1130

Author: elizarov

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

@@ -529,7 +529,6 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
             if (state !is Incomplete) {
                 // already complete -- select result
                 if (select.trySelect(null)) {
-                    select.completion.context.checkCompletion() // always check for our completion
                     block.startCoroutineUnintercepted(select.completion)
                 }
                 return

kotlinx-coroutines-core/common/src/flow/operators/Delay.kt

@@ -64,13 +64,8 @@ public fun <T> Flow<T>.debounce(timeoutMillis: Long): Flow<T> {
         coroutineScope {
             val values = Channel<Any?>(Channel.CONFLATED) // Actually Any, KT-30796
             // Channel is not closed deliberately as there is no close with value
-            val collector = launch {
-                try {
-                    collect { value -> values.send(value ?: NullSurrogate) }
-                } catch (e: Throwable) {
-                    values.close(e) // Workaround for #1130
-                    throw e
-                }
+            val collector = async {
+                collect { value -> values.send(value ?: NullSurrogate) }
             }
 
             var isDone = false
@@ -89,7 +84,7 @@ public fun <T> Flow<T>.debounce(timeoutMillis: Long): Flow<T> {
                     }
 
                     // Close with value 'idiom'
-                    collector.onJoin {
+                    collector.onAwait {
                         if (lastValue != null) emit(NullSurrogate.unbox(lastValue))
                         isDone = true
                     }

kotlinx-coroutines-core/common/test/selects/SelectLoopTest.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+@file:Suppress("NAMED_ARGUMENTS_NOT_ALLOWED") // KT-21913
+
+package kotlinx.coroutines.selects
+
+import kotlinx.coroutines.*
+import kotlinx.coroutines.channels.*
+import kotlin.test.*
+
+class SelectLoopTest : TestBase() {
+    // https://github.com/Kotlin/kotlinx.coroutines/issues/1130
+    @Test
+    fun testChannelSelectLoop() = runTest(
+        expected = { it is TestException }
+    ) {
+        expect(1)
+        val channel = Channel<Unit>()
+        val job = launch {
+            expect(2)
+            channel.send(Unit)
+            expect(3)
+            throw TestException()
+        }
+        var isDone = false
+        while (!isDone) {
+            select<Unit> {
+                channel.onReceiveOrNull {
+                    expect(4)
+                    assertEquals(Unit, it)
+                }
+                job.onJoin {
+                    expect(5)
+                    isDone = true
+                }
+            }
+        }
+        finish(6)
+    }
+}