Introduce InlineList to simplify helpClose logic, reverse helpClose resume order

Author: qwwdfsad

kotlinx-coroutines-core/common/src/channels/AbstractChannel.kt

@@ -304,8 +304,7 @@ internal abstract class AbstractSendChannel<E> : SendChannel<E> {
          * Now if T2's close resumes T1's receive_2 then it's receive gets "closed for receive" exception, but
          * its subsequent attempt to send successfully rendezvous with receive_1, producing non-linearizable execution.
          */
-        var closedNode: Receive<E>? = null // used when one node was closed to avoid extra memory allocation
-        var closedList: ArrayList<Receive<E>>? = null // used when more nodes were closed
+        var closedList = InlineList<Receive<E>>()
         while (true) {
             // Break when channel is empty or has no receivers
             @Suppress("UNCHECKED_CAST")
@@ -316,19 +315,14 @@ internal abstract class AbstractSendChannel<E> : SendChannel<E> {
                 previous.helpRemove() // make sure remove is complete before continuing
                 continue
             }
-            // add removed nodes to a separate list 
-            if (closedNode == null) {
-                closedNode = previous
-            } else {
-                val list = closedList ?: ArrayList<Receive<E>>().also { closedList = it }
-                list += previous
-            }
-        }
-        // now notify all removed nodes that the channel was closed
-        if (closedNode != null) {
-            closedNode.resumeReceiveClosed(closed)
-            closedList?.forEach { it.resumeReceiveClosed(closed) }
+            // add removed nodes to a separate list
+            closedList += previous
         }
+        /*
+         * Now notify all removed nodes that the channel was closed
+         * in the order they were added to the channel
+         */
+        closedList.forEachReversed { it.resumeReceiveClosed(closed) }
         // and do other post-processing
         onClosedIdempotent(closed)
     }

kotlinx-coroutines-core/common/src/internal/InlineList.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+@file:Suppress("UNCHECKED_CAST")
+
+package kotlinx.coroutines.internal
+
+import kotlinx.coroutines.assert
+
+/*
+ * Inline class that represents a mutable list, but does not allocate an underlying storage
+ * for zero and one elements.
+ * Cannot be parametrized with `List<*>`.
+ */
+internal inline class InlineList<E>(private val holder: Any? = null) {
+    public operator fun plus(element: E): InlineList<E>  {
+        assert { element !is List<*> } // Lists are prohibited
+        return when (holder) {
+            null -> InlineList(element)
+            is ArrayList<*> -> {
+                (holder as ArrayList<E>).add(element)
+                InlineList(holder)
+            }
+            else -> {
+                val list = ArrayList<E>(4)
+                list.add(holder as E)
+                list.add(element)
+                InlineList(list)
+            }
+        }
+    }
+
+    public inline fun forEachReversed(action: (E) -> Unit) {
+        when (holder) {
+            null -> return
+            !is ArrayList<*> -> action(holder as E)
+            else -> {
+                val list = holder as ArrayList<E>
+                for (i in (list.size - 1) downTo 0) {
+                    action(list[i])
+                }
+            }
+        }
+    }
+}

kotlinx-coroutines-core/common/test/channels/ChannelsTest.kt

@@ -19,6 +19,37 @@ class ChannelsTest: TestBase() {
         assertEquals(testList, testList.asReceiveChannel().toList())
     }
 
+    @Test
+    fun testCloseWithMultipleWaiters() = runTest {
+        val channel = Channel<Int>()
+        launch {
+            try {
+                expect(2)
+                channel.receive()
+                expectUnreached()
+            } catch (e: ClosedReceiveChannelException) {
+                expect(5)
+            }
+        }
+
+        launch {
+            try {
+                expect(3)
+                channel.receive()
+                expectUnreached()
+            } catch (e: ClosedReceiveChannelException) {
+                expect(6)
+            }
+        }
+
+        expect(1)
+        yield()
+        expect(4)
+        channel.close()
+        yield()
+        finish(7)
+    }
+
     @Test
     fun testAssociate() = runTest {
         assertEquals(testList.associate { it * 2 to it * 3 },