ConflatedChannel with lock to protect the one-element buffer, double-…

…linked list used for suspending receivers only.

kotlinx-coroutines-core/build.gradle

@@ -107,7 +107,7 @@ task jdk16Test(type: Test, dependsOn: [compileTestKotlinJvm, checkJdk16]) {
     testClassesDirs = files { jvmTest.testClassesDirs }
     executable = "$System.env.JDK_16/bin/java"
     exclude '**/*LFStressTest.*' // lock-freedom tests use LockFreedomTestEnvironment which needs JDK8
-    exclude '**/*LCStressTest.*' // lic-check tests use LinChecker which needs JDK8
+    exclude '**/*LCStressTest.*' // lin-check tests use LinChecker which needs JDK8
     exclude '**/exceptions/**'   // exceptions tests check suppressed exception which needs JDK8
     exclude '**/ExceptionsGuideTest.*'
 }

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

@@ -1,11 +1,9 @@
-/*
- * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
- */
-
 package kotlinx.coroutines.channels
 
-import kotlinx.coroutines.selects.*
+import kotlinx.coroutines.*
 import kotlinx.coroutines.internal.*
+import kotlinx.coroutines.selects.*
+import kotlin.native.concurrent.SharedImmutable
 
 /**
  * Channel that buffers at most one element and conflates all subsequent `send` and `offer` invocations,
@@ -15,80 +13,124 @@ import kotlinx.coroutines.internal.*
  * Sender to this channel never suspends and [offer] always returns `true`.
  *
  * This channel is created by `Channel(Channel.CONFLATED)` factory function invocation.
- *
- * This implementation is fully lock-free.
  */
 internal open class ConflatedChannel<E> : AbstractChannel<E>() {
-    protected final override val isBufferAlwaysEmpty: Boolean get() = true
-    protected final override val isBufferEmpty: Boolean get() = true
+    protected final override val isBufferAlwaysEmpty: Boolean get() = false
+    protected final override val isBufferEmpty: Boolean get() = value === EMPTY
     protected final override val isBufferAlwaysFull: Boolean get() = false
     protected final override val isBufferFull: Boolean get() = false
 
-    override fun onClosedIdempotent(closed: LockFreeLinkedListNode) {
-        @Suppress("UNCHECKED_CAST")
-        (closed.prevNode as? SendBuffered<E>)?.let { lastBuffered ->
-            conflatePreviousSendBuffered(lastBuffered)
-        }
-    }
+    override val isEmpty: Boolean get() = lock.withLock { isEmptyImpl }
 
-    /**
-     * Queues conflated element, returns null on success or
-     * returns node reference if it was already closed or is waiting for receive.
-     */
-    private fun sendConflated(element: E): ReceiveOrClosed<*>? {
-        val node = SendBuffered(element)
-        queue.addLastIfPrev(node) { prev ->
-            if (prev is ReceiveOrClosed<*>) return@sendConflated prev
-            true
-        }
-        conflatePreviousSendBuffered(node)
-        return null
+    private val lock = ReentrantLock()
+
+    private var value: Any? = EMPTY
+
+    private companion object {
+        @SharedImmutable
+        private val EMPTY = Symbol("EMPTY")
     }
 
-    private fun conflatePreviousSendBuffered(node: SendBuffered<E>) {
-        // Conflate all previous SendBuffered, helping other sends to conflate
-        var prev = node.prevNode
-        while (prev is SendBuffered<*>) {
-            if (!prev.remove()) {
-                prev.helpRemove()
+    // result is `OFFER_SUCCESS | Closed`
+    protected override fun offerInternal(element: E): Any {
+        var receive: ReceiveOrClosed<E>? = null
+        lock.withLock {
+            closedForSend?.let { return it }
+            // if there is no element written in buffer
+            if (value === EMPTY) {
+                // check for receivers that were waiting on the empty buffer
+                loop@ while(true) {
+                    receive = takeFirstReceiveOrPeekClosed() ?: break@loop // break when no receivers queued
+                    if (receive is Closed) {
+                        return receive!!
+                    }
+                    val token = receive!!.tryResumeReceive(element, null)
+                    if (token != null) {
+                        assert { token === RESUME_TOKEN }
+                        return@withLock
+                    }
+                }
             }
-            prev = prev.prevNode
+            value = element
+            return OFFER_SUCCESS
         }
+        // breaks here if offer meets receiver
+        receive!!.completeResumeReceive(element)
+        return receive!!.offerResult
     }
 
-    // result is always `OFFER_SUCCESS | Closed`
-    protected override fun offerInternal(element: E): Any {
-        while (true) {
-            val result = super.offerInternal(element)
-            when {
-                result === OFFER_SUCCESS -> return OFFER_SUCCESS
-                result === OFFER_FAILED -> { // try to buffer
-                    when (val sendResult = sendConflated(element)) {
-                        null -> return OFFER_SUCCESS
-                        is Closed<*> -> return sendResult
+    // result is `ALREADY_SELECTED | OFFER_SUCCESS | Closed`
+    protected override fun offerSelectInternal(element: E, select: SelectInstance<*>): Any {
+        var receive: ReceiveOrClosed<E>? = null
+        lock.withLock {
+            closedForSend?.let { return it }
+            if (value === EMPTY) {
+                loop@ while(true) {
+                    val offerOp = describeTryOffer(element)
+                    val failure = select.performAtomicTrySelect(offerOp)
+                    when {
+                        failure == null -> { // offered successfully
+                            receive = offerOp.result
+                            return@withLock
+                        }
+                        failure === OFFER_FAILED -> break@loop // cannot offer -> Ok to queue to buffer
+                        failure === RETRY_ATOMIC -> {} // retry
+                        failure === ALREADY_SELECTED || failure is Closed<*> -> return failure
+                        else -> error("performAtomicTrySelect(describeTryOffer) returned $failure")
                     }
-                    // otherwise there was receiver in queue, retry super.offerInternal
                 }
-                result is Closed<*> -> return result
-                else -> error("Invalid offerInternal result $result")
             }
+            // try to select sending this element to buffer
+            if (!select.trySelect()) {
+                return ALREADY_SELECTED
+            }
+            value = element
+            return OFFER_SUCCESS
         }
+        // breaks here if offer meets receiver
+        receive!!.completeResumeReceive(element)
+        return receive!!.offerResult
     }
 
-    // result is always `ALREADY_SELECTED | OFFER_SUCCESS | Closed`.
-    protected override fun offerSelectInternal(element: E, select: SelectInstance<*>): Any {
-        while (true) {
-            val result = if (hasReceiveOrClosed)
-                super.offerSelectInternal(element, select) else
-                (select.performAtomicTrySelect(describeSendConflated(element)) ?: OFFER_SUCCESS)
-            when {
-                result === ALREADY_SELECTED -> return ALREADY_SELECTED
-                result === OFFER_SUCCESS -> return OFFER_SUCCESS
-                result === OFFER_FAILED -> {} // retry
-                result === RETRY_ATOMIC -> {} // retry
-                result is Closed<*> -> return result
-                else -> error("Invalid result $result")
+    // result is `E | POLL_FAILED | Closed`
+    protected override fun pollInternal(): Any? {
+        var result: Any? = null
+        lock.withLock {
+            if (value === EMPTY) return closedForSend ?: POLL_FAILED
+            result = value
+            value = EMPTY
+        }
+        return result
+    }
+
+    // result is `E | POLL_FAILED | Closed`
+    protected override fun pollSelectInternal(select: SelectInstance<*>): Any? {
+        var result: Any? = null
+        lock.withLock {
+            if (value === EMPTY) return closedForSend ?: POLL_FAILED
+            if (!select.trySelect())
+                return ALREADY_SELECTED
+            result = value
+            value = EMPTY
+        }
+        return result
+    }
+
+    protected override fun onCancelIdempotent(wasClosed: Boolean) {
+        if (wasClosed) {
+            lock.withLock {
+                value = EMPTY
             }
         }
+        super.onCancelIdempotent(wasClosed)
     }
-}
+
+    override fun enqueueReceiveInternal(receive: Receive<E>): Boolean = lock.withLock {
+        super.enqueueReceiveInternal(receive)
+    }
+
+    // ------ debug ------
+
+    override val bufferDebugString: String
+        get() = "(value=$value)"
+}

kotlinx-coroutines-core/jvm/test/channels/ChannelLFStressTest.kt

@@ -41,14 +41,6 @@ class ChannelLFStressTest : TestBase() {
         checkAllReceived()
     }
 
-    @Test
-    fun testConflatedLockFreedom() {
-        // This test does not really verify that all sent elements were received
-        // and checks only LF property
-        channel = Channel(Channel.CONFLATED)
-        performLockFreedomTest()
-    }
-
     private fun performLockFreedomTest() {
         env.onCompletion {
             // We must cancel the channel to abort both senders & receivers

kotlinx-coroutines-core/jvm/test/linearizability/ChannelsLCStressTest.kt

@@ -17,7 +17,6 @@ import org.jetbrains.kotlinx.lincheck.paramgen.*
 import org.jetbrains.kotlinx.lincheck.verifier.*
 import org.junit.*
 
-
 class RendezvousChannelLCStressTest : ChannelLCStressTestBase(
     c = Channel(RENDEZVOUS),
     sequentialSpecification = SequentialRendezvousChannel::class.java
@@ -48,7 +47,6 @@ class ConflatedChannelLCStressTest : ChannelLCStressTestBase(
 )
 class SequentialConflatedChannel : SequentialIntChannelBase(CONFLATED)
 
-
 @Param.Params(
     Param(name = "value", gen = IntGen::class, conf = "1:5"),
     Param(name = "closeToken", gen = IntGen::class, conf = "1:3")
@@ -105,10 +103,10 @@ abstract class ChannelLCStressTestBase(private val c: Channel<Int>, private val
     // @Operation
     fun cancel(@Param(name = "closeToken") token: Int) = c.cancel(NumberedCancellationException(token))
 
-//    @Operation
+    // @Operation
     fun isClosedForReceive() = c.isClosedForReceive
 
-//    @Operation
+    // @Operation
     fun isClosedForSend() = c.isClosedForSend
 
     // TODO: this operation should be (and can be!) linearizable, but is not