Fix LinkedListChannel.onUndeliveredElement call on channel cancel

Fixes #2435

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

@@ -477,7 +477,14 @@ internal abstract class AbstractSendChannel<E>(
         override val pollResult: Any? get() = element
         override fun tryResumeSend(otherOp: PrepareOp?): Symbol? = RESUME_TOKEN.also { otherOp?.finishPrepare() }
         override fun completeResumeSend() {}
-        override fun resumeSendClosed(closed: Closed<*>) {}
+
+        /**
+         * This method should be never called, see special logic in [LinkedListChannel.onCancelIdempotentList].
+         */
+        override fun resumeSendClosed(closed: Closed<*>) {
+            assert { false }
+        }
+
         override fun toString(): String = "SendBuffered@$hexAddress($element)"
     }
 }
@@ -669,6 +676,13 @@ internal abstract class AbstractChannel<E>(
             // Add to the list only **after** successful removal
             list += previous as Send
         }
+        onCancelIdempotentList(list, closed)
+    }
+
+    /**
+     * This method is overriden by [LinkedListChannel] to handle cancellation of [SendBuffered] elements from the list.
+     */
+    protected open fun onCancelIdempotentList(list: InlineList<Send>, closed: Closed<*>) {
         list.forEachReversed { it.resumeSendClosed(closed) }
     }
 

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

@@ -298,7 +298,7 @@ internal open class ArrayChannel<E>(
         }
         // then clean all queued senders
         super.onCancelIdempotent(wasClosed)
-        undeliveredElementException?.let { throw it } // throw cancel exception at the end if there was one
+        undeliveredElementException?.let { throw it } // throw UndeliveredElementException at the end if there was one
     }
 
     // ------ debug ------

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

@@ -120,7 +120,7 @@ internal open class ConflatedChannel<E>(onUndeliveredElement: OnUndeliveredEleme
             undeliveredElementException = updateValueLocked(EMPTY)
         }
         super.onCancelIdempotent(wasClosed)
-        undeliveredElementException?.let { throw it } // throw exception at the end if there was one
+        undeliveredElementException?.let { throw it } // throw UndeliveredElementException at the end if there was one
     }
 
     private fun updateValueLocked(element: Any?): UndeliveredElementException? {

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

@@ -58,5 +58,19 @@ internal open class LinkedListChannel<E>(onUndeliveredElement: OnUndeliveredElem
             }
         }
     }
+
+    override fun onCancelIdempotentList(list: InlineList<Send>, closed: Closed<*>) {
+        var undeliveredElementException: UndeliveredElementException? = null
+        list.forEachReversed {
+            when (it) {
+                is SendBuffered<*> -> {
+                    @Suppress("UNCHECKED_CAST")
+                    undeliveredElementException = onUndeliveredElement?.callUndeliveredElementCatchingException(it.element as E, undeliveredElementException)
+                }
+                else -> it.resumeSendClosed(closed)
+            }
+        }
+        undeliveredElementException?.let { throw it } // throw UndeliveredElementException at the end if there was one
+    }
 }
 

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

@@ -50,6 +50,20 @@ class ChannelUndeliveredElementTest : TestBase() {
         assertTrue(resA.isCancelled) // now cancelled in buffer
     }
 
+    @Test
+    fun testUnlimitedChannelCancelled() = runTest {
+        val channel = Channel<Resource>(Channel.UNLIMITED) { it.cancel() }
+        val resA = Resource("A")
+        val resB = Resource("B")
+        channel.send(resA) // goes to buffer
+        channel.send(resB) // goes to buffer
+        assertFalse(resA.isCancelled) // it is in buffer, not cancelled
+        assertFalse(resB.isCancelled) //  it is in buffer, not cancelled
+        channel.cancel() // now cancel the channel
+        assertTrue(resA.isCancelled) // now cancelled in buffer
+        assertTrue(resB.isCancelled) // now cancelled in buffer
+    }
+
     @Test
     fun testConflatedResourceCancelled() = runTest {
         val channel = Channel<Resource>(Channel.CONFLATED) { it.cancel() }