Fix context support in Publisher.asFlow.flowOn

* When using asFlow().flowOn(...) context is now properly tracked and taken into account for both execution context of the reactive subscription and for injection into Reactor context.
* Publisher.asFlow slow-path implementation is simplified. It does not sure specialized openSubscription anymore, but always uses the same flow request logic.

Fixes #1765

reactive/kotlinx-coroutines-reactive/src/ReactiveFlow.kt

@@ -27,7 +27,7 @@ import kotlin.coroutines.*
  * see its documentation for additional details.
  */
 public fun <T : Any> Publisher<T>.asFlow(): Flow<T> =
-    PublisherAsFlow(this, 1)
+    PublisherAsFlow(this)
 
 /**
  * Transforms the given flow to a reactive specification compliant [Publisher].
@@ -39,30 +39,11 @@ public fun <T : Any> Flow<T>.asPublisher(): Publisher<T> = FlowAsPublisher(this)
 
 private class PublisherAsFlow<T : Any>(
     private val publisher: Publisher<T>,
-    capacity: Int
-) : ChannelFlow<T>(EmptyCoroutineContext, capacity) {
+    context: CoroutineContext = EmptyCoroutineContext,
+    capacity: Int = 1
+) : ChannelFlow<T>(context, capacity) {
     override fun create(context: CoroutineContext, capacity: Int): ChannelFlow<T> =
-        PublisherAsFlow(publisher, capacity)
-
-    override fun produceImpl(scope: CoroutineScope): ReceiveChannel<T> {
-        // use another channel for conflation (cannot do openSubscription)
-        if (capacity < 0) return super.produceImpl(scope)
-        // Open subscription channel directly
-        val channel = publisher
-            .injectCoroutineContext(scope.coroutineContext)
-            .openSubscription(capacity)
-        val handle = scope.coroutineContext[Job]?.invokeOnCompletion(onCancelling = true) { cause ->
-            channel.cancel(cause?.let {
-                it as? CancellationException ?: CancellationException("Job was cancelled", it)
-            })
-        }
-        if (handle != null && handle !== NonDisposableHandle) {
-            (channel as SendChannel<*>).invokeOnClose {
-                handle.dispose()
-            }
-        }
-        return channel
-    }
+        PublisherAsFlow(publisher, context, capacity)
 
     private val requestSize: Long
         get() = when (capacity) {
@@ -73,8 +54,26 @@ private class PublisherAsFlow<T : Any>(
         }
 
     override suspend fun collect(collector: FlowCollector<T>) {
+        val collectContext = coroutineContext
+        val newDispatcher = context[ContinuationInterceptor]
+        if (newDispatcher == null || newDispatcher == collectContext[ContinuationInterceptor]) {
+            // fast path -- subscribe directly in this dispatcher
+            return collectImpl(collectContext + context, collector)
+        }
+        // slow path -- produce in a separate dispatcher
+        collectSlowPath(collector)
+    }
+
+    private suspend fun collectSlowPath(collector: FlowCollector<T>) {
+        coroutineScope {
+            collector.emitAll(produceImpl(this + context))
+        }
+    }
+
+    private suspend fun collectImpl(injectContext: CoroutineContext, collector: FlowCollector<T>) {
         val subscriber = ReactiveSubscriber<T>(capacity, requestSize)
-        publisher.injectCoroutineContext(coroutineContext).subscribe(subscriber)
+        // inject subscribe context into publisher
+        publisher.injectCoroutineContext(injectContext).subscribe(subscriber)
         try {
             var consumed = 0L
             while (true) {
@@ -90,9 +89,9 @@ private class PublisherAsFlow<T : Any>(
         }
     }
 
-    // The second channel here is used only for broadcast
+    // The second channel here is used for produceIn/broadcastIn and slow-path (dispatcher change)
     override suspend fun collectTo(scope: ProducerScope<T>) =
-        collect(SendingCollector(scope.channel))
+        collectImpl(scope.coroutineContext, SendingCollector(scope.channel))
 }
 
 @Suppress("SubscriberImplementation")

reactive/kotlinx-coroutines-reactive/test/PublisherAsFlowTest.kt

@@ -120,7 +120,7 @@ class PublisherAsFlowTest : TestBase() {
                     7 -> try {
                         send(value)
                     } catch (e: CancellationException) {
-                        finish(6)
+                        expect(5)
                         throw e
                     }
                     else -> expectUnreached()
@@ -143,6 +143,6 @@ class PublisherAsFlowTest : TestBase() {
                 }
             }
         }
-        expect(5)
+        finish(6)
     }
 }

reactive/kotlinx-coroutines-reactor/test/FlowAsFluxTest.kt

@@ -4,16 +4,17 @@ import kotlinx.coroutines.*
 import kotlinx.coroutines.flow.*
 import kotlinx.coroutines.reactive.*
 import org.junit.Test
-import reactor.core.publisher.Mono
+import reactor.core.publisher.*
 import reactor.util.context.Context
-import kotlin.test.assertEquals
+import kotlin.test.*
 
 class FlowAsFluxTest : TestBase() {
     @Test
-    fun testFlowToFluxContextPropagation() {
+    fun testFlowAsFluxContextPropagation() {
         val flux = flow<String> {
             (1..4).forEach { i -> emit(createMono(i).awaitFirst()) }
-        }   .asFlux()
+        }
+            .asFlux()
             .subscriberContext(Context.of(1, "1"))
             .subscriberContext(Context.of(2, "2", 3, "3", 4, "4"))
         val list = flux.collectList().block()!!
@@ -24,4 +25,47 @@ class FlowAsFluxTest : TestBase() {
         val ctx = coroutineContext[ReactorContext]!!.context
         ctx.getOrDefault(i, "noValue")
     }
+
+    @Test
+    fun testFluxAsFlowContextPropagationWithFlowOn() = runTest {
+        expect(1)
+        Flux.create<String> {
+            it.next("OK")
+            it.complete()
+        }
+            .subscriberContext { ctx ->
+                expect(2)
+                assertEquals("CTX", ctx.get(1))
+                ctx
+            }
+            .asFlow()
+            .flowOn(ReactorContext(Context.of(1, "CTX")))
+            .collect {
+                expect(3)
+                assertEquals("OK", it)
+            }
+        finish(4)
+    }
+
+    @Test
+    fun testFluxAsFlowContextPropagationFromScope() = runTest {
+        expect(1)
+        withContext(ReactorContext(Context.of(1, "CTX"))) {
+            Flux.create<String> {
+                    it.next("OK")
+                    it.complete()
+                }
+            .subscriberContext { ctx ->
+                expect(2)
+                assertEquals("CTX", ctx.get(1))
+                ctx
+            }
+            .asFlow()
+            .collect {
+                expect(3)
+                assertEquals("OK", it)
+            }
+        }
+        finish(4)
+    }
 }

reactive/kotlinx-coroutines-reactor/test/FluxContextTest.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines.reactor
+
+import kotlinx.coroutines.*
+import kotlinx.coroutines.flow.*
+import kotlinx.coroutines.reactive.*
+import org.junit.*
+import org.junit.Test
+import reactor.core.publisher.*
+import kotlin.test.*
+
+class FluxContextTest : TestBase() {
+    private val dispatcher = newSingleThreadContext("FluxContextTest")
+
+    @After
+    fun tearDown() {
+        dispatcher.close()
+    }
+
+    @Test
+    fun testFluxCreateAsFlowThread() = runTest {
+        expect(1)
+        val mainThread = Thread.currentThread()
+        val dispatcherThread = withContext(dispatcher) { Thread.currentThread() }
+        assertTrue(dispatcherThread != mainThread)
+        Flux.create<String> {
+            assertEquals(dispatcherThread, Thread.currentThread())
+            it.next("OK")
+            it.complete()
+        }
+            .asFlow()
+            .flowOn(dispatcher)
+            .collect {
+                expect(2)
+                assertEquals("OK", it)
+                assertEquals(mainThread, Thread.currentThread())
+            }
+        finish(3)
+    }
+}

reactive/kotlinx-coroutines-rx2/test/FlowableContextTest.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines.rx2
+
+import io.reactivex.*
+import kotlinx.coroutines.*
+import kotlinx.coroutines.flow.*
+import kotlinx.coroutines.reactive.*
+import org.junit.*
+import org.junit.Test
+import kotlin.test.*
+
+class FlowableContextTest : TestBase() {
+    private val dispatcher = newSingleThreadContext("FlowableContextTest")
+
+    @After
+    fun tearDown() {
+        dispatcher.close()
+    }
+
+    @Test
+    fun testFlowableCreateAsFlowThread() = runTest {
+        expect(1)
+        val mainThread = Thread.currentThread()
+        val dispatcherThread = withContext(dispatcher) { Thread.currentThread() }
+        assertTrue(dispatcherThread != mainThread)
+        Flowable.create<String>({
+            assertEquals(dispatcherThread, Thread.currentThread())
+            it.onNext("OK")
+            it.onComplete()
+        }, BackpressureStrategy.BUFFER)
+            .asFlow()
+            .flowOn(dispatcher)
+            .collect {
+                expect(2)
+                assertEquals("OK", it)
+                assertEquals(mainThread, Thread.currentThread())
+            }
+        finish(3)
+    }
+}