Improve HotDataFlow behavior (EXPOSUREAPP-3777) (#1612)

* Fix HotDataFlow behavior.
* Prevent re-execution of past submissions when the observable goes cold.
* Guard internal value updates with a mutex in case the value update is not as thread-safe as we think it is.

* Fix problematic testcases.

Corona-Warn-App/src/main/java/de/rki/coronawarnapp/util/flow/HotDataFlow.kt

@@ -6,7 +6,6 @@ import kotlinx.coroutines.channels.BufferOverflow
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.SharingStarted
-import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.channelFlow
 import kotlinx.coroutines.flow.collect
 import kotlinx.coroutines.flow.distinctUntilChanged
@@ -15,6 +14,8 @@ import kotlinx.coroutines.flow.onCompletion
 import kotlinx.coroutines.flow.onStart
 import kotlinx.coroutines.flow.shareIn
 import kotlinx.coroutines.plus
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import timber.log.Timber
 import kotlin.coroutines.CoroutineContext
 
@@ -36,29 +37,35 @@ class HotDataFlow<T : Any>(
         extraBufferCapacity = Int.MAX_VALUE,
         onBufferOverflow = BufferOverflow.SUSPEND
     )
+    private val valueGuard = Mutex()
 
     private val internalFlow = channelFlow {
-        var currentValue = startValueProvider().also {
-            Timber.tag(tag).v("startValue=%s", it)
-            send(it)
+        var currentValue = valueGuard.withLock {
+            startValueProvider().also { send(it) }
         }
+        Timber.tag(tag).v("startValue=%s", currentValue)
 
-        updateActions.collect { updateAction ->
-            currentValue = updateAction(currentValue).also {
-                currentValue = it
-                send(it)
+        updateActions
+            .onCompletion {
+                Timber.tag(tag).v("updateActions onCompletion -> resetReplayCache()")
+                updateActions.resetReplayCache()
             }
-        }
+            .collect { updateAction ->
+                currentValue = valueGuard.withLock {
+                    updateAction(currentValue).also { send(it) }
+                }
+            }
+
+        Timber.tag(tag).v("internal channelFlow finished.")
     }
 
     val data: Flow<T> = internalFlow
         .distinctUntilChanged()
         .onStart { Timber.tag(tag).v("internal onStart") }
-        .catch {
-            Timber.tag(tag).e(it, "internal Error")
-            throw it
+        .onCompletion { err ->
+            if (err != null) Timber.tag(tag).w(err, "internal onCompletion due to error")
+            else Timber.tag(tag).v("internal onCompletion")
         }
-        .onCompletion { Timber.tag(tag).v("internal onCompletion") }
         .shareIn(
             scope = scope + coroutineContext,
             replay = 1,

Corona-Warn-App/src/test/java/de/rki/coronawarnapp/util/flow/HotDataFlowTest.kt

 package de.rki.coronawarnapp.util.flow
 
+import de.rki.coronawarnapp.util.mutate
 import io.kotest.matchers.shouldBe
 import io.kotest.matchers.types.instanceOf
 import io.mockk.coEvery
@@ -122,6 +123,48 @@ class HotDataFlowTest : BaseTest() {
         coVerify(exactly = 1) { valueProvider.invoke(any()) }
     }
 
+    data class TestData(
+        val number: Long = 1
+    )
+
+    @Test
+    fun `check multi threading value updates with more complex data`() {
+        val testScope = TestCoroutineScope()
+        val valueProvider = mockk<suspend CoroutineScope.() -> Map<String, TestData>>()
+        coEvery { valueProvider.invoke(any()) } returns mapOf("data" to TestData())
+
+        val hotData = HotDataFlow(
+            loggingTag = "tag",
+            scope = testScope,
+            startValueProvider = valueProvider,
+            sharingBehavior = SharingStarted.Lazily
+        )
+
+        val testCollector = hotData.data.test(startOnScope = testScope)
+        testCollector.silent = true
+
+        (1..10).forEach { _ ->
+            thread {
+                (1..400).forEach { _ ->
+                    hotData.updateSafely {
+                        mutate {
+                            this["data"] = getValue("data").copy(
+                                number = getValue("data").number + 1
+                            )
+                        }
+                    }
+                }
+            }
+        }
+
+        runBlocking {
+            testCollector.await { list, l -> list.size == 4001 }
+            testCollector.latestValues.map { it.values.single().number } shouldBe (1L..4001L).toList()
+        }
+
+        coVerify(exactly = 1) { valueProvider.invoke(any()) }
+    }
+
     @Test
     fun `only emit new values if they actually changed updates`() {
         val testScope = TestCoroutineScope()
@@ -163,9 +206,9 @@ class HotDataFlowTest : BaseTest() {
 
 
         testScope.runBlockingTest2(permanentJobs = true) {
-            val sub1 = hotData.data.test().start(scope = this)
-            val sub2 = hotData.data.test().start(scope = this)
-            val sub3 = hotData.data.test().start(scope = this)
+            val sub1 = hotData.data.test(tag = "sub1", startOnScope = this)
+            val sub2 = hotData.data.test(tag = "sub2", startOnScope = this)
+            val sub3 = hotData.data.test(tag = "sub3", startOnScope = this)
 
             hotData.updateSafely { "A" }
             hotData.updateSafely { "B" }
@@ -181,4 +224,47 @@ class HotDataFlowTest : BaseTest() {
         }
         coVerify(exactly = 1) { valueProvider.invoke(any()) }
     }
+
+    @Test
+    fun `update queue is wiped on completion`() = runBlockingTest2(permanentJobs = true) {
+        val valueProvider = mockk<suspend CoroutineScope.() -> Long>()
+        coEvery { valueProvider.invoke(any()) } returns 1
+
+        val hotData = HotDataFlow(
+            loggingTag = "tag",
+            scope = this,
+            coroutineContext = this.coroutineContext,
+            startValueProvider = valueProvider,
+            sharingBehavior = SharingStarted.WhileSubscribed(replayExpirationMillis = 0)
+        )
+
+        val testCollector1 = hotData.data.test(tag = "collector1", startOnScope = this)
+        testCollector1.silent = false
+
+        (1..10).forEach { _ ->
+            hotData.updateSafely {
+                this + 1L
+            }
+        }
+
+        advanceUntilIdle()
+
+        testCollector1.await { list, _ -> list.size == 11 }
+        testCollector1.latestValues shouldBe (1L..11L).toList()
+
+        testCollector1.cancel()
+        testCollector1.awaitFinal()
+
+        val testCollector2 = hotData.data.test(tag = "collector2", startOnScope = this)
+        testCollector2.silent = false
+
+        advanceUntilIdle()
+
+        testCollector2.cancel()
+        testCollector2.awaitFinal()
+
+        testCollector2.latestValues shouldBe listOf(1L)
+
+        coVerify(exactly = 2) { valueProvider.invoke(any()) }
+    }
 }

Corona-Warn-App/src/test/java/testhelpers/coroutines/FlowTest.kt

@@ -16,14 +16,17 @@ import kotlinx.coroutines.flow.onStart
 import kotlinx.coroutines.runBlocking
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
+import kotlinx.coroutines.test.TestCoroutineScope
+import kotlinx.coroutines.withTimeout
+import org.joda.time.Duration
 import timber.log.Timber
 
 fun <T> Flow<T>.test(
     tag: String? = null,
-    startOnScope: CoroutineScope
-): TestCollector<T> = test(tag ?: "FlowTest").start(scope = startOnScope)
+    startOnScope: CoroutineScope = TestCoroutineScope()
+): TestCollector<T> = createTest(tag ?: "FlowTest").start(scope = startOnScope)
 
-fun <T> Flow<T>.test(
+fun <T> Flow<T>.createTest(
     tag: String? = null
 ): TestCollector<T> = TestCollector(this, tag ?: "FlowTest")
 
@@ -74,9 +77,14 @@ class TestCollector<T>(
     val latestValues: List<T>
         get() = collectedValues
 
-    fun await(condition: (List<T>, T) -> Boolean): T = runBlocking {
-        emissions().first {
-            condition(collectedValues, it)
+    fun await(
+        timeout: Duration = Duration.standardSeconds(10),
+        condition: (List<T>, T) -> Boolean
+    ): T = runBlocking {
+        withTimeout(timeMillis = timeout.millis) {
+            emissions().first {
+                condition(collectedValues, it)
+            }
         }
     }
 
@@ -94,6 +102,8 @@ class TestCollector<T>(
     }
 
     fun cancel() {
+        if (job.isCompleted) throw IllegalStateException("Flow is already canceled.")
+
         runBlocking {
             job.cancelAndJoin()
         }