fix(DataStore): serialize IncomingAsyncSubscriptionEventPublisher events

AmplifyPlugins/DataStore/Sources/AWSDataStorePlugin/Sync/SubscriptionSync/IncomingAsyncSubscriptionEventPublisher.swift

@@ -43,7 +43,7 @@ final class IncomingAsyncSubscriptionEventPublisher: AmplifyCancellable {
     private let awsAuthService: AWSAuthServiceBehavior
 
     private let consistencyQueue: DispatchQueue
-
+    private let taskQueue: TaskQueue<Void>
     private let modelName: ModelName
 
     init(modelSchema: ModelSchema,
@@ -58,6 +58,7 @@ final class IncomingAsyncSubscriptionEventPublisher: AmplifyCancellable {
         self.consistencyQueue = DispatchQueue(
             label: "com.amazonaws.Amplify.RemoteSyncEngine.\(modelSchema.name)"
         )
+        self.taskQueue = TaskQueue()
         self.modelName = modelSchema.name
 
         self.connectionStatusQueue = OperationQueue()
@@ -170,26 +171,26 @@ final class IncomingAsyncSubscriptionEventPublisher: AmplifyCancellable {
 
     func sendConnectionEventIfConnected(event: Event) {
         if combinedConnectionStatusIsConnected {
-            incomingSubscriptionEvents.send(event)
+            send(event)
         }
     }
 
     func genericValueListenerHandler(event: Event, cancelAwareBlock: CancelAwareBlockOperation) {
         if case .connection = event {
             connectionStatusQueue.addOperation(cancelAwareBlock)
         } else {
-            incomingSubscriptionEvents.send(event)
+            send(event)
         }
     }
 
     func genericCompletionListenerHandler(result: Result<Void, APIError>) {
         switch result {
         case .success:
-            incomingSubscriptionEvents.send(completion: .finished)
+            send(completion: .finished)
         case .failure(let apiError):
             log.verbose("[InitializeSubscription.1] API.subscribe failed for `\(modelName)` error: \(apiError.errorDescription)")
             let dataStoreError = DataStoreError(error: apiError)
-            incomingSubscriptionEvents.send(completion: .failure(dataStoreError))
+            send(completion: .failure(dataStoreError))
         }
     }
 
@@ -237,6 +238,20 @@ final class IncomingAsyncSubscriptionEventPublisher: AmplifyCancellable {
         incomingSubscriptionEvents.subscribe(subscriber)
     }
 
+    func send(_ event: Event) {
+        taskQueue.async { [weak self] in
+            guard let self else { return }
+            self.incomingSubscriptionEvents.send(event)
+        }
+    }
+
+    func send(completion: Subscribers.Completion<DataStoreError>) {
+        taskQueue.async { [weak self] in
+            guard let self else { return }
+            self.incomingSubscriptionEvents.send(completion: completion)
+        }
+    }
+
     func cancel() {
         consistencyQueue.sync {
             genericCompletionListenerHandler(result: .successfulVoid)

AmplifyPlugins/DataStore/Tests/AWSDataStorePluginTests/Sync/SubscriptionSync/IncomingAsyncSubscriptionEventPublisherTests.swift

@@ -0,0 +1,61 @@
+//
+// Copyright Amazon.com Inc. or its affiliates.
+// All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+import XCTest
+@testable import Amplify
+@testable import AmplifyTestCommon
+@testable import AWSPluginsCore
+@testable import AWSDataStorePlugin
+
+final class IncomingAsyncSubscriptionEventPublisherTests: XCTestCase {
+    var apiPlugin: MockAPICategoryPlugin!
+    override func setUp() {
+        apiPlugin = MockAPICategoryPlugin()
+    }
+
+    /// This test was written to to reproduce a bug where the subscribe would miss events emitted by the publisher.
+    /// The pattern in this test using the publisher (`IncomingAsyncSubscriptionEventPublisher`) and subscriber
+    /// (`IncomingAsyncSubscriptionEventToAnyModelMapper`) are identical to the usage in `AWSModelReconciliationQueue.init()`.
+    ///
+    /// See the changes in this PR: https://github.com/aws-amplify/amplify-swift/pull/3489
+    ///
+    /// Before the PR changes, the publisher would emit events concurrently which caused some of them to be missed
+    /// by the subscriber even though the subscriber applied back pressure to process one event at a time (demand
+    /// of `max(1)`). For more details regarding back-pressure, see
+    /// https://developer.apple.com/documentation/combine/processing-published-elements-with-subscribers
+    ///
+    /// The change, to publish the events though the same TaskQueue ensures that the events are properly buffered
+    /// and sent only when the subscriber demands for it.
+    func testSubscriberRecievedEvents() async throws {
+        let expectedEvents = expectation(description: "Expected number of ")
+        let numberOfEvents = 50
+        expectedEvents.expectedFulfillmentCount = numberOfEvents
+        let asyncEvents = await IncomingAsyncSubscriptionEventPublisher(
+            modelSchema: Post.schema,
+            api: apiPlugin,
+            modelPredicate: nil,
+            auth: nil,
+            authModeStrategy: AWSDefaultAuthModeStrategy(),
+            awsAuthService: nil)
+        let mapper = IncomingAsyncSubscriptionEventToAnyModelMapper()
+        asyncEvents.subscribe(subscriber: mapper)
+        let sink = mapper
+            .publisher
+            .sink(
+                receiveCompletion: { _ in },
+                receiveValue: { _ in
+                    expectedEvents.fulfill()
+                }
+            )
+        DispatchQueue.concurrentPerform(iterations: numberOfEvents) { index in
+            asyncEvents.send(.connection(.connected))
+        }
+
+        await fulfillment(of: [expectedEvents], timeout: 2)
+        sink.cancel()
+    }
+}