[in_app_purchase] Write to the transactions update queue from the main thread (flutter#9068)

Equeue to the channel from the main thread to avoid channel safety issues.

Background: `sendTransactionUpdate` can be called from the main thread, where it is safe to send messages on the `onTransactionsUpdated` channel. This seems to happen when the purchase code path succeeds and generates a transaction. However `sendTransactionUpdate` can also be called on non-main threads serving background executors, when transaction callbacks result from restoring purchases or other transactions are reported to the transaction subscription (eg. a purchase made outside of the app, or a refund is issued -- this can be triggered from XCode's StoreKit Test transaction log). In these cases, at least sometimes, an error is logged about writing to a channel on a non-platform thread (see the issue below).

Enqueue on the main thread to avoid channel thread-safety issues. This could be optimized further, but I believe this is safe.

Resolves flutter/flutter#166493

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: edpizzi

packages/in_app_purchase/in_app_purchase_storekit/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.3.22+1
+
+* Fix a channel thread-safety issue when StoreKit2 is enabled.
+
 ## 0.3.22
 
 * Adds `sync()` and `countryCode()`.

packages/in_app_purchase/in_app_purchase_storekit/darwin/in_app_purchase_storekit/Sources/in_app_purchase_storekit/InAppPurchasePlugin.swift

@@ -44,7 +44,7 @@ public class InAppPurchasePlugin: NSObject, FlutterPlugin, FIAInAppPurchaseAPI {
     self._updateListenerTask = task
   }
 
-  var transactionCallbackAPI: InAppPurchase2CallbackAPI? = nil
+  var transactionCallbackAPI: InAppPurchase2CallbackAPIProtocol? = nil
 
   public static func register(with registrar: FlutterPluginRegistrar) {
     #if os(iOS)

packages/in_app_purchase/in_app_purchase_storekit/darwin/in_app_purchase_storekit/Sources/in_app_purchase_storekit/StoreKit2/InAppPurchasePlugin+StoreKit2.swift

@@ -207,12 +207,14 @@ extension InAppPurchasePlugin: InAppPurchase2API {
   /// Sends an transaction back to Dart. Access these transactions with `purchaseStream`
   private func sendTransactionUpdate(transaction: Transaction, receipt: String? = nil) {
     let transactionMessage = transaction.convertToPigeon(receipt: receipt)
-    self.transactionCallbackAPI?.onTransactionsUpdated(newTransactions: [transactionMessage]) {
-      result in
-      switch result {
-      case .success: break
-      case .failure(let error):
-        print("Failed to send transaction updates: \(error)")
+    Task { @MainActor in
+      self.transactionCallbackAPI?.onTransactionsUpdated(newTransactions: [transactionMessage]) {
+        result in
+        switch result {
+        case .success: break
+        case .failure(let error):
+          print("Failed to send transaction updates: \(error)")
+        }
       }
     }
   }

packages/in_app_purchase/in_app_purchase_storekit/example/shared/RunnerTests/InAppPurchaseStoreKit2PluginTests.swift

@@ -7,8 +7,17 @@ import XCTest
 
 @testable import in_app_purchase_storekit
 
-@available(iOS 15.0, macOS 12.0, *)
+final class FakeIAP2Callback: InAppPurchase2CallbackAPIProtocol {
+  func onTransactionsUpdated(
+    newTransactions newTransactionsArg: [in_app_purchase_storekit.SK2TransactionMessage],
+    completion: @escaping (Result<Void, in_app_purchase_storekit.PigeonError>) -> Void
+  ) {
+    // We should only write to a flutter channel from the main thread.
+    XCTAssertTrue(Thread.isMainThread)
+  }
+}
 
+@available(iOS 15.0, macOS 12.0, *)
 final class InAppPurchase2PluginTests: XCTestCase {
   private var session: SKTestSession!
   private var plugin: InAppPurchasePlugin!
@@ -24,6 +33,7 @@ final class InAppPurchase2PluginTests: XCTestCase {
     plugin = InAppPurchasePluginStub(receiptManager: FIAPReceiptManagerStub()) { request in
       DefaultRequestHandler(requestHandler: FIAPRequestHandler(request: request))
     }
+    plugin.transactionCallbackAPI = FakeIAP2Callback()
     try plugin.startListeningToTransactions()
   }
 
@@ -296,6 +306,8 @@ final class InAppPurchase2PluginTests: XCTestCase {
         XCTFail("Purchase should NOT fail. Failed with \(error)")
       }
     }
+    await fulfillment(of: [purchaseExpectation], timeout: 5)
+
     plugin.restorePurchases { result in
       switch result {
       case .success():
@@ -304,8 +316,7 @@ final class InAppPurchase2PluginTests: XCTestCase {
         XCTFail("Restore purchases should NOT fail. Failed with \(error)")
       }
     }
-
-    await fulfillment(of: [restoreExpectation, purchaseExpectation], timeout: 5)
+    await fulfillment(of: [restoreExpectation], timeout: 5)
   }
 
   func testFinishTransaction() async throws {

packages/in_app_purchase/in_app_purchase_storekit/pubspec.yaml

@@ -2,7 +2,7 @@ name: in_app_purchase_storekit
 description: An implementation for the iOS and macOS platforms of the Flutter `in_app_purchase` plugin. This uses the StoreKit Framework.
 repository: https://github.com/flutter/packages/tree/main/packages/in_app_purchase/in_app_purchase_storekit
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+in_app_purchase%22
-version: 0.3.22
+version: 0.3.22+1
 
 environment:
   sdk: ^3.4.0