Merge pull request #21008 from wordpress-mobile/perform-query-runtime…

…-check

Add warnings about incorrect `performQuery` usages

WordPress/Classes/Utility/CoreDataHelper.swift

@@ -196,6 +196,8 @@ extension ContextManager.ContextManagerError: LocalizedError, CustomDebugStringC
 
 extension CoreDataStack {
     /// Perform a query using the `mainContext` and return the result.
+    ///
+    /// - Warning: Do not return `NSManagedObject` instances from the closure.
     func performQuery<T>(_ block: @escaping (NSManagedObjectContext) -> T) -> T {
         mainContext.performAndWait { [mainContext] in
             block(mainContext)
@@ -358,3 +360,69 @@ extension CoreDataStack {
         try ContextManager.migrateDataModelsIfNecessary(storeURL: databaseLocation, objectModel: objectModel)
     }
 }
+
+/// This extension declares many `performQuery` usages that may introduce Core Data concurrency issues.
+///
+/// The context object used by the `performQuery` function is opaque to the caller. The caller should not assume what
+/// the context object is, nor the context queue type (the main queue or a background queue). That means the caller
+/// does not have enough information to guarantee safe access to the returned `NSManagedObject` instances.
+///
+/// The closure passed to the `performQuery` function should use the context to query objects and return non- Core Data
+/// types. Here is an example of how it should be used.
+///
+/// ```
+/// // Wrong:
+/// let account = coreDataStack.performQuery { context in
+///     return Account.lookUp(in: context)
+/// }
+/// let name = account.username
+///
+/// // Right:
+/// let name = coreDataStack.performQuery { context in
+///     let account = Account.lookUp(in: context)
+///     return account.username
+/// }
+/// ```
+extension CoreDataStack {
+    @available(*, deprecated, message: "Returning `NSManagedObject` instances may introduce Core Data concurrency issues.")
+    func performQuery<T>(_ block: @escaping (NSManagedObjectContext) -> T) -> T where T: NSManagedObject {
+        mainContext.performAndWait { [mainContext] in
+            block(mainContext)
+        }
+    }
+
+    @available(*, deprecated, message: "Returning `NSManagedObject` instances may introduce Core Data concurrency issues.")
+    func performQuery<T>(_ block: @escaping (NSManagedObjectContext) -> T?) -> T? where T: NSManagedObject {
+        mainContext.performAndWait { [mainContext] in
+            block(mainContext)
+        }
+    }
+
+    @available(*, deprecated, message: "Returning `NSManagedObject` instances may introduce Core Data concurrency issues.")
+    func performQuery<T>(_ block: @escaping (NSManagedObjectContext) -> T) -> T where T: Sequence, T.Element: NSManagedObject {
+        mainContext.performAndWait { [mainContext] in
+            block(mainContext)
+        }
+    }
+
+    @available(*, deprecated, message: "Returning `NSManagedObject` instances may introduce Core Data concurrency issues.")
+    func performQuery<T>(_ block: @escaping (NSManagedObjectContext) -> T?) -> T? where T: Sequence, T.Element: NSManagedObject {
+        mainContext.performAndWait { [mainContext] in
+            block(mainContext)
+        }
+    }
+
+    @available(*, deprecated, message: "Returning `NSManagedObject` instances may introduce Core Data concurrency issues.")
+    func performQuery<T, E>(_ block: @escaping (NSManagedObjectContext) -> Result<T, E>) -> Result<T, E> where T: NSManagedObject, E: Error {
+        mainContext.performAndWait { [mainContext] in
+            block(mainContext)
+        }
+    }
+
+    @available(*, deprecated, message: "Returning `NSManagedObject` instances may introduce Core Data concurrency issues.")
+    func performQuery<T, E>(_ block: @escaping (NSManagedObjectContext) -> Result<T, E>?) -> Result<T, E>? where T: NSManagedObject, E: Error {
+        mainContext.performAndWait { [mainContext] in
+            block(mainContext)
+        }
+    }
+}