'Simultaneous accesses to 0x1c0a7f0f8, but modification requires exclusive access error on Xcode 9 beta 4
my project uses both Objective-C and Swift code. When a user logs in, it calls a set of apis for user preference, I have a DataCoordinator.swift class which schedules the API operation and I make this calls from UserDetailViewController.m class to load user preferences. This use to work fine before I migrated my code to Swift 4 using Xcode 9 beta 4. Now when I login it crashes by giving me this error in my DataCoordinator class. Below is a sample of my DataCoordinator and Viewcontroller class.
DataCoordinator.swift
import UIKit
@objcMembers
class DataCoordinator: NSObject {
//MARK:- Private
fileprivate var user = myDataStore.sharedInstance().user
fileprivate var preferenceFetchOperations = [FetchOperation]()
fileprivate func scheduleFetchOperation(_ operation:FetchOperation, inFetchOperations operations:inout [FetchOperation]) {
guard operations.index(of: operation) == nil else { return }
operations.append(operation)
}
fileprivate func completeFetchOperation(_ fetchOperation:FetchOperation, withError error:Error?, andCompletionHandler handler:@escaping FetchCompletionHandler) {
func removeOperation(_ operation:FetchOperation, fromOperations operations:inout [FetchOperation]) {
if operations.count > 0 {
operations.remove(at: operations.index(of: fetchOperation)!)
handler(error)
}
}
if preferenceFetchOperations.contains(fetchOperation) {
removeOperation(fetchOperation, fromOperations: &preferenceFetchOperations)
}
}
fileprivate func schedulePreferencesFetchOperation(_ serviceName:String, fetch:@escaping FetchOperationBlock){
let operation = FetchOperation(name: serviceName, fetch: fetch);
scheduleFetchOperation(operation, inFetchOperations: &preferenceFetchOperations)
}
fileprivate func runOperationsIn(_ fetchOperations:inout [FetchOperation]) {
for var operation in fetchOperations {
guard operation.isActivated == false else { continue }
operation.isActivated = true
operation.execute()
}
}
//MARK:- Non-Private
typealias FetchCompletionHandler = (_ error:Error?)->Void
var numberOfPreferencesFetchCalls:Int {
get { return preferenceFetchOperations.count }
}
// MARK: -
func fetchPreferences(_ completionHandler:@escaping FetchCompletionHandler) -> Void {
defer {
runOperationsIn(&preferenceFetchOperations)
}
schedulePreferencesFetchOperation("com.fetchPreferences.type1") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType1Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
schedulePreferencesFetchOperation("com.fetchPreferences.type2") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType2Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
schedulePreferencesFetchOperation("com.fetchPreferences.type3") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType3Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
schedulePreferencesFetchOperation("com.fetchPreferences.type4") {[unowned self] (operation:FetchOperation) in
WebServiceManager.getType4Detail(for: user) {[unowned self] (error) in
self.completeFetchOperation(operation, withError: error, andCompletionHandler: completionHandler)
}
}
}
}
// MARK:- Fetch Operation Struct
private typealias FetchOperationBlock = (_ operation:FetchOperation)->Void
private struct FetchOperation:Hashable {
fileprivate var runToken = 0
fileprivate let fetchBlock:FetchOperationBlock
let name:String!
var isActivated:Bool {
get {
return runToken == 0 ? false : true
}
mutating set {
if runToken == 0 && newValue == true {
runToken = 1
}
}
}
fileprivate var hashValue: Int {
get {
return name.hashValue
}
}
func execute() -> Void {
fetchBlock(self)
}
init (name:String, fetch:@escaping FetchOperationBlock) {
self.name = name
self.fetchBlock = fetch
}
}
private func ==(lhs: FetchOperation, rhs: FetchOperation) -> Bool {
return lhs.hashValue == rhs.hashValue
}
//This is how I call it in my viewcontrollers viewDidLoad method
__weak UserDetailViewController *weakSelf = self;
[self.dataCoordinator fetchPreferences:^(NSError * _Nullable error) {
if (error == nil) {
[weakSelf didFetchPrefrences];
}
else {
// handle error
}
}];
//completion response
- (void)didFetchPrefrences {
//when api calls complete load data
if (self.dataCoordinator.numberOfPreferencesFetchCalls == 0) {
//Load details
}
}
I'm not sure how to proceed on this, I saw a bug report at https://bugs.swift.org/browse/SR-5119 but it seems to be fixed in Xcode 9 beta 3. Any help is appreciated
Solution 1:[1]
I think this 'bug' may be a Swift 4 'feature', specifically something they call 'Exclusive access to Memory'.
Check out this WWDC video. Around the 50 minute mark, the long-haired speaker explains it.
https://developer.apple.com/videos/play/wwdc2017/402/?time=233
You could try turning the thread sanitizer off in your scheme settings if you're happy to ignore it. However, the debugger is trying to tell you about a subtle threading issue so it's probably a better use of your time to try to figure out why you've got something writing to your array at the same time it's being read from.
Solution 2:[2]
Under the target's Build Settings. Select No Enforcement for Exclusive Access to Memory from Swift Compiler - Code Generation
Solution 3:[3]
Only in Swift 4 and when using .initial option for your KVO Settings
If you check your context in observeValue method, just make your context variable static. This blog post describes this bug in detail.
Solution 4:[4]
In Swift 5.0, this will be the default behavior when running your application in Release mode. Before 5.0 (Swift 4.2.1 as of today and lower) this behavior is only running when in Debug mode.
Your application will fail in release mode if you ignored this error.
Consider this example:
func modifyTwice(_ value: inout Int, by modifier: (inout Int) -> ()) {
modifier(&value)
modifier(&value)
}
func testCount() {
var count = 1
modifyTwice(&count) { $0 += count }
print(count)
}
What is the value of count, when the print(count) line is printed? Well I don't know either and the compiler gives unpredicatable results when you run this code. This isn't allowed in Swift 4.0 in debug mode and in Swift 5.0 it crashes even in runtime.
Solution 5:[5]
Swift 5 here.
I was calling a function from a property didSet and testing another property from the same object and I got this error.
I fixed it by calling my function from within another thread:
DispatchQueue.global(qos: .userInitiated).async {
// do something
}
Basic fix but it works.
Solution 6:[6]
The answers by @Mark Bridges and @geek1706 are good answers but I would like to add my 2 cents about this matter and give a general example.
As stated above this is a feature in Swift 4 SE-176.
The implementation should still be permitted to detect concurrent conflicting accesses, of course. Some programmers may wish to use an opt-in thread-safe enforcement mechanism instead, at least in some build configurations.
The exclusive access enforces that every write mutation of vars must be exclusive when accessing that variable. In a multithread environment, multiple threads accessing a shared var and one or more can modify it.
There's nothing like a good example:
If we try to mutate a shared value in a multi-threaded environment using an abstraction (mutation occurs on a protocol type) between 2 objects and the Exclusive Access to Memory is on, our app will crash.
protocol Abstraction {
var sharedProperty: String {get set}
}
class MyClass: Abstraction {
var sharedProperty: String
init(sharedProperty: String) {
self.sharedProperty = sharedProperty
}
func myMutatingFunc() {
// Invoking this method from a background thread
sharedProperty = "I've been changed"
}
}
class MainClass {
let myClass: Abstraction
init(myClass: Abstraction) {
self.myClass = myClass
}
func foobar() {
DispatchQueue.global(qos: .background).async {
self.myClass.myMutatingFunc()
}
}
}
let myClass = MyClass(sharedProperty: "Hello")
let mainClass = MainClass(myClass: myClass)
// This will crash
mainClass.foobar()
Since we didn't state that the Abstraction protocol is class bound, during runtime, inside myMutatingFunc, the capture of self will be treated as struct even though we injected an actual class (MyClass).
Escaping variables generally require dynamic enforcement instead of static enforcement. This is because Swift cannot reason about when an escaping closure will be called and thus when the variable will be accessed.
The solution is to bound the Abstraction protocol to class:
protocol Abstraction: class
Solution 7:[7]
What I would do is change FetchOperation to a class instead of struct.
Solution 8:[8]
In my case, Swift 4 actually uncovered a kind of bug that I wouldn't have noticed until I started calling a function from more than one place. My function was passed an inout global array and it was referencing both that parameter and the global name. When I changed the function to reference only the parameter, the "simultaneous access" error went away.
Solution 9:[9]
What fixed it for me, is adding lazy to the var
Solution 10:[10]
Returning zero in the numberOfSections override function will cause this crash:
override func numberOfSections(in collectionView: UICollectionView) -> Int {
// This causes a crash!
return 0
}
Simple solution - return 1 in the function above and then return 0 in the collectionView(_:numberOfItemsInSection:) function.
override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
Solution 11:[11]
In my case, I changed the table height during process of build the project. At that time my device was connected with network. I deleted the derived data and it resolved the issue for me.
Solution 12:[12]
This was happening for me when, when I was modifying the predicate as per search query and also reloading the table so its sort of reentry (access) to the same class object code. Please check my stack trace, its setting the 'searchPhrase' and from DB worker again triggering the table reload which in-turn comes back to DB worker again (for Item count. after reloadData).
0 libswiftCore.dylib 0x000000010a325590 swift_beginAccess + 568
1 Groupe 0x00000001063d5cf0 SCPickUsersInteractor.dbWorker.getter + 59
2 Groupe 0x00000001063d5830 SCPickUsersInteractor.count.getter + 58
3 Groupe 0x00000001063d8550 protocol witness for SCPickUsersInteractorProtocol.count.getter in conformance SCPickUsersInteractor + 14
4 Groupe 0x0000000105a340d0 SCPickUsersViewController.tableView(_:numberOfRowsInSection:) + 278
5 Groupe 0x0000000105a34280 @objc SCPickUsersViewController.tableView(_:numberOfRowsInSection:) + 76
6 UIKitCore 0x00007fff482a3537 -[UITableView _numberOfRowsInSection:] + 62
7 UIKitCore 0x00007fff482b3c42 -[UISectionRowData refreshWithSection:tableView:tableViewRowData:] + 1938
8 UIKitCore 0x00007fff482b85cd -[UITableViewRowData numberOfRows] + 67
9 UIKitCore 0x00007fff4827362d -[UITableView noteNumberOfRowsChanged] + 117
10 UIKitCore 0x00007fff48271b8b -[UITableView reloadData] + 1426
11 Groupe 0x0000000105a35890 SCPickUsersViewController.reloadTableView() + 152
12 Groupe 0x0000000105a37060 protocol witness for SCListUpdatesProtocol.reloadTableView() in conformance SCPickUsersViewController + 9
13 Groupe 0x00000001068a14f0 SCPickUsersPresenter.reloadTableView() + 158
14 Groupe 0x00000001068a2350 protocol witness for SCListUpdatesProtocol.reloadTableView() in conformance SCPickUsersPresenter + 17
15 Groupe 0x0000000105a8bc90 SCPickUsersDBWorker.searchPhrase.didset + 911
16 Groupe 0x0000000105a8c0e0 SCPickUsersDBWorker.searchPhrase.setter + 356
17 Groupe 0x0000000105a8ffb0 protocol witness for SCFRCProtocol.searchPhrase.setter in conformance SCPickUsersDBWorker + 37
18 Groupe 0x00000001063d6500 SCPickUsersInteractor.searchPhrase.setter + 274
19 Groupe 0x00000001063d8630 protocol witness for SCPickUsersInteractorProtocol.searchPhrase.setter in conformance SCPickUsersInteractor + 17
20 Groupe 0x0000000105a34eb0 SCPickUsersViewController.searchBar(_:textDidChange:) + 322
21 Groupe 0x0000000105a35020 @objc SCPickUsersViewController.searchBar(_:textDidChange:) + 105
Solution worked for me: Called 'reloadData' from DB worker class after few milli seconds or half a second.
Solution 13:[13]
I was facing the same issue with computed property in which I was using nested iterations over my data, i.e. compactMap, FlatMap and filter.
I had to change
.receive(on: RunLoop.main)
to
.receive(on: DispatchQueue.main)
in my bindings and crash resolved.
Sources
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Source: Stack Overflow