GCD serial queue and race condition - ios

Question

I have two methods which run on a serial queue. Each method return a copy of some class. I'm trying to achieve thread safety solution while also mainting data integrity.
for example:
-(Users *) getAllUsers
{
__block copiedUsers;
dispatch_sync(_backgroundQueue, ^{
copiedUsers = [self.users copy]; // return copy object to calling thread.
});
return copiedUsers;
}
-(Orders *) getAllOrders
{
__block copiedOrders;
dispatch_sync(_backgroundQueue, ^{
copiedOrders = [self.Orders copy]; // return copy object to calling thread.
});
return copiedOrders;
}
In addition to this two methods, I have a worker class that add/remove users and orders, all done via a serial queue backgroundQueue.
If in the main thread I call getAllUsers and then getAllOrders right after the other my data integrity isn't safe because between the two calls the worker class might have changed the model.
my question is how can I make to the caller a nice interface that allows multiple methods to run atomically?

Answer 1

Model is only updated from backgroundQueue serial queue.
Client talks to model via a method that receives a block that runs in the background queue.
In addition, not to freeze main thread, I create another queue and run a block that talks with the gateway method.
P.S - attention that dispatch_sync is called only in runBlockAndGetNeededDataSafely to avoid deadlocks.
Code sample:
aViewController.m
ManagerClass *m = [ManagerClass new];
dispatch_queue_t q = dispatch_queue_create("funnelQueue", DISPATCH_QUEUE_SERIAL);
dispatch_block_t block_q = ^{
__Users *users;
__Orders *orders;
[manager runBlockAndGetNeededDataSafely:^
{
users = [manager getUsers];
orders = [manager getOrders];
dispatch_async(dispatch_get_main_queue(),
^{
// got data safely - no thread issues, copied objects. update UI!
[self refreshViewWithUsers:users
orders:orders];
});
}];
}
dispatch_async(q, block_q);
Manager.m implementation:
-(void) runBlockInBackground:(dispatch_block_t) block
{
dispatch_sync(self.backgroundQueue, block);
}
-(Users *) getAllUsers
{
return [self.users copy];
}
-(Orders *) getAllOrders
{
return [self.Orders copy];
}

Answer 2

To answer your question about how to checking the current queue:
First when you create the queue, give it a tag:
static void* queueTag = &queueTag;
dispatch_queue_t queue = dispatch_queue_create("a queue", 0);
dispatch_queue_set_specific(queue, queueTag, queueTag, NULL);
and then run a block like this:
-(void)runBlock:(void(^)()) block
{
if (dispatch_get_specific(queueTag) != NULL) {
block();
}else {
dispatch_async(self.queue, block);
}
}

Answer 3

Your example doesn't work. I suggest to use completion callback. You should have an option to know when the worker finish his job to return to value.
- (void)waitForCompletion:(BOOL*)conditions length:(int)len timeOut:(NSInteger)timeoutSecs {
NSDate *timeoutDate = [NSDate dateWithTimeIntervalSinceNow:timeoutSecs];
BOOL done = YES;
for (int i = 0; i < len; i++) {
done = done & *(conditions+i);
}
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:timeoutDate];
if([timeoutDate timeIntervalSinceNow] < 0.0)
break;
//update done
done = YES;
for (int i = 0; i < len; i++) {
done = done & *(conditions+i);
}
} while (!done);
}
-(void) getAllUsers:(void(^)(User* user, NSError* error))completion
{
dispatch_async(_backgroundQueue, ^{
BOOL condition[2] = [self.userCondition, self.orderCondition];
[self waitForCompletion: &condition[0] length:2 timeOut:60];
if (completion) {
completion([self.users copy], nil);
}
});
}