I am currently implementing a multithreaded application and I encounter a strange race condition (leading to an ARC problem: error for object 0x7f8bcbd6a1c0: pointer being freed was not allocated).
The app creates multiple NSOperations, each one is downloading and processing information. As in every one of these NSOperations an error may occur (e.g., the web service is not available), I want to propagate the error up so I can handle it. However, I seem to encounter a race condition and the operations try to access invalid memory.
A minimalistic example which shows the memory access problem is the following one:
- (void) createError: (NSError**) err {
*err = [[NSError alloc] initWithDomain:#"Test" code:12345 userInfo:nil];
}
- (void)viewDidLoad {
[super viewDidLoad];
__block NSError *globalError = nil;
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
NSOperation *op = nil;
for (int i=0; i<10; i++) {
op = [NSBlockOperation blockOperationWithBlock:^{
NSError *err = nil;
[self createError:&err];
// This loop increases the chance to get the race condition
for (int j=0; j<10000;j++) {
#synchronized(globalError) {
globalError = err;
}
}
}];
[queue addOperation:op];
}
[queue waitUntilAllOperationsAreFinished];
if (globalError) {
NSLog(#"An error occured in at least one of the operations");
}
}
Whenever I run this program, I get an exception. Mostly it is error for object 0x7fc0b860e860: pointer being freed was not allocated, however, sometimes I also got an EXC_BAD_ACCESS (code=EXC_I386_GPFLT) break in the debugger.
I have added the for-Loop over 10000 iterations only to increase the chance that the race condition occurs. If I leave it, the error only occurs in rare occasions (but still has to be fixed, obviously I am doing something wrong here).
The #synchronized directive uses the object you pass to control the synchronization, so, as you note, you're trying to synchronize on nil. And even if it wasn't nil, the object referenced by the #synchronized directive is a different object every time, largely defeating the attempt to synchronize.
The easiest change is to use #synchronized(self), instead. Or, create a NSLock object and then lock and unlock it when you access globalError. Or, if this was a high demand situation, use GCD queue (either serial queue or reader-writer pattern).
Related
I have been asking and trying to understand how completion handlers work. Ive used quite a few and I've read many tutorials. i will post the one I use here, but I want to be able to create my own without using someone else's code as a reference.
I understand this completion handler where this caller method:
-(void)viewDidLoad{
[newSimpleCounter countToTenThousandAndReturnCompletionBLock:^(BOOL completed){
if(completed){
NSLog(#"Ten Thousands Counts Finished");
}
}];
}
and then in the called method:
-(void)countToTenThousandAndReturnCompletionBLock:(void (^)(BOOL))completed{
int x = 1;
while (x < 10001) {
NSLog(#"%i", x);
x++;
}
completed(YES);
}
Then I sorta came up with this one based on many SO posts:
- (void)viewDidLoad{
[self.spinner startAnimating];
[SantiappsHelper fetchUsersWithCompletionHandler:^(NSArray *users) {
self.usersArray = users;
[self.tableView reloadData];
}];
}
which will reload the tableview with the received data users after calling this method:
typedef void (^Handler)(NSArray *users);
+(void)fetchUsersWithCompletionHandler:(Handler)handler {
NSURL *url = [NSURL URLWithString:#"http://www.somewebservice.com"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalAndRemoteCacheData timeoutInterval:10];
[request setHTTPMethod: #"GET"];
**// We dispatch a queue to the background to execute the synchronous NSURLRequest**
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
// Perform the request
NSURLResponse *response;
NSError *error = nil;
NSData *receivedData = [NSURLConnection sendSynchronousRequest:request
returningResponse:&response
error:&error];
if (error) { **// If an error returns, log it, otherwise log the response**
// Deal with your error
if ([response isKindOfClass:[NSHTTPURLResponse class]]) {
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;
NSLog(#"HTTP Error: %d %#", httpResponse.statusCode, error);
return;
}
NSLog(#"Error %#", error);
return;
}
**// So this line won't get processed until the response from the server is returned?**
NSString *responseString = [[NSString alloc] initWithData:receivedData encoding:NSUTF8StringEncoding];
NSArray *usersArray = [[NSArray alloc] init];
usersArray = [NSJSONSerialization JSONObjectWithData:[responseString dataUsingEncoding:NSASCIIStringEncoding] options:0 error:nil];
// Finally when a response is received and this line is reached, handler refers to the block passed into this called method...so it dispatches back to the main queue and returns the usersArray
if (handler){
dispatch_sync(dispatch_get_main_queue(), ^{
handler(usersArray);
});
}
});
}
I can see it in the counter example, that the called method (with the passed block) will never exit the loop until it is done. Thus the 'completion' part actually depends on the code inside the called method, not the block passed into it?
In this case the 'completion' part depends on the fact that the call to NSURLRequest is synchronous. What if it was asynchronous? How would I be able to hold off calling the block until my data was populated by the NSURLResponse?
Your first example is correct and complete and the best way to understand completion blocks. There is no further magic to them. They do not automatically get executed ever. They are executed when some piece of code calls them.
As you note, in the latter example, it is easy to call the completion block at the right time because everything is synchronous. If it were asynchronous, then you need to store the block in an instance variable, and call it when the asynchronous operation completed. It is up to you to arrange to be informed when the operation completes (possibly using its completion handler).
Do be careful when you store a block in an ivar. One of your examples includes:
self.usersArray = users;
The call to self will cause the block to retain self (the calling object). This can easily create a retain loop. Typically, you need to take a weak reference to self like this:
- (void)viewDidLoad{
[self.spinner startAnimating];
__weak typeof(self) weakSelf = self;
[SantiappsHelper fetchUsersWithCompletionHandler:^(NSArray *users) {
typeof(self) strongSelf = weakSelf;
if (strongSelf) {
[strongSelf setUsersArray:users];
[[strongSelf tableView] reloadData];
}
}];
}
This is a fairly pedantic version of the weakSelf/strongSelf pattern, and it could be done a little simpler in this case, but it demonstrates all the pieces you might need. You take a weak reference to self so that you don't create a retain loop. Then, in the completely block, you take a strong reference so that self so that it can't vanish on you in the middle of your block. Then you make sure that self actually still exists, and only then proceed. (Since messaging nil is legal, you could have skipped the strongSelf step in this particular case, and it would be the same.)
Your first example (countToTenThousandAndReturnCompletionBLock) is actually a synchronous method. A completion handler doesn't make much sense here: Alternatively, you could call that block immediately after the hypothetical method countToTenThousand (which is basically the same, just without the completion handler).
Your second example fetchUsersWithCompletionHandler: is an asynchronous method. However, it's actually quite suboptimal:
It should somehow signal the call-site that the request may have failed. That is, either provide an additional parameter to the completion handler, e.g. " NSError* error or us a single parameter id result. In the first case, either error or array is not nil, and in the second case, the single parameter result can be either an error object (is kind of NSError) or the actual result (is kind of NSArray).
In case your request fails, you miss to signal the error to the call-site.
There are code smells:
As a matter of fact, the underlying network code implemented by the system is asynchronous. However, the utilized convenient class method sendSynchronousRequest: is synchronous. That means, as an implementation detail of sendSynchronousRequest:, the calling thread is blocked until after the result of the network response is available. And this_blocking_ occupies a whole thread just for waiting. Creating a thread is quite costly, and just for this purpose is a waste. This is the first code smell. Yes, just using the convenient class method sendSynchronousRequest: is by itself bad programming praxis!
Then in your code, you make this synchronous request again asynchronous through dispatching it to a queue.
So, you are better off using an asynchronous method (e.g. sendAsynchronous...) for the network request, which presumable signals the completion via a completion handler. This completion handler then may invoke your completion handler parameter, taking care of whether you got an actual result or an error.
I am new to asynchronous callbacks and have been given differing advice. I need to perform asynchronous callbacks and after many hours of research I still do not know if I should use blocks or GCD and queues. Any pointers would be welcome.
OK. So what I was really asking is:
"in order to use an 'asynchronous' callbacks, do I need to use GCD and queues?"
What I am gathering from the answers is that the answer is YES. Definitely GCD and queues inside of blocks.
My confusion stemmed from the fact that I had been given the direction that all I needed was a block, like the code below:
[UIView animateWithDuration:.4f
animations:^{
flashView.alpha = 0.f;
}
completion:^(BOOL finished){
[flashView removeFromSuperview];
}
];
But what I am seeing in the answers here is that the above block in not sufficient for making 'asynchronous' callbacks. Instead I DO in-fact need to use GCD and queues inside a block, like the code below:
- (void)invokeAsync:(id (^)(void))asyncBlock resultBlock:(void (^)(id))resultBlock errorBlock:(void (^)(id))errorBlock {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
id result = nil;
id error = nil;
#try {
result = asyncBlock();
} #catch (NSException *exception) {
NSLog(#"caught exception: %#", exception);
error = exception;
}
// tell the main thread
dispatch_async(dispatch_get_main_queue(), ^{
NSAutoreleasePool *secondaryPool = [[NSAutoreleasePool alloc] init];
if (error != nil) {
errorBlock(error);
} else {
resultBlock(result);
}
[secondaryPool release];
});
[pool release];
});
}
An asynchronous callback is one where your current thread keeps executing statements, and you detach the execution of code in a different thread to be ran later.
There are several technologies to accomplish this. On this example I'm calling the method cacheImage:, with parameter image (just an example) in 4 different asynchronous ways.
// 1. NSThread
[NSThread detachNewThreadSelector:#selector(cacheImage:) toTarget:self withObject:image];
// 2. performSelector...
[self performSelectorInBackground:#selector(cacheImage:) withObject:image];
// 3. NSOperationQueue
NSInvocationOperation *invOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(cacheImage:) object:image];
NSOperationQueue *opQueue = [[NSOperationQueue alloc] init];
[opQueue addOperation:invOperation];
// 4. GCD
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self cacheImage:image];
});
By far the more simple way is to use GCD because it already has a thread ready for you to use, instead having to create it yourself with the other options.
However, because blocks are implemented as objects, you could indeed use blocks without GCD, for example:
// block definition
typedef void (^hello_t)();
// method that uses a block as parameter
-(void) runBlock:(hello_t)hello {
hello();
}
// asynchronous execution of a block
[NSThread detachNewThreadSelector:#selector(runBlock) toTarget:self withObject:^(){
NSLog(#"hi");
}];
PS: you don't need to use NSAutoreleasePool manually unless you create many many objects and you want to free memory immediately. Also, #try #catch are rarely used in Objective-C
Not any point of confusion here. GCD also has a block execution.
GCD API, which supports the asynchronous execution of operations at the Unix level of the system.
Block objects are a C-level syntactic and runtime feature. They are similar to standard C functions, but in addition to executable code they may also contain variable bindings to automatic (stack) or managed (heap) memory. A block can therefore maintain a set of state (data) that it can use to impact behavior when executed.
Apple designed blocks with the explicit goal of making it easier to write programs for the Grand Central Dispatch threading architecture, although it is independent of that architecture and can be used in much the same way as closures in other languages. Apple has implemented blocks both in their own branch of the GNU Compiler Collection and in the Clang LLVM compiler front end. Language runtime library support for blocks is also available as part of the LLVM project.
Therefore you can use any one of them given you same functionality.
From the docs:
The completion block you provide is executed when the value returned by the isFinished method changes to YES. Thus, this block is executed by the operation object after the operation’s primary task is finished or cancelled.
I'm using RestKit/AFNetworking, if that matters.
I have multiple dependencies in my NSOperation in a OperationQueue. I use the completion block to set some variables (appending the results to an array) that my child requires.
(task1,...,taskN) -> taskA
taskA addDependency: task1-taskN
Will taskA receive incomplete data since the child can execute before the completion block is fired?
Reference
Do NSOperations and their completionBlocks run concurrently?
I did a simple test by adding a sleep in my completion block and I had a different result. The completion block runs in the main thread. While all the completion block are sleeping, the child task ran.
As I discuss below under "a few observations", you have no assurances that this final dependent operation will not start before your other sundry AFNetworking completion blocks have finished. It strikes me that if this final operation really needs to wait for these completion blocks to finish, then you have a couple of alternatives:
Use semaphores within each of the n the completion blocks to signal when they're done and have the completion operation wait for n signals; or
Don't queue this final operation up front, but rather have your completion blocks for the individual uploads keep track of how many pending uploads are still incomplete, and when it falls to zero, then initiate the final "post" operation.
As you pointed out in your comments, you could wrap your invocation of the AFNetworking operation and its completion handler in your own operation, at which point you can then use the standard addDependency mechanism.
You could abandon the addDependency approach (which adds an observer on the isFinished key of the operation upon which this operation is dependent, and once all those dependencies are resolved, performs the isReady KVN; the problem being that this can theoretically happen before your completion block is done) and replace it with your own isReady logic. For example, imagine you had a post operation which you could add your own key dependencies and remove them manually in your completion block, rather than having them removed automatically upon isFinished. Thus, you custom operation
#interface PostOperation ()
#property (nonatomic, getter = isReady) BOOL ready;
#property (nonatomic, strong) NSMutableArray *keys;
#end
#implementation PostOperation
#synthesize ready = _ready;
- (void)addKeyDependency:(id)key {
if (!self.keys)
self.keys = [NSMutableArray arrayWithObject:key];
else
[self.keys addObject:key];
self.ready = NO;
}
- (void)removeKeyDependency:(id)key {
[self.keys removeObject:key];
if ([self.keys count] == 0)
self.ready = YES;
}
- (void)setReady:(BOOL)ready {
if (ready != _ready) {
[self willChangeValueForKey:#"isReady"];
_ready = ready;
[self didChangeValueForKey:#"isReady"];
}
}
- (void)addDependency:(NSOperation *)operation{
NSAssert(FALSE, #"You should not use addDependency with this custom operation");
}
Then, your app code could do something like, using addKeyDependency rather than addDependency, and explicitly either removeKeyDependency or cancel in the completion blocks:
PostOperation *postOperation = [[PostOperation alloc] init];
for (NSInteger i = 0; i < numberOfImages; i++) {
NSURL *url = ...
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSString *key = [url absoluteString]; // or you could use whatever unique value you want
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
[operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
// update your model or do whatever
// now inform the post operation that this operation is done
[postOperation removeKeyDependency:key];
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
// handle the error any way you want
// perhaps you want to cancel the postOperation; you'd either cancel it or remove the dependency
[postOperation cancel];
}];
[postOperation addKeyDependency:key];
[queue addOperation:operation];
}
[queue addOperation:postOperation];
This is using AFHTTPRequestOperation, and you'd obviously replace all of this logic with the appropriate AFNetworking operation for your upload, but hopefully it illustrates the idea.
Original answer:
A few observations:
As I think you concluded, when your operation completes, it (a) initiates its completion block; (b) makes the queue available for other operations (either operations that had not yet started because of maxConcurrentOperationCount, or because of dependencies between the operations). I do not believe that you have any assurances that the completion block will be done before that next operation commences.
Empirically, it looks like the dependent operation does not actually trigger until after the completion blocks are done, but (a) I don't see that documented anywhere and (b) this is moot because if you're using AFNetworking's own setCompletionBlockWithSuccess, it ends up dispatching the block asynchronously to the main queue (or the defined successCallbackQueue), thereby thwarting any (undocumented) assurances of synchrony.
Furthermore, you say that the completion block runs in the main thread. If you're talking about the built in NSOperation completion block, you have no such assurances. In fact, the setCompletionBlock documentation says:
The exact execution context for your completion block is not guaranteed but is typically a secondary thread. Therefore, you should not use this block to do any work that requires a very specific execution context. Instead, you should shunt that work to your application’s main thread or to the specific thread that is capable of doing it. For example, if you have a custom thread for coordinating the completion of the operation, you could use the completion block to ping that thread.
But if you're talking about one of AFNetworking's custom completion blocks, e.g. those that you might set with AFHTTPRequestOperation's setCompletionBlockWithSuccess, then, yes, it's true that those are generally dispatched back to the main queue. But AFNetworking does this using the standard completionBlock mechanism, so the above concerns still apply.
It matters if your NSOperation is a subclass of AFHTTPRequestOperation. AFHTTPRequestOperation uses the NSOperation's property completionBlock for its own purpose in method setCompletionBlockWithSuccess:failure. In that case, don't set the property completionBlock yourself!
It seems, AFHTTPRequestOperation's success and failure handler will run on the main thread.
Otherwise, the execution context of NSOperation's completion block is "undefined". That means, the completion block can execute on any thread/queue. In fact it executes on some private queue.
IMO, this is the preferred approach, unless the execution context shall be explicitly specified by the call-site. Executing completion handlers on threads or queues which instances are accessible (the main thread for example) can easily cause dead locks by an unwary developer.
Edit:
If you want to start a dependent operation after the completion block of the parent operation has been finished, you can solve that by making the completion block content itself a NSBlockOperation (a new parent) and add this operation as a dependency to the children operation and start it in a queue. You may realize, that this quickly becomes unwieldy, though.
Another approach would require an utility class or class library which is especially suited to solve asynchronous problems in a more concise and easy way. ReactiveCocoa would be capable to solve such (an easy) problem. However, it's unduly complex and it actually has a "learning curve" - and a steep one. I wouldn't recommend it, unless you agree to spend a few weeks in learning it and have a lot other asynchronous use cases and even much more complex ones.
A simpler approach would utilize "Promises" which are pretty common in JavaScript, Python, Scala and a few other languages.
Now, please read carefully, the (easy) solution is actually below:
"Promises" (sometimes called Futures or Deferred) represent the eventual result of an asynchronous task. Your fetch request is such asynchronous task. But instead specifying a completion handler, the asynchronous method/task returns a Promise:
-(Promise*) fetchThingsWithURL:(NSURL*)url;
You obtain the result - or the error - with registering a success handler block or a failure handler block like so:
Promise* thingsPromise = [self fetchThingsWithURL:url];
thingsPromise.then(successHandlerBlock, failureHandlerBlock);
or, the blocks inlined:
thingsPromise.then(^id(id things){
// do something with things
return <result of success handler>
}, ^id(NSError* error){
// Ohps, error occurred
return <result of failure handler>
});
And shorter:
[self fetchThingsWithURL:url]
.then(^id(id result){
return [self.parser parseAsync:result];
}, nil);
Here, parseAsync: is an asynchronous method which returns a Promise. (Yes, a Promise).
You might wonder how to get the result from the parser?
[self fetchThingsWithURL:url]
.then(^id(id result){
return [self.parser parseAsync:result];
}, nil)
.then(^id(id parserResult){
NSLog(#"Parser returned: %#", parserResult);
return nil; // result not used
}, nil);
This actually starts async task fetchThingsWithURL:. Then when finished successfully, it starts async task parseAsync:. Then when this finished successfully, it prints the result, otherwise it prints the error.
Invoking several asynchronous tasks sequentially, one after the other, is called "continuation" or "chaining".
Note that the whole statement above is asynchronous! That is, when you wrap the above statement into a method, and execute it, the method returns immediately.
You might wonder how to catch any errors, say fetchThingsWithURL: fails, or parseAsync::
[self fetchThingsWithURL:url]
.then(^id(id result){
return [self.parser parseAsync:result];
}, nil)
.then(^id(id parserResult){
NSLog(#"Parser returned: %#", parserResult);
return nil; // result not used
}, nil)
.then(/*succes handler ignored*/, ^id (NSError* error){
// catch any error
NSLog(#"ERROR: %#", error);
return nil; // result not used
});
Handlers execute after the corresponding task has been finished (of course). If the task succeeds, the success handler will be called (if any). If the tasks fails, the error handler will be called (if any).
Handlers may return a Promise (or any other object). For example, if an asynchronous task finished successfully, its success handler will be invoked which starts another asynchronous task, which returns the promise. And when this is finished, yet another one can be started, and so force. That's "continuation" ;)
You can return anything from a handler:
Promise* finalResult = [self fetchThingsWithURL:url]
.then(^id(id result){
return [self.parser parseAsync:result];
}, nil)
.then(^id(id parserResult){
return #"OK";
}, ^id(NSError* error){
return error;
});
Now, finalResult will either eventually become the value #"OK" or an NSError.
You can save the eventual results into an array:
array = #[
[self task1],
[self task2],
[self task3]
];
and then continue when all tasks have been finished successfully:
[Promise all:array].then(^id(results){
...
}, ^id (NSError* error){
...
});
Setting a promise's value will be called: "resolving". You can resolve a promise only ONCE.
You may wrap any asynchronous method with a completion handler or completion delegates into a method which returns a promise:
- (Promise*) fetchUserWithURL:(NSURL*)url
{
Promise* promise = [Promise new];
HTTPOperation* op = [[HTTPOperation alloc] initWithRequest:request
success:^(NSData* data){
[promise fulfillWithValue:data];
}
failure:^(NSError* error){
[promise rejectWithReason:error];
}];
[op start];
return promise;
}
Upon completion of the task, the promise can be "fulfilled" passing it the result value, or it can be "rejected" passing it the reason (error).
Depending on the actual implementation, a Promise can also be cancelled. Say, you hold a reference to a request operation:
self.fetchUserPromise = [self fetchUsersWithURL:url];
You can cancel the asynchronous task as follows:
- (void) viewWillDisappear:(BOOL)animate {
[super viewWillDisappear:animate];
[self.fetchUserPromise cancel];
self.fetchUserPromise = nil;
}
In order to cancel the associated async task, register a failure handler in the wrapper:
- (Promise*) fetchUserWithURL:(NSURL*)url
{
Promise* promise = [Promise new];
HTTPOperation* op = ...
[op start];
promise.then(nil, ^id(NSError* error){
if (promise.isCancelled) {
[op cancel];
}
return nil; // result unused
});
return promise;
}
Note: you can register success or failure handlers, when, where and as many as you want.
So, you can do a lot with promises - and even more than in this brief introduction. If you read up to here, you might get an idea how to solve your actual problem. It's right there - and it's a few lines of code.
I admit, that this short introduction into promises was quite rough and it's also quite new to Objective-C developers, and may sound uncommon.
You can read a lot about promises in the JS community. There are one or three implementations in Objective-C. The actual implementation won't exceed a few hundred lines of code. It happens, that I'm the author of one of it:
RXPromise.
Take it with a grain of salt, I'm probably totally biased, and apparently all others ever dealt with Promises, too. ;)
I have this (rare) odd case where my objective-c iOS program is locking up. When I break into the debugger, there are two threads and both of them are stuck at a #synchronized().
Unless I am completely misunderstanding #synchronized, I didn't think that was possible and the whole point of the command.
I have a main thread and worker thread that both need access to a sqlite database, so I wrap the chunks of code that are accessing the db in #synchronized(myDatabase) blocks. Not much else happens in these blocks except the db access.
I'm also using the FMDatabase framework to access sqlite, I don't know if that matters.
The myDatabase is a global variable that contains the FMDatabase object. It is created once at the start of the program.
I know I'm late to the party with this, but I've found a strange combination of circumstances that #synchronized handles poorly and is probably responsible for your problem. I don't have a solution to it, besides to change the code to eliminate the cause once you know what it is.
I will be using this code below to demonstrate.
- (int)getNumberEight {
#synchronized(_lockObject) {
// Point A
return 8;
}
}
- (void)printEight {
#synchronized(_lockObject) {
// Point B
NSLog(#"%d", [self getNumberEight]);
}
}
- (void)printSomethingElse {
#synchronized(_lockObject) {
// Point C
NSLog(#"Something Else.");
}
}
Generally, #synchronized is a recursively-safe lock. As such, calling [self printEight] is ok and will not cause deadlocks. What I've found is an exception to that rule. The following series of events will cause deadlock and is extremely difficult to track down.
Thread 1 enters -printEight and acquires the lock.
Thread 2 enters -printSomethingElse and attempts to acquire the lock. The lock is held by Thread 1, so it is enqueued to wait until the lock is available and blocks.
Thread 1 enter -getNumberEight and attempts to acquire the lock. The lock is held already and someone else is in the queue to get it next, so Thread 1 blocks. Deadlock.
It appears that this functionality is an unintended consequence of the desire to bound starvation when using #synchronized. The lock is only recursively safe when no other thread is waiting for it.
The next time you hit deadlock in your code, examine the call stacks on each thread to see if either of the deadlocked threads already holds the lock. In the sample code above, by adding long sleeps at Point A, B, and C, the deadlock can be recreated with almost 100% consistency.
EDIT:
I'm no longer able to demonstrate the previous issue, but there is a related situation that still causes issues. It has to do with the dynamic behavior of dispatch_sync.
In this code, there are two attempts to acquire the lock recursively. The first calls from the main queue into a background queue. The second calls from the background queue into the main queue.
What causes the difference in behavior is the distinction between dispatch queues and threads. The first example calls onto a different queue, but never changes threads, so the recursive mutex is acquired. The second changes threads when it changes queues, so the recursive mutex cannot be acquired.
To emphasize, this functionality is by design, but it behavior may be unexpected to some that do not understand GCD as well as they could.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
NSObject *lock = [[NSObject alloc] init];
NSTimeInterval delay = 5;
NSLog(#"Example 1:");
dispatch_async(queue, ^{
NSLog(#"Starting %d seconds of runloop for example 1.", (int)delay);
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:delay]];
NSLog(#"Finished executing runloop for example 1.");
});
NSLog(#"Acquiring initial Lock.");
#synchronized(lock) {
NSLog(#"Acquiring recursive Lock.");
dispatch_sync(queue, ^{
NSLog(#"Deadlock?");
#synchronized(lock) {
NSLog(#"No Deadlock!");
}
});
}
NSLog(#"\n\nSleeping to clean up.\n\n");
sleep(delay);
NSLog(#"Example 2:");
dispatch_async(queue, ^{
NSLog(#"Acquiring initial Lock.");
#synchronized(lock) {
NSLog(#"Acquiring recursive Lock.");
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Deadlock?");
#synchronized(lock) {
NSLog(#"Deadlock!");
}
});
}
});
NSLog(#"Starting %d seconds of runloop for example 2.", (int)delay);
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:delay]];
NSLog(#"Finished executing runloop for example 2.");
I stumbled into this recently, assuming that #synchronized(_dataLock) does what it's supposed to do, since it is such a fundamental thing after all.
I went on investigating the _dataLock object, in my design I have several Database objects that will do their locking independently so I was simply creating _dataLock = [[NSNumber numberWithInt:1] retain] for each instance of Database.
However the [NSNumber numberWithInt:1] returns the same object, as in same pointer!!!
Which means what I thought was a localized lock for only one instance of Database is not a global lock for all instances of Database.
Of course this was never the intended design and I am sure this was the cause of issues.
I will change the
_dataLock = [[NSNumber numberWithInt:1] retain]
with
_dataLock = [[NSUUID UUID] UUIDString] retain]
So, I am using [NSThread detachNewThreadSelector] to spawn a new thread and I am getting "autoreleased with no pool in place " errors in the console. I know this can happen if you fail to create an auto release pool, but the thing is, I am creating one. I use similar code in other parts of the same app and do NOT get these errors.
Here is the relevant code:
- (void) startThread:(NSString*)strURL
{
// start new thread to load image
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[NSThread detachNewThreadSelector:#selector(loadImageFromURL:) toTarget:self withObject:strURL];
[pool release];
}
- (void) loadImageFromURL:(NSString*)strURL
{
NSNumber* nn = [NSNumber numberWithInt:self.tag];
NSLog(#"loadURL: Tag number == %i", [nn intValue]);
// other code here actually does the work
}
Now, there was more code in loadImageFromURL which actually does the work (of loading an image from a remote server) - but the problem manifests itself without that code, so I've removed it (just so you don't think I have a pointless thread which does nothing!). I left in just one line of code which demonstrates the problem - it creates an autoreleased NSNumber object.
When this code runs, it reports this to the console:
__NSAutoreleaseNoPool(): Object 0x535c0e0 of class NSCFNumber autoreleased with no pool in place - just leaking
Of course, the real code creates many other AR objects and all of them get reported as well.
Would be grateful for any tips or pointers which might help!
Thanks!
When you create a new thread, you need to also create a new autorelease pool for it. In your case, that looks as simple as adding:
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
at the beginning of loadImageFromURL: and
[pool drain];
at the end.
You probably don't need or want the pool you're creating in startThread:. Check out the Threading Programming Guide, particularly the "Writing Your Thread Entry Routine" section.
On your code, - (void) startThread:(NSString*)strURL is running in the main thread, while - (void) loadImageFromURL:(NSString*)strURL is running on the background thread you are detaching.
The main thread already has a NSAutoreleasePool, so the one you are creating in startThread: is probably unneeded. However, the background thread will not create a NSAutoreleasePool, so you'd need to create it yourself.
In your code, that would look like:
- (void) startThread:(NSString*)strURL
{
// start new thread to load image
[NSThread detachNewThreadSelector:#selector(loadImageFromURL:) toTarget:self withObject:strURL];
}
- (void) loadImageFromURL:(NSString*)strURL
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSNumber* nn = [NSNumber numberWithInt:self.tag];
NSLog(#"loadURL: Tag number == %i", [nn intValue]);
// other code here actually does the work
[pool drain];
}
Also, as #Carl Norum suggested, you should use drain instead of release when you are done using the autorelelase pool.
Solution for a similar problem but using ARC.
If using ARC, you could get an error "'NSAutoreleasePool' is unavailable: not available in automatic reference counting mode".
Use:
- (void) startThread:(NSString*)strURL
{
// start new thread to load image
[NSThread detachNewThreadSelector:#selector(loadImageFromURL:) toTarget:self withObject:strURL];
}
- (void) loadImageFromURL:(NSString*)strURL
{
#autoreleasepool {
NSNumber* nn = [NSNumber numberWithInt:self.tag];
NSLog(#"loadURL: Tag number == %i", [nn intValue]);
// other code here actually does the work
}
}