Accessing currentUser on multiple threads causing deadlock

[eeallen1]

When I'm launching my app, there are several places where I'm using PFUser.CurrentUser - mostly for queries, but also a couple of writes. It looks like the deadlock occurs when one thread is setting an object on currentUser while another is using it in a query, but I'm not entirely sure.

Here is a stack trace:

I know that there have been similar issues opened and resolved for other releases, and it may be that 1.8 has nothing to do with it, but it only started happening in the last couple of days (after I updated).

Some more details:

• Deadlock doesn't occur every launch, and it appears to happen much more frequently on older devices (iphone 4s) than the newer models. It almost never occurs on the simulator.
• I've tried commenting out dozens of lines of code that the stack trace points to, but it always end up getting hung somewhere else. I haven't procedurally uncommented and tested every call to currentUser because so much code depends on it, but that'll be my next step for troubleshooting.

Edit
Is it possible that the deadlock is occurring during Facebook authentication while I'm trying to query/modify currentUser elsewhere? Take a look at this:

[nlutsenko]

Hey @eeallen1, thanks for reporting this.
I've tried to reproduce the problem, but it doesn't repro for me, I am going to try investigating it further and there is a single piece that I think the bug must be hiding behind.
I'll update this issue once I have more information or attach a PR to this.

[eeallen1]

@nlutsenko If there's anything specific I can look for that would help you, let me know. I tried creating an "empty" version of my app with the same number of Parse requests, but I can't seem to recreate the error when I take out the processor-intensive parts of the app.

I don't know much about how multithreading works, but I'm guessing this issue only arises when the processor can't switch quickly enough between threads. I could be totally off base, but the deadlock does occur significantly less frequently on the simulator and the newest iPhone than on the iPhone 4s and 5.

[kross50]

happens to me too on 1.8.0 and 1.8.1 (as has happened on some previous versions of iOS SDK)... so i'm staying on 1.7.5 where it doesn't happen... seems to happen right after Facebook login after which i immediately make 5 PFQuerys serially in rapid succession (after one completes the next is kicked off). It freezes on the 3rd or 4th PFQuery. PFUser properties are assessed multiple times in this workflow.

[ahashp]

I was using 1.7.5, and I tried updating to 1.8.1 And immediately, I notice my app locks up very very frequently, oftening locking up more than 10 seconds at at time. I, too, have lots of PFQuery(s) after app launch, all of them has equal criteria on currentUser, some of them write to properties on currentUser. Looking at this repo's issue this, this issue seems to describe what I am experiencing. (I'm running this on iPhone 6 Plus, iOS 8.4)

[arunv]

+1. I'm seeing deadlock. On PFUser. I have PFSubclassing on user, if it makes a difference.

Update: agreeing with @eeallen1 , my specific error case also seems to be coming from [PFFacebookAuthenticationProvider restoreAuthenticationWithAuthData:]

I have an already-logged-in facebook user. I'm guessing something in that function is indirectly trying to get to PFUser.sessionToken or PFUser._state, both of which are waiting on (self.lock), acquired in ([PFUser synchronizeAuthDataWithAuthType:])

[richardjrossiii]

I have found the issue and am working on a fix.

[nlutsenko]

Hey guys, just wanted to update on this issue with a workaround:
The problem only happens with ParseFacebookUtils that is built to work with an old (v3) version of Facebook SDK. If you migrate to using the latest Facebook SDK (aka v4) - the problem will be fixed.

We are working on the fix and it should be out with 1.8.2 of the SDK very soon.

[ahashp]

@nlutsenko I am using ParseFacebookUtilsV4.framework and I still see the deadlock issue with 1.8.1 I do not think it is limited to v3. You guys might want to investigate further to have a complete handle on this bug.