Multiple realms on concurrent background dispatch queue

MongoDB Atlas App Services & Realm
1

Hi,

I’m working on an iOS application which needs to run multiple API requests concurrently and store their results to realm database.

So I’m using a OperationQueue which sends API requests concurrently (max 4) and then delegates their results to concurrent DispatchQueue. On this queue I always create a new Realm instance and write results to database.

The realm object has some raw data (strings and dates) and some relationships (either 1-1 or 1-many), so it also contains direct links to other realm objects or LinkingObjects.

At some point during the asynchronous writes I check one of the LinkingObjects count and I get a crash there which says:

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x000001a1d093cb4d
VM Region Info: 0x1a1d093cb4d is not in any region. Bytes after previous region: 1783226420046
REGION TYPE START - END [ VSIZE] PRT/MAX SHRMOD REGION DETAIL
MALLOC_NANO 0000000280000000-00000002a0000000 [512.0M] rw-/rwx SM=PRV
—>
UNUSED SPACE AT END

Termination Signal: Segmentation fault: 11
Termination Reason: Namespace SIGNAL, Code 0xb
Terminating Process: exc handler [2158]
Triggered by Thread: 5

The code that crashes looks like this:

let realm = try Realm(configuration: config)
realm.write {
    // Creation/editing of new/existing object, assigning some base values and relationships

    var attachments = self.children.filter(...)

    guard attachments.count > 0 else {
        self.mainAttachment = nil
        return
    }

    let pdfs = attachments.filter(...).sorted(byKeyPath: "dateAdded", ascending: true)

    if pdfs.count > 0 {	// <- this count check crashes
        self.mainAttachment = pdfs.first
        return
    }

    // Other checks
    ...
}

So I wanted to ask whether the problem could be that I’m creating multiple Realm instances which then perform writes on a single concurrent DispatchQueue.

2

@Michal_Rentka Can you file an issue here with a reproduction case please: