Deleting a bunch of objects at once may cause a crash in migration

[anlaital]

Goals

Delete objects in a migration.

Expected Results

Objects get deleted.

Actual Results

Realm crashes with the following stack trace:

This crash is caused by the following method (array index out-of-bounds):

- (void)deleteObjectsMarkedForDeletion {
    for (NSString *className in deletedObjectIndices.allKeys) {
        RLMResults *objects = [_realm allObjects:className]; // objects.count == 811 at start, decreasing by one with each deletion as this is a self-updating type
        for (NSNumber *index in deletedObjectIndices[className]) {
            // here the sort order is random (or implicitly user-defined); this means that lower indices may get deleted before higher ones, causing the crash
            RLMObject *object = objects[index.longValue]; 
            [_realm deleteObject:object];
        }
    }
}

Here's a very simple fixed version that sorts the indices before looping (this fixes the crash, haven't verified that it works semantically correctly):

- (void)deleteObjectsMarkedForDeletion {
    for (NSString *className in deletedObjectIndices.allKeys) {
        RLMResults *objects = [_realm allObjects:className];
        // Delete objects with indices sorted in descending order to prevent array index out-of-bounds crashes.
        NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:nil ascending:NO];
        NSArray *sortedIndices = [deletedObjectIndices[className] sortedArrayUsingDescriptors:@[sort]];
        for (NSNumber *index in sortedIndices) {
            RLMObject *object = objects[index.longValue];
            [_realm deleteObject:object];
        }
    }
}

Version of Realm and Tooling

Realm framework version: 3.0.2

[bdash]

Can you please share some code that reproduces the issue so we'll be able to verify that whatever fix we make addresses the problem you're encountering?

[anlaital]

This was simple to reproduce. First create some data (doesn't matter what the data is):

class O: Object {
    
    @objc dynamic var i = 0
    
}

var conf = Realm.Configuration.defaultConfiguration

let realm = try! Realm(configuration: conf)

try! realm.write {
    for _ in 0..<1000 {
        realm.add(O())
    }
}

Then increment version number and implement the following migration block:

conf.schemaVersion = 1
conf.migrationBlock = {
    migration, oldSchemaVersion in
    
    var toDelete = [MigrationObject]()
    
    migration.enumerateObjects(ofType: O.className()) {
        oldObject, newObject in
        toDelete.append(newObject!)
    }
    
    // deleting objects in reversed order causes the crash
    for object in toDelete.reversed() {
        migration.delete(object)
    }
}

[bdash]

Thank you!

[fmorau]

@anlaital did you find any workaround for this? :)

[anlaital]

Maintaining the enumeration order when deleting the objects doesn't crash.

[fmorau]

@anlaital 👍🏻👍🏻 Thank you, will try :)