Realm-java: Index out of bounds in migration transformation

[hardysim]

Goal

Run transformation in a migration for all elements. If a linking object (Bar) is found, I want to set a the string from Bar.bbb on the DynamicRealmObject (Foo.aaa) and if not, I want to remove the Foo-object.

schema.get("Foo")?.apply {
    addField("aaa", String::class.java)

    transform { dro ->
        dro.linkingObjects("Bar", "foo")
            ?.firstOrNull()
            ?.apply {
                dro.setString("aaa", getString("bbb"))
            }
            ?: dro.deleteFromRealm()
    }
}

Actual Results

This works fine on a few test devices I could test but crashlytics reports many crashes for end users.
I get 2 crashes:

Fatal Exception: java.lang.IllegalStateException: Row index out of range
Exception backtrace:
<backtrace not supported on this platform>
       at io.realm.internal.CheckedRow.nativeSetString(CheckedRow.java)
       at io.realm.internal.UncheckedRow.setString(UncheckedRow.java:234)
       at io.realm.DynamicRealmObject.setString(DynamicRealmObject.java:689)
       at de.loewen.lcsmobile.migration.MigrateToVersion23Kt$migrateToVersion23$1$1.apply(MigrateToVersion23Kt.java:36)
       at io.realm.MutableRealmObjectSchema.transform(MutableRealmObjectSchema.java:288)
       at de.loewen.lcsmobile.migration.MigrateToVersion23Kt.migrateToVersion23(MigrateToVersion23Kt.java:26)
       at de.loewen.lcsmobile.migration.RealmMigrationManager.migrate(RealmMigrationManager.java:50)

and

Fatal Exception: java.lang.ArrayIndexOutOfBoundsException: rowIndex > available rows: 54 > 53
       at io.realm.internal.Table.nativeMoveLastOver(Table.java)
       at io.realm.internal.Table.moveLastOver(Table.java:330)
       at io.realm.RealmObject.deleteFromRealm(RealmObject.java:115)
       at io.realm.RealmObject.deleteFromRealm(RealmObject.java:87)
       at de.loewen.lcsmobile.migration.MigrateToVersion23Kt$migrateToVersion23$1$1.apply(MigrateToVersion23Kt.java:46)
       at io.realm.MutableRealmObjectSchema.transform(MutableRealmObjectSchema.java:288)
       at de.loewen.lcsmobile.migration.MigrateToVersion23Kt.migrateToVersion23(MigrateToVersion23Kt.java:26)
       at de.loewen.lcsmobile.migration.RealmMigrationManager.migrate(RealmMigrationManager.java:50)

Steps & Code to Reproduce

This seems to happen everytime the user opens the app (and a migration is needed). As said, I cannot reproduce it myself because it's working on my devices.

Version of Realm and tooling

Realm version(s): 6.0.0

Realm Sync feature enabled: No

Android Studio version: 3.6 Beta 3

Android Build Tools version: 29.0.2

Gradle version: 5.6.4

Which Android version and device(s): 8.0.0 (Galaxy S7) crashes on the user; I've tested an S7 with 8.0.0 as well which worked. Also tested Android 10, 9, 8, 7, 6, 5 - all working.

[Zhuinden]

Out of curiosity, is encryption enabled?

[hardysim]

Out of curiosity, is encryption enabled?

No (I saw the fix for it in 6.0.1)

[Zhuinden]

Interesting problem you have, but overall the trick seems to be that transform is applied to all elements within that given object schema through a regular iterator, rather than over a snapshot collection.

The solution would be to save the "candidates for deletion" into a Set<? extends RealmObject> and delete the objects only after the transform {} is actually done.

Currently I don't have a Realm-based project open so I don't see the actual generated implementation for the transform { call, but this is my guess.

[hardysim]

So the problem is that removing an entry of the still-running iterator causes those 2 errors? Sounds plausible. I'll add your "candidates for deletion" approach. Thanks!

[hardysim]

Seems like this fix worked. No more crashes. Closing.

[Bhullnatik]

Could this issue be reopened?
This doesn't feel intuitive at all since deleteFromRealm is available in the obj in transform, I'd expect it to work.

[hardysim]

Hmm, so the solution would be to remove DynamicRealmObject.deleteFromRealm()? This might be a good idea to prevent such errors like mine.

[Zhuinden]

Doesn't mean you shouldn't be able to delete items using the DynamicRealm API.

The trick is that transform should iterate backwards.

[cmelchior]

I guess we didn't account for people deleting objects in the transform method. We should fix this by either making a snapshot or iterating backward. It doesn't look like we guarantee any specific order of the elements being returned, so that should work.

But yes, this looks like a bug on our end.

[Zhuinden]

Fixed in 6.1.0