Illegal attempt to work with the core-data or default zone in the shared database

Question

The setup:
My app uses CoreData+CloudKit to mirror iCloud content.
I am implementing record zone sharing and am testing it using 2 iPhones. iPhone 1 executes the app standalone, iPhone 2 under Xcode control.
When iPhone 1 user invites iPhone 2 user to share his records, iPhone 2 user can accept the share.
This mirrors as expected the iCloud records to the shared database.

The problem:
During mirroring, I get several times the log:

[error] fault: Illegal attempt to work with the core-data or default zone  
in the shared database: <NSSQLCore: 0x105907720>  
(URL: file:///var/mobile/Containers/Data/Application/  
66941553-40FC-4180-928C-BDF85D44AE84/Library/Application%20Support/Shared)  

I have no idea what is illegal, nor what consequences this illegal attempt has.

Any ideas?

EDIT 1:
When I enable run time breakpoints, the app stops in:

Thread 17 Queue : NSManagedObjectContext 0x283f73b50 (serial)  

at:

+[NSCKRecordZoneMetadata zoneMetadataForZoneID:inDatabaseWithScope:forStore:inContext:error:]   

So it seems that the metadata of the iCloud shared database zone that is used for mirroring cannot be read.

EDIT 2:
One more observation, if zone sharing is active and I lookup my iCloud container in the Dashboard:
If I select Zones, and there the Shared Database, the shared zone com.apple.coredata.cloudkit.zone of type REGULAR_CUSTOM_ZONE is displayed. If I tap on the zone name, the Zone Details should be displayed.
Instead I get the error
„BadRequestException: endpoint not applicable in the database type 'sharedb‘“.
This seems to me to be the same error that I get in my app.

Answer 1

By now I believe this error can be ignored.
My app uses CoreData+CloudKit mirroring for the private and the shared databases.
It now monitors NSPersistentCloudKitContainer.eventChangedNotification and logs events as follows:

func printSyncEvent(_ event: NSPersistentCloudKitContainer.Event) {
    let id = event.identifier
    let storeID = event.storeIdentifier
    let eventType: String
    switch event.type {
        case .setup:
            eventType = "setup"
        case .import:
            eventType = "import"
        case .export:
            eventType = "export"
        @unknown default:
            eventType = "unknown"
    }
    let startDate = "\(event.startDate)"
    guard let endDate = event.endDate else {
        print("Event: id: \(id), storeID: \(storeID), type: \(eventType), start: \(startDate)")
        return
    }
    guard event.succeeded else {
        let error = event.error!
        print("Event: id: \(id), storeID: \(storeID), type: \(eventType), start: \(startDate), end: \(endDate), error: \(error)")
        return
    }
    print("Event: id: \(id), storeID: \(storeID), type: \(eventType), start: \(startDate), end: \(endDate)")
}  

A sample log (only the relevant parts) looks like this:

PrivateStore: ID = 85D7AF77-64C3-47D6-A30E-06FDD034F6C9
SharedStore: ID = 87582E5F-1F92-4A57-ADEB-E8520190423F

Event: id: 2363CEBA-CF4D-4A10-84A9-6DB0C078C6C0, storeID: 85D7AF77-64C3-47D6-A30E-06FDD034F6C9, type: setup, start: 2022-04-05 09:43:26 +0000
Event: id: 2363CEBA-CF4D-4A10-84A9-6DB0C078C6C0, storeID: 85D7AF77-64C3-47D6-A30E-06FDD034F6C9, type: setup, start: 2022-04-05 09:43:26 +0000, end: 2022-04-05 09:43:27 +0000
Event: id: B0F2EAFE-3FBC-4AAD-B881-DBCCDB1E54E1, storeID: 87582E5F-1F92-4A57-ADEB-E8520190423F, type: setup, start: 2022-04-05 09:43:27 +0000
Event: id: B0F2EAFE-3FBC-4AAD-B881-DBCCDB1E54E1, storeID: 87582E5F-1F92-4A57-ADEB-E8520190423F, type: setup, start: 2022-04-05 09:43:27 +0000, end: 2022-04-05 09:43:27 +0000
Event: id: B5B1577B-671F-4094-914B-BD77C9B9E25B, storeID: 85D7AF77-64C3-47D6-A30E-06FDD034F6C9, type: import, start: 2022-04-05 09:43:27 +0000

Event: id: 7EA39D4B-FC72-428E-AFD3-2C97EFD07D30, storeID: 87582E5F-1F92-4A57-ADEB-E8520190423F, type: import, start: 2022-04-05 09:43:27 +0000
2022-04-05 11:43:28.103640+0200 ShopEasy[50306:2763534] [error] fault: Illegal attempt to work with the core-data or default zone in the shared database: <NSSQLCore: 0x13b021340> (URL: file:///Users/reiner/Library/Developer/CoreSimulator/Devices/0E3AD15A-7079-4BED-82D4-58BB5C26AE61/data/Containers/Data/Application/64189142-A8C8-476D-BD0D-6758E259A100/Library/Application%20Support/Shared)
2022-04-05 11:43:28.103867+0200 ShopEasy[50306:2763534] [error] CoreData: Illegal attempt to work with the core-data or default zone in the shared database: <NSSQLCore: 0x13b021340> (URL: file:///Users/reiner/Library/Developer/CoreSimulator/Devices/0E3AD15A-7079-4BED-82D4-58BB5C26AE61/data/Containers/Data/Application/64189142-A8C8-476D-BD0D-6758E259A100/Library/Application%20Support/Shared)
2022-04-05 11:43:28.108523+0200 ShopEasy[50306:2763534] [error] fault: Illegal attempt to work with the core-data or default zone in the shared database: <NSSQLCore: 0x13b021340> (URL: file:///Users/reiner/Library/Developer/CoreSimulator/Devices/0E3AD15A-7079-4BED-82D4-58BB5C26AE61/data/Containers/Data/Application/64189142-A8C8-476D-BD0D-6758E259A100/Library/Application%20Support/Shared)
2022-04-05 11:43:28.108784+0200 ShopEasy[50306:2763534] [error] CoreData: Illegal attempt to work with the core-data or default zone in the shared database: <NSSQLCore: 0x13b021340> (URL: file:///Users/reiner/Library/Developer/CoreSimulator/Devices/0E3AD15A-7079-4BED-82D4-58BB5C26AE61/data/Containers/Data/Application/64189142-A8C8-476D-BD0D-6758E259A100/Library/Application%20Support/Shared)
Event: id: 7EA39D4B-FC72-428E-AFD3-2C97EFD07D30, storeID: 87582E5F-1F92-4A57-ADEB-E8520190423F, type: import, start: 2022-04-05 09:43:27 +0000, end: 2022-04-05 09:43:28 +0000

Event: id: 2C4409CE-9E46-4F5C-A14E-FF7612FDE3B8, storeID: 87582E5F-1F92-4A57-ADEB-E8520190423F, type: export, start: 2022-04-05 09:43:28 +0000
Event: id: 2C4409CE-9E46-4F5C-A14E-FF7612FDE3B8, storeID: 87582E5F-1F92-4A57-ADEB-E8520190423F, type: export, start: 2022-04-05 09:43:28 +0000, end: 2022-04-05 09:43:28 +0000  

This indicates to me that the errors were created by an import event from the shared database.
There are 4 consecutive errors of the same type (Illegal attempt to work with the core-data or default zone in the shared database) but then the import is finished without error. It seems that the importer retries the import several times until it succeeds.
I am afraid we cannot do anything about it except to ask Apple not to log such errors…