Realm-cocoa: Load of null pointer of type 'RLMAccessorContext *' error

[kristiyandobrev]

I am getting the following errors/warnings in XCode, every time I run the my app:

Along with these warnings sometimes my app hangs on startup and I have to restart it.
Any ideas what might potentially cause those issues ?

---

Realm framework version: Latest

Xcode version: 9.1

iOS/OSX version: 11.1

Dependency manager + version: CocoaPods

[austinzheng]

Unfortunately that screenshot isn't enough information for us to understand what sort of problem you're facing. If you could provide us with a reproduction case that allows us to see this issue for ourselves, that would be highly appreciated.

[kristiyandobrev]

I found the problem. The warning appears because I was looking up for a Realm object by it's primary key (String) and I was providing a nil instead of a proper primary key.... But it looks weird to me that it didn't crash but instead it threw some warnings 🤨

[kristiyandobrev]

@austinzheng Actually here is what I have:

import RealmSwift

final class Usage: Object {
    @objc dynamic var id = ""
    @objc dynamic var user: User? //<--- Realm object 
    @objc dynamic var startTime: Date!
    @objc dynamic var endTime: Date?
    @objc dynamic var machine: Machine? //<--- Realm object 

    override static func primaryKey() -> String? {
        return #keyPath(Usage.id)
    }

    override static func indexedProperties() -> [String] {
        return [#keyPath(Usage.id)]
    }
}

extension Usage {
    convenience init(usage: UsageResponse) {
        self.init()
        self.id = usage.id

        self.startTime = Date.Formatter.iso8601.date(from: usage.startTime)
        self.endTime = Date.Formatter.iso8601.date(from: usage.endTime)

        guard let realm = try? Realm() else {
            return
        }

        let machineId = usage.machineId
        if let machine = realm.object(ofType: Machine.self, forPrimaryKey: machineId) {
            self.machine = machine
        }

        let userId = usage.user.id
        if let user = realm.object(ofType: User.self, forPrimaryKey: userId) {
            self.user = user
        } else {
            self.user = User(user: usage.user)
        }

    }
}

The problem occurs when I try to assign the machine and user properties. Interesting because even though the warnings are thrown I can see with Realm Browser that the objects are saved correctly 🤔. Can you see anything wrong with the code aboce @austinzheng ?

[hhanesand]

Just saw this issue as well. Will report if I find anything useful.

[hhanesand]

@austinzheng Here is a stack trace for the ubsan report : https://gist.github.com/hhanesand/3e67226caed0a6122e75db991ffe7521

The error message "Reference binding to null pointer of type RLMClassInfo" occurs here. For some reason, both the _realm and the _info pointer are nil :

I am willing to share my project if this is of any priority.

[austinzheng]

If you could send us a project (or even a cut down repro case), and repro instructions to [email protected], that would be immensely appreciated!

[Fogh]

Just to follow up on this issue. I work with @kristiyandobrev on a project where this problem occurs. It only seem to happen on the iOS simulator not on a real device if that is to any help.

[hhanesand]

I've seen it happen on device.

[nevil]

@austinzheng

I get an UBSan warning in RLMGetObject() on both Simulator and real device with RealmSwift 3.0.2 and 2.10.2. Seems same/related to this issue.

Thread 1: Null pointer use
Reference binding to null pointer of type 'RLMAccessorContext'

This is the line:

 auto obj = realm::Object::get_for_primary_key(*c, realm->_realm, *info.objectSchema,
                                                      key ?: NSNull.null);

If we check the full function we see that c is assigned nullptr and later dereferenced when calling get_for_primary_key().

id RLMGetObject(RLMRealm *realm, NSString *objectClassName, id key) {
    RLMVerifyRealmRead(realm);

    RLMAccessorContext *c = nullptr;
    auto& info = realm->_info[objectClassName];
    if (RLMProperty *prop = info.propertyForPrimaryKey()) {
        RLMValidateValueForProperty(key, info.rlmObjectSchema, prop);
    }
    try {
---->        auto obj = realm::Object::get_for_primary_key(*c, realm->_realm, *info.objectSchema,
                                                      key ?: NSNull.null);
        if (!obj.is_valid())
            return nil;
        return RLMCreateObjectAccessor(realm, info, obj.row());
    }
    catch (std::exception const& e) {
        @throw RLMException(e);
    }
}

[hhanesand]

Figured out what was causing this on my end. I was accessing a RLMLinkingObjects that was created by a unmanaged realm object, which caused the _info pointer to be nil. Doesn't seem like this is an error on your end @austinzheng, but maybe Realm can handle it a little better.

I see methods such as lastObject and firstObject on RLMResults that return early if the _info pointer is nil. An early return in valueForKey: that checks if the _info pointer is nil would avoid the warning in my case. Thoughts?

@nevil @Fogh @kristiyandobrev Could you all post the full stack traces so we can figure out if we're seeing similar problems?

[austinzheng]

I can't comment in any official capacity any longer, but if a crash occurs in Realm-related code at runtime and there isn't an associated RLMException (or, at worst, C++ exception), then it's certainly a bug and needs to be addressed.

[h-anders-unext]

@tgoyne
Do you have any comment on what I wrote earlier?
https://github.com/realm/realm-cocoa/issues/5478#issuecomment-353907644

UBSan is warning because of dereferencing a null ptr:

RLMAccessorContext *c = nullptr;
...
auto obj = realm::Object::get_for_primary_key(*c, realm->_realm, *info.objectSchema,
                                                      key ?: NSNull.null);

[uson1x]

Seeing these errors in runtime on device:

Pods/Realm/Realm/RLMObjectStore.mm:238:55: runtime error: reference binding to null pointer of type 'RLMAccessorContext'
Pods/Realm/include/object_accessor.hpp:309:47: runtime error: reference binding to null pointer of type 'RLMAccessorContext'
Pods/Realm/include/RLMAccessor.hpp:83:10: runtime error: load of null pointer of type 'RLMAccessorContext *'
Pods/Realm/Realm/RLMAccessor.mm:687:39: runtime error: load of null pointer of type 'RLMAccessorContext *'

and here is a print of some vars when app hits a breakpoint in RLMGetObject(RLMRealm *realm, NSString *objectClassName, id key)

(lldb) p info
(RLMClassInfo) $0 = {
  realm = 0x00000001c04bd700
  rlmObjectSchema = 0x00000001c40bb5a0
  objectSchema = 0x00000001129cee20
  observedObjects = size=0 {}
  m_table = 0x0000000000000000
  m_linkTargets = size=0 {}
}

(lldb) po key
26fc9999-b72a-4a7d-b589-13942612620e

(lldb) po info.propertyForPrimaryKey()
guid {
    type = string;
    objectClassName = (null);
    linkOriginPropertyName = (null);
    indexed = YES;
    isPrimary = YES;
    array = NO;
    optional = YES;
}

(lldb) p *info.objectSchema
(const realm::ObjectSchema) $5 = {
  name = "Note"
  persisted_properties = size=13 {
    [0] = {
      name = "identifier"
      type = Int
      object_type = ""
      link_origin_property_name = ""
      is_primary = (m_value = false)
      is_indexed = (m_value = true)
      table_column = 1
    }
    [1] = {
      name = "guid"
      type = 66
      object_type = ""
      link_origin_property_name = ""
      is_primary = (m_value = true)
      is_indexed = (m_value = true)
      table_column = 0
    }
    [2] = {
      name = "modified"
      type = 68
      object_type = ""
      link_origin_property_name = ""
      is_primary = (m_value = false)
      is_indexed = (m_value = false)
      table_column = 2
    }
    ... <some other columns here>
  }
  computed_properties = size=0 {}
  primary_key = "guid"
}

_(this is in Realm 3.1.1)_

[nevil]

@uson1x
Isn't your log is the same as mine which is is caused by RLMAccessorContext *c = nullptr; in RLMGetObject()?

[uson1x]

@nevil yeah, you are right, forgot to check the context pointer. It is 0x0000000000000000 indeed.