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[CBL-6981] Crash While Accessing Weak Cache of Nested Dictionary, Array Or Blob

Yuvraj
Updated 
Product: Couchbase Lite
Component: couchbase-lite
Issue Link: CBL-6981
Affects Version: 3.2.x
Fix Version: 3.2.4

Summary

  • A rare but persistent crash in Couchbase Lite Swift (version 3.2.1) was found, primarily when converting a document to a dictionary using Document.toDictionary().
  • The issue is traced to Couchbase Lite internals (in weak reference handling) and often involves nested data structures such as arrays or dictionaries within documents.
  • This crash can occur even without apparent multi-threaded access or memory pressure and has led to regular, low-volume application crashes in production.
  • This issue usually manifests as rare, low-frequency production crashes. 

Symptoms 

  • Crash occurs and the stack trace points to Objective-C runtime and CouchbaseLite Swift internals (e.g., CBLDictionary.mm:42, objc_loadWeakRetained, objc_retain)
  • The crash may occur even if the document is not accessed concurrently elsewhere

Triggers

  • Accessing a Couchbase Lite document containing arrays or other nested data structures
  • Use of toDictionary() on a document, like: 
    extension Document {
    func toLegacyDictionary() -> [String: Any] {
        var mutableDictionary = self.toDictionary() // <--- crash here
        mutableDictionary["_id"] = id
        return mutableDictionary
    }
}
  • Code may not explicitly involve multiple threads
  • Occurs under normal operation, not memory pressure

Verification

  • Crash logs showing errors around CBLDictionary.mm:42, DictionaryObject.swift : 72, objc_loadWeakRetained, objc_retain, or related CouchbaseLiteSwift/Objective-C bridging code.

For example:

stacktrace
Thread 23 crashed
0     libobjc.A.dylib  0x000000019b1f9ce8     objc_loadWeakRetained
1     CouchbaseLiteSwift     0x000000010ae25f3f     [CBLDictionary swiftObject] (CBLDictionary.mm : 42)
2     CouchbaseLiteSwift     0x000000010ad85f3f     static CouchbaseLiteSwift.DataConverter.convertGETValue(Any?) -> Any? (DataConverter.swift : 38)
3     CouchbaseLiteSwift     0x000000010ad95a27     closure #1 () -> Any? in CouchbaseLiteSwift.ArrayObject.makeIterator() -> Swift.AnyIterator<Any> (ArrayObject.swift : 73)
4     CouchbaseLiteSwift     0x000000010ad957bf      CouchbaseLiteSwift.ArrayObject.toArray() -> [Any] (<compiler-generated>)
5     CouchbaseLiteSwift     0x000000010ada529b      CouchbaseLiteSwift.Document.toDictionary() -> [Swift.String : Any] (Document.swift : 219)
6     Inspect     0x0000000102e9e247     (extension in Inspect):CouchbaseLiteSwift.Document.toLegacyDictionary() -> [Swift.String : Any] (Document+Extension.swift : 17)
7     Inspect     0x0000000102c9726f     generic specialization <Inspect.FDAFacilityMappingDocument> of closure #1 () -> () in (extension in Inspect):CouchbaseLiteSwift.Database.readDocument<A where A: Swift.Decodable>(withID: Swift.String, queue: __C.OS_dispatch_queue, completion: (Swift.Result<A, Swift.Error>) -> ()) -> () (Database+Extension.swift : 13)
8     Inspect     0x0000000102e62e4b     reabstraction thunk helper from @escaping @callee_guaranteed () -> () to @escaping @callee_unowned @convention(block) () -> () (<compiler-generated>)
stacktrace
Thread 0 crashed
0  libobjc.A.dylib      0x000000018caf1c54   objc_retain
1  CouchbaseLiteSwift   0x000000010ae0c9df   CouchbaseLiteSwift.DictionaryObject.toDictionary() -> [Swift.String : Any] (DictionaryObject.swift : 72)
2  CouchbaseLiteSwift   0x000000010ae19687   CouchbaseLiteSwift.ArrayObject.toArray() -> [Any] (DataConverter.swift : 107)
3  CouchbaseLiteSwift   0x000000010ae0cc57   CouchbaseLiteSwift.DictionaryObject.toDictionary() -> [Swift.String : Any] (DictionaryObject.swift : 218)
4  CouchbaseLiteSwift   0x000000010ae0cb6f   CouchbaseLiteSwift.DictionaryObject.toDictionary() -> [Swift.String : Any] (DictionaryObject.swift : 216)
5  CouchbaseLiteSwift   0x000000010ae291b3   CouchbaseLiteSwift.Document.toDictionary() -> [Swift.String : Any] (Document.swift : 217)
6  Inspect              0x0000000102ebf3af   (extension in Inspect):CouchbaseLiteSwift.Document.toLegacyDictionary() -> [Swift.String : Any] (Document+Extension.swift : 17)
  • Confirm the affected environment is using Couchbase Lite less than version 3.2.4

Workarounds

  • There is no good workaround worth suggesting: one can limit calls to access arrays, other nested data structures, or toDictionary(); or refactor code to avoid converting documents with nested arrays/dictionaries unless necessary.
  • If encountered, we strongly recommend upgrading to Couchbase Lite 3.2.4 or a newer version, where the weak reference handling is fixed.

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