`TypeCastException` in `KeychainSettings.keys` during `clear()`

[Skeptick]

We've seen a single production crash during a migration from KeychainSettings to DataStore. The crash happens while running migration cleanup, when calling clear().

Kotlin: 2.3.10
Coroutines: 1.10.2
multiplatform-settings: 1.3.0

Fatal Exception: kotlin.TypeCastException
ThrowTypeCastException + 54 (RuntimeUtils.kt:54)
kfun:com.russhwolf.settings.KeychainSettings#<get-keys>(){}kotlin.collections.Set<kotlin.String> + 154 (KeychainSettings.kt:154)
kfun:com.russhwolf.settings.KeychainSettings#clear(){} + 1915 (_Collections.kt:1915)
kfun:com.russhwolf.settings.coroutines.SuspendSettingsWrapper.SuspendSettingsWrapper$clear$2.invoke#internal + 52 (Converters.kt:52)
kfun:kotlinx.coroutines.intrinsics.startUndspatched#internal + 1 ([K][Suspend]Functions:1)
kfun:kotlinx.coroutines#withContext#suspend(kotlin.coroutines.CoroutineContext;kotlin.coroutines.SuspendFunction1<kotlinx.coroutines.CoroutineScope,0:0>;kotlin.coroutines.Continuation<0:0>){0§<kotlin.Any?>}kotlin.Any? + 157 (Builders.common.kt:157)
kfun:com.russhwolf.settings.coroutines.SuspendSettingsWrapper.clear#internal + 52 (Converters.kt:52)
kfun:com.example.CommonDataMigration#cleanUp#suspend(kotlin.coroutines.Continuation<kotlin.Unit>){}kotlin.Any + 41 (PreferencesMigration.common.kt:41)
kfun:androidx.datastore.core.DataMigrationInitializer.Companion.DataMigrationInitializer$Companion$runMigrations$2.DataMigrationInitializer$Companion$runMigrations$2$invoke$$inlined$fold$1.invoke#internal + 67 (_Collections.kt:67)
kfun:androidx.datastore.core.DataMigrationInitializer.Companion.$runMigrationsCOROUTINE$1.invokeSuspend#internal + 1 ([K][Suspend]Functions:1)
kfun:androidx.datastore.core.DataMigrationInitializer.Companion.DataMigrationInitializer$Companion$getInitializer$1.invoke#internal + 66 (DataMigrationInitializer.kt:66)
kfun:androidx.datastore.core.DataStoreImpl.InitDataStore.DataStoreImpl$InitDataStore$doRun$initData$1.$invokeCOROUTINE$1.invokeSuspend#internal + 1 ([K][Suspend]Functions:1)
kfun:androidx.datastore.core.DataStoreImpl.InitDataStore.DataStoreImpl$InitDataStore$doRun$initData$1.invoke#internal + 488 (DataStoreImpl.kt:488)
kfun:androidx.datastore.core.SingleProcessCoordinator.$lockCOROUTINE$0.invokeSuspend#internal + 1 ([K][Suspend]Functions:1)
kfun:androidx.datastore.core.SingleProcessCoordinator#lock#suspend(kotlin.coroutines.SuspendFunction0<0:0>;kotlin.coroutines.Continuation<0:0>){0§<kotlin.Any?>}kotlin.Any? + 41 (SingleProcessCoordinator.kt:41)
kfun:androidx.datastore.core.DataStoreImpl.InitDataStore.$doRunCOROUTINE$2.invokeSuspend#internal + 46 (InterProcessCoordinator.kt:46)
kfun:androidx.datastore.core.DataStoreImpl.InitDataStore.doRun#internal + 491 (DataStoreImpl.kt:491)
kfun:androidx.datastore.core.RunOnce.$runIfNeededCOROUTINE$22.invokeSuspend#internal + 509 (DataStoreImpl.kt:509)
kfun:androidx.datastore.core.DataStoreImpl.$readAndInitOrPropagateAndThrowFailureCOROUTINE$18.invokeSuspend#internal + 513 (DataStoreImpl.kt:513)
kfun:androidx.datastore.core.DataStoreImpl.readAndInitOrPropagateAndThrowFailure#internal + 291 (DataStoreImpl.kt:291)
kfun:androidx.datastore.core.DataStoreImpl.DataStoreImpl$readState$2.$invokeCOROUTINE$6.invokeSuspend#internal + 232 (DataStoreImpl.kt:232)
kfun:androidx.datastore.core.DataStoreImpl.DataStoreImpl$readState$2.invoke#internal + 242 (DataStoreImpl.kt:242)
kfun:kotlin.coroutines.intrinsics.createCoroutineUnintercepted$$inlined$createCoroutineFromSuspendFunction$2.invokeSuspend#internal (IntrinsicsNative.kt)
kfun:kotlin.coroutines.native.internal.BaseContinuationImpl#resumeWith(kotlin.Result<kotlin.Any?>){} + 50 (ContinuationImpl.kt:50)
kfun:kotlinx.coroutines.DispatchedTask#run(){} + 109 (DispatchedTask.kt:109)
kfun:kotlinx.coroutines.internal.LimitedDispatcher.Worker.run#internal + 128 (LimitedDispatcher.kt:128)
kfun:kotlinx.coroutines.MultiWorkerDispatcher.MultiWorkerDispatcher$workerRunLoop$1.$invokeCOROUTINE$0.invokeSuspend#internal + 12 (Runnable.kt:12)
kfun:kotlin.coroutines.native.internal.BaseContinuationImpl#resumeWith(kotlin.Result<kotlin.Any?>){} + 50 (ContinuationImpl.kt:50)
kfun:kotlinx.coroutines.DispatchedTask#run(){} + 109 (DispatchedTask.kt:109)
kfun:kotlinx.coroutines.EventLoopImplBase#processNextEvent(){}kotlin.Long + 15 (ObjectiveCUtils.kt:15)
kfun:kotlinx.coroutines#runBlocking(kotlin.coroutines.CoroutineContext;kotlin.coroutines.SuspendFunction1<kotlinx.coroutines.CoroutineScope,0:0>){0§<kotlin.Any?>}0:0 + 49 (EventLoop.common.kt:49)
kfun:kotlinx.coroutines.MultiWorkerDispatcher.MultiWorkerDispatcher$1.MultiWorkerDispatcher$1$invoke$$inlined$apply$1.$<bridge-DN>invoke(){}#internal + 123 (MultithreadedDispatchers.kt:123)

[russhwolf]

Seems strange. It's throwing a TypeCastException on this line. In my understanding, cfKey should always be a CFString there (it's already been null-checked), which should always lead to CFBridgingRelease(cfKey) being a non-null NSString. But iOS APIs love their strange undocumented edge-cases.

If it's only happened once, it's hard to know what to check next. If you're able to reproduce consistently, I'd be interested to know more about the value of the key causing the crash.

[Skeptick]

I can say that over the month since that version was released, this crash affected 11 users, which is about 0.05% of all iOS users. Unfortunately, I can’t provide any more detailed information.
I decided not to roll back the version and not to implement any specific workaround for this error.
I think the issue can probably be closed, but I’ll leave that up to you. :)

[russhwolf]

I'll leave it open because it does sound like there's a real issue here, even if it's rare. Can you provide any information about the devices where this occurs?

[Skeptick]

Yes, here are all the devices and OS versions where this occurred:

iPhone XR         — iOS 18.7.4
iPhone 11         — iOS 17.7.2
iPhone 11         — iOS 18.5.0
iPhone 11         — iOS 26.2.1
iPhone 12 Mini    — iOS 18.5.0
iPhone 14         — iOS 26.2.0
iPhone 15 Pro     — iOS 26.3.0
iPhone 15 Pro Max — iOS 26.3.0
iPhone 16         — iOS 18.5.0
iPhone 16 Pro     — iOS 26.2.1
iPhone 16 Pro Max — iOS 26.3.0

It looks like a fairly normal distribution across devices and OS versions, so it seems pretty random.

[rocketraman]

If it's only happened once, it's hard to know what to check next. If you're able to reproduce consistently, I'd be interested to know more about the value of the key causing the crash.

I reported issue #244 which appears to be similar or the same, and it happens to me consistently in my development environment. To know the value of the key causing the crash I guess I'd need to create a debug build that logs it right?

[rocketraman]

I added debugging to KeychainSettings. The debug output was:

KeychainSettings keys(): typeId=CFString value=device_uuid
KeychainSettings keys(): typeId=CFString value=_version
KeychainSettings keys(): typeId=CFData description=<CFData 0x13b545740 [0x1345c0ee0]>{length = 27, capacity = 27, bytes = 0x4558446576696365496e7374616c6c61 ... 6e555549444b6579}
Function doesn't have or inherit @Throws annotation and thus exception isn't propagated from Kotlin to Objective-C/Swift as NSError.
It is considered unexpected and unhandled instead. Program will be terminated.
Uncaught Kotlin exception: kotlin.TypeCastException: class __NSCFData cannot be cast to class platform.Foundation.NSString

The bytes of the CFData key that is causing the problem translate to "EXDeviceInstalla ... nUUIDKey" so I'm guessing this is because some other part of the app wrote a CFData key rather than CFString to the default settings (in my case this appears to have come from Expo / React Native). This unexpected data now causes keychain.keys to bork.

My suggestion would be that the code should ignore keys it can't parse / are not in the right format.

FYI this was my debug patch:

diff --git a/multiplatform-settings/src/appleMain/kotlin/com/russhwolf/settings/KeychainSettings.kt b/multiplatform-settings/src/appleMain/kotlin/com/russhwolf/settings/KeychainSettings.kt
index dde77af..b994337 100644
--- a/multiplatform-settings/src/appleMain/kotlin/com/russhwolf/settings/KeychainSettings.kt
+++ b/multiplatform-settings/src/appleMain/kotlin/com/russhwolf/settings/KeychainSettings.kt
@@ -26,12 +26,15 @@ import kotlinx.cinterop.ExperimentalForeignApi
 import kotlinx.cinterop.MemScope
 import kotlinx.cinterop.UnsafeNumber
 import kotlinx.cinterop.alloc
+import kotlinx.cinterop.allocArray
 import kotlinx.cinterop.allocArrayOf
 import kotlinx.cinterop.convert
 import kotlinx.cinterop.memScoped
 import kotlinx.cinterop.ptr
 import kotlinx.cinterop.reinterpret
 import kotlinx.cinterop.value
+import kotlinx.cinterop.ByteVar
+import kotlinx.cinterop.toKString
 import platform.CoreFoundation.CFArrayGetCount
 import platform.CoreFoundation.CFArrayGetValueAtIndex
 import platform.CoreFoundation.CFArrayRefVar
@@ -41,7 +44,14 @@ import platform.CoreFoundation.CFDictionaryRef
 import platform.CoreFoundation.CFStringRef
 import platform.CoreFoundation.CFTypeRef
 import platform.CoreFoundation.CFTypeRefVar
+import platform.CoreFoundation.CFCopyDescription
+import platform.CoreFoundation.CFDataGetTypeID
+import platform.CoreFoundation.CFGetTypeID
+import platform.CoreFoundation.CFRelease
+import platform.CoreFoundation.CFStringGetCString
+import platform.CoreFoundation.CFStringGetTypeID
 import platform.CoreFoundation.kCFAllocatorDefault
+import platform.CoreFoundation.kCFStringEncodingUTF8
 import platform.CoreFoundation.kCFBooleanFalse
 import platform.CoreFoundation.kCFBooleanTrue
 import platform.Foundation.CFBridgingRelease
@@ -149,8 +159,26 @@ public class KeychainSettings : Settings {
             return buildSet {
                 for (i in 0..<CFArrayGetCount(attributes.value)) {
                     val item: CFDictionaryRef? = CFArrayGetValueAtIndex(attributes.value, i.convert())?.reinterpret()
-                    val cfKey: CFStringRef? = CFDictionaryGetValue(item, kSecAttrAccount)?.reinterpret()
-                    if (cfKey != null) {
+                    val rawValue: CFTypeRef? = CFDictionaryGetValue(item, kSecAttrAccount)?.reinterpret()
+                    if (rawValue != null) {
+                        val typeId = CFGetTypeID(rawValue)
+                        when {
+                            typeId == CFStringGetTypeID() -> {
+                                val cfKey: CFStringRef = rawValue.reinterpret()
+                                val buf = allocArray<ByteVar>(512)
+                                val str = if (CFStringGetCString(cfKey, buf, 512, kCFStringEncodingUTF8)) buf.toKString() else "<CFStringGetCString failed>"
+                                println("KeychainSettings keys(): typeId=CFString value=$str")
+                            }
+                            typeId == CFDataGetTypeID() -> {
+                                val desc = CFCopyDescription(rawValue)
+                                val buf = allocArray<ByteVar>(1024)
+                                val descStr = if (desc != null && CFStringGetCString(desc, buf, 1024, kCFStringEncodingUTF8)) buf.toKString() else "<unknown>"
+                                if (desc != null) CFRelease(desc)
+                                println("KeychainSettings keys(): typeId=CFData description=$descStr")
+                            }
+                            else -> println("KeychainSettings keys(): unexpected typeId=$typeId")
+                        }
+                        val cfKey: CFStringRef? = rawValue.reinterpret()
                         val nsKey = CFBridgingRelease(cfKey) as NSString
                         add(nsKey.toKString())
                     }