MXKAccountManager: accounts are stored in a discardable cache files when no app group id is available

[giomfo]

When no application group identifier is defined, the Matrix account credentials are stored in a discardable cache files (Library/Caches).

Indeed the [MXKAppSettings cacheFolder] used in MXKAccountManager:

matrix-ios-kit/MatrixKit/Models/Account/MXKAccountManager.m

Line 496 in 2a57b54

NSString *matrixKitCacheFolder = [MXKAppSettings cacheFolder];

returns this folder when no group id is defined, see:

matrix-ios-kit/MatrixKit/Models/MXKAppSettings.m

Line 77 in 2a57b54

NSArray *cacheDirList = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);

A new method should be added to MXKAppSettings to return an applicationSupportFolder (based on NSApplicationSupportDirectory), then the MXKAccountManager will be able to use it to store account data (when no group id is defined).

[tladesignz]

We experienced this, too.

See here, why that is a bad idea:

https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html
(Under "Where You Should Put Your App’s Files")

It seems, on storage constrained devices, this can happen fairly regularly.
My guess is, esp. when the App Store app loads updates and needs to keep these somewhere until they're complete.

It obviously copies this behavior from MatrixSDK:
https://github.com/matrix-org/matrix-ios-sdk/blob/develop/MatrixSDK/Data/Store/MXFileStore/MXFileStore.m#L802

I'd recommend cross-checking in MatrixSDK, if the encryption keys are lost, when the cache is purged. That's also not nice...