fix: Replace subscriptpattern by calling setValue

[hivinau]

There is a crash that occurs in iOS 11 (iPhone 6S) doing :

let tracker = ATInternet.sharedInstance.tracker(Constants.trackerName, configuration: configuration)
tracker.context.level2 = Constants.iosLevel2Id

Here, key contains s2 as value. By doing, po buffer.persistentParameters[key], it returns nil.
I don't really know the reason for this, but calling setValue(value :, forKeyPath :) instead of the subscript method, there are no more crashes

[nsagnett]

Sorry but i can't reproduce your crash...have you any other input to reproduce ?

[hivinau]

Unfortunately, we cannot reproduce this with concrete data. However, I would like to point out that this crash only occurs on iOS 11 and iOS 12.

[ghugues]

We're seeing the same crash. It happens when processSetParam(_:value:options:) is called simultaneously (or almost simultaneously) from multiple threads. This is actually not surprising since the volatileParameters is not protected agains simultaneous accesses and mutating a Swift dictionary from multiple threads is undefined behavior and most likely to crash.

The question is : Is the SDK supposed to be thread safe ? In which case this is a SDK bug. Or is it not ? In which case it is a programmer error to use it simultaneously from multiple threads.

[nsagnett]

Unfortunately, the SDK is not thread safe