Fix: Crash when calling GrowthBookSDK.getFeatureValue concurrently (t…

[vazarkevych]

• Introduced NSRecursiveLock plus getEvalContext, copyEvalContextLocked, and updateEvalContext(with:) to guard GrowthBookSDK.evalContext against concurrent access.
• getFeatureValue now grabs a locked copy of the evaluation context, runs the FeatureEvaluator, and merges the results back, eliminating race conditions when called from multiple threads.
• The same lock-aware flow now covers evalFeature, run, setAttributes, appendAttributes, setForcedFeatures, setForcedVariations, etc., so every code path that mutates shared state stays thread-safe.

[madhuchavva]

The approach here is to:

1. Lock → Copy → Unlock
2. Evaluate (unlocked)
3. Lock → Merge back → Unlock

I think this is a bit complex way to manage the state and If two threads evaluate simultaneously, the second thread's updateEvalContext will overwrite the first thread's changes??

can we have a simple approach of using single lock around entire evaluation? that way it serializes the entire evaluation? here's an eg:

private let evaluationLock = NSLock() // Regular lock is fine

public func getFeatureValue(feature id: String, default defaultValue: JSON) -> JSON {
    evaluationLock.lock()
    defer { evaluationLock.unlock() }
    
    let result = FeatureEvaluator(context: evalContext, featureKey: id).evaluateFeature()
    return result.value ?? defaultValue
}

public func evalFeature(id: String) -> FeatureResult {
    evaluationLock.lock()
    defer { evaluationLock.unlock() }
    
    return FeatureEvaluator(context: evalContext, featureKey: id).evaluateFeature()
}

public func run(experiment: Experiment) -> ExperimentResult {
    evaluationLock.lock()
    defer { evaluationLock.unlock() }
    
    return ExperimentEvaluator().evaluateExperiment(context: evalContext, experiment: experiment)
}