feat!: Add ImmutableContext implementation (#70)

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## This PR
- Don't expose `MutableContext`, only use it in tests
- Expose a new `ImmutableContext`, with methods to safely edit it by
returning new immutable copies
- Remove `deepCopy` operations, not needed with the new ImmutableContext

### Related Issues
We've received crash reports on the Confidence Provider that we believe
are caused by the passed context object being mutated at runtime.

---------

Signed-off-by: Fabrizio Demaria <[email protected]>

Author: fabriziodemaria

Sources/OpenFeature/EvaluationContext.swift

@@ -4,5 +4,4 @@ import Foundation
 public protocol EvaluationContext: Structure {
     func getTargetingKey() -> String
     func deepCopy() -> EvaluationContext
-    func setTargetingKey(targetingKey: String)
 }

Sources/OpenFeature/ImmutableContext.swift

@@ -0,0 +1,67 @@
+import Foundation
+
+/// The ``ImmutableContext`` is an ``EvaluationContext`` implementation which is immutable and thread-safe.
+/// It provides read-only access to context data and cannot be modified after creation.
+public struct ImmutableContext: EvaluationContext {
+    private let targetingKey: String
+    private let structure: ImmutableStructure
+
+    public init(targetingKey: String = "", structure: ImmutableStructure = ImmutableStructure()) {
+        self.targetingKey = targetingKey
+        self.structure = structure
+    }
+
+    public init(attributes: [String: Value]) {
+        self.init(structure: ImmutableStructure(attributes: attributes))
+    }
+
+    public func deepCopy() -> EvaluationContext {
+        return ImmutableContext(targetingKey: targetingKey, structure: structure.deepCopy())
+    }
+
+    public func getTargetingKey() -> String {
+        return targetingKey
+    }
+
+    public func keySet() -> Set<String> {
+        return structure.keySet()
+    }
+
+    public func getValue(key: String) -> Value? {
+        return structure.getValue(key: key)
+    }
+
+    public func asMap() -> [String: Value] {
+        return structure.asMap()
+    }
+
+    public func asObjectMap() -> [String: AnyHashable?] {
+        return structure.asObjectMap()
+    }
+}
+
+extension ImmutableContext {
+    public func withTargetingKey(_ targetingKey: String) -> ImmutableContext {
+        return ImmutableContext(targetingKey: targetingKey, structure: structure)
+    }
+
+    public func withAttribute(key: String, value: Value) -> ImmutableContext {
+        var newAttributes = structure.asMap()
+        newAttributes[key] = value
+        return ImmutableContext(targetingKey: targetingKey, structure: ImmutableStructure(attributes: newAttributes))
+    }
+
+    public func withAttributes(_ attributes: [String: Value]) -> ImmutableContext {
+        var newAttributes = structure.asMap()
+        for (key, value) in attributes {
+            newAttributes[key] = value
+        }
+        return ImmutableContext(targetingKey: targetingKey, structure: ImmutableStructure(attributes: newAttributes))
+    }
+
+    public func withoutAttribute(key: String) -> ImmutableContext {
+        var newAttributes = structure.asMap()
+        newAttributes.removeValue(forKey: key)
+        return ImmutableContext(targetingKey: targetingKey, structure: ImmutableStructure(attributes: newAttributes))
+    }
+}

Sources/OpenFeature/ImmutableStructure.swift

@@ -0,0 +1,54 @@
+import Foundation
+
+/// The ``ImmutableStructure`` is a ``Structure`` implementation which is immutable and thread-safe.
+/// It provides read-only access to structured data and cannot be modified after creation.
+public class ImmutableStructure: Structure {
+    private let attributes: [String: Value]
+
+    public init(attributes: [String: Value] = [:]) {
+        self.attributes = attributes
+    }
+
+    public func keySet() -> Set<String> {
+        return Set(attributes.keys)
+    }
+
+    public func getValue(key: String) -> Value? {
+        return attributes[key]
+    }
+
+    public func asMap() -> [String: Value] {
+        return attributes
+    }
+
+    public func asObjectMap() -> [String: AnyHashable?] {
+        return attributes.mapValues(convertValue)
+    }
+
+    public func deepCopy() -> ImmutableStructure {
+        return ImmutableStructure(attributes: attributes)
+    }
+}
+
+extension ImmutableStructure {
+    private func convertValue(value: Value) -> AnyHashable? {
+        switch value {
+        case .boolean(let value):
+            return value
+        case .string(let value):
+            return value
+        case .integer(let value):
+            return value
+        case .double(let value):
+            return value
+        case .date(let value):
+            return value
+        case .list(let value):
+            return value.map(convertValue)
+        case .structure(let value):
+            return value.mapValues(convertValue)
+        case .null:
+            return nil
+        }
+    }
+}

Sources/OpenFeature/OpenFeatureAPI.swift

@@ -156,11 +156,11 @@ public class OpenFeatureAPI {
         self.providerSubject.send(provider)
 
         if let initialContext = initialContext {
-            self.evaluationContext = initialContext.deepCopy()
+            self.evaluationContext = initialContext
         }
 
         do {
-            try await provider.initialize(initialContext: initialContext?.deepCopy())
+            try await provider.initialize(initialContext: initialContext)
             self.providerStatus = .ready
             self.eventHandler.send(.ready)
         } catch {
@@ -177,13 +177,13 @@ public class OpenFeatureAPI {
 
     private func updateContext(evaluationContext: EvaluationContext) async {
         do {
-            let oldContext = self.evaluationContext?.deepCopy()
-            self.evaluationContext = evaluationContext.deepCopy()
+            let oldContext = self.evaluationContext
+            self.evaluationContext = evaluationContext
             self.providerStatus = .reconciling
             eventHandler.send(.reconciling)
             try await self.providerSubject.value?.onContextSet(
                 oldContext: oldContext,
-                newContext: evaluationContext.deepCopy()
+                newContext: evaluationContext
             )
             self.providerStatus = .ready
             eventHandler.send(.contextChanged)

Sources/OpenFeature/MutableContext.swift renamed to Tests/OpenFeatureTests/Helpers/MutableContext.swift

@@ -1,5 +1,7 @@
 import Foundation
 
+@testable import OpenFeature
+
 /// The ``MutableContext`` is an ``EvaluationContext`` implementation which is threadsafe, and whose attributes can
 /// be modified after instantiation.
 public class MutableContext: EvaluationContext {
@@ -28,12 +30,6 @@ public class MutableContext: EvaluationContext {
         }
     }
 
-    public func setTargetingKey(targetingKey: String) {
-        queue.sync {
-            self.targetingKey = targetingKey
-        }
-    }
-
     public func keySet() -> Set<String> {
         return queue.sync {
             structure.keySet()
@@ -57,12 +53,18 @@ public class MutableContext: EvaluationContext {
             structure.asObjectMap()
         }
     }
+
+    public func setTargetingKey(targetingKey: String) {
+        queue.sync {
+            self.targetingKey = targetingKey
+        }
+    }
 }
 
 extension MutableContext {
     @discardableResult
     public func add(key: String, value: Value) -> MutableContext {
-        queue.sync {
+        _ = queue.sync {
             self.structure.add(key: key, value: value)
         }
         return self