[Swift 6] Add AtomicBox to CoreInternal (#14594)

Author: ncooke3

FirebaseCore/Internal/Sources/HeartbeatLogging/HeartbeatStorage.swift

@@ -59,27 +59,23 @@ final class HeartbeatStorage: Sendable, HeartbeatStorageProtocol {
     // `nonisolated(unsafe)` to disable concurrency-safety checks. The
     // property's access is protected by an external synchronization mechanism
     // (see `instancesLock` property).
-    private nonisolated(unsafe) static var cachedInstances: [
-      String: WeakContainer<HeartbeatStorage>
-    ] = [:]
+    private nonisolated(unsafe) static var cachedInstances: AtomicBox<
+      [String: WeakContainer<HeartbeatStorage>]
+    > = AtomicBox([:])
   #else
     // TODO(Xcode 16): Delete this block when minimum supported Xcode is
     // Xcode 16.
-    private static var cachedInstances: [
-      String: WeakContainer<HeartbeatStorage>
-    ] = [:]
+    static var cachedInstances: AtomicBox<[String: WeakContainer<HeartbeatStorage>]> =
+      AtomicBox([:])
   #endif // compiler(>=6)
 
-  /// Used to synchronize concurrent access to  the `cachedInstances` property.
-  private static let instancesLock = NSLock()
-
   /// Gets an existing `HeartbeatStorage` instance with the given `id` if one exists. Otherwise,
   /// makes a new instance with the given `id`.
   ///
   /// - Parameter id: A string identifier.
   /// - Returns: A `HeartbeatStorage` instance.
   static func getInstance(id: String) -> HeartbeatStorage {
-    instancesLock.withLock {
+    cachedInstances.withLock { cachedInstances in
       if let cachedInstance = cachedInstances[id]?.object {
         return cachedInstance
       } else {
@@ -110,8 +106,8 @@ final class HeartbeatStorage: Sendable, HeartbeatStorageProtocol {
 
   deinit {
     // Removes the instance if it was cached.
-    _ = Self.instancesLock.withLock {
-      Self.cachedInstances.removeValue(forKey: id)
+    Self.cachedInstances.withLock { value in
+      value.removeValue(forKey: id)
     }
   }
 

FirebaseCore/Internal/Sources/Utilities/AtomicBox.swift

@@ -0,0 +1,45 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import Foundation
+
+final class AtomicBox<T> {
+  private var _value: T
+  private let lock = NSLock()
+
+  public init(_ value: T) {
+    _value = value
+  }
+
+  public func value() -> T {
+    lock.withLock {
+      _value
+    }
+  }
+
+  @discardableResult
+  public func withLock(_ mutatingBody: (_ value: inout T) -> Void) -> T {
+    lock.withLock {
+      mutatingBody(&_value)
+      return _value
+    }
+  }
+
+  @discardableResult
+  public func withLock<R>(_ mutatingBody: (_ value: inout T) throws -> R) rethrows -> R {
+    try lock.withLock {
+      try mutatingBody(&_value)
+    }
+  }
+}