[Storage] Addressing Swift 6 issues with Storage's instance management (#13445)

Author: ncooke3

FirebaseStorage/Sources/Storage.swift

@@ -73,15 +73,7 @@ import FirebaseCore
   }
 
   private class func storage(app: FirebaseApp, bucket: String) -> Storage {
-    os_unfair_lock_lock(&instancesLock)
-    defer { os_unfair_lock_unlock(&instancesLock) }
-
-    if let instance = instances[bucket] {
-      return instance
-    }
-    let newInstance = FirebaseStorage.Storage(app: app, bucket: bucket)
-    instances[bucket] = newInstance
-    return newInstance
+    return InstanceCache.shared.storage(app: app, bucket: bucket)
   }
 
   /// The `FirebaseApp` associated with this Storage instance.
@@ -249,6 +241,31 @@ import FirebaseCore
 
   // MARK: - Internal and Private APIs
 
+  private final class InstanceCache: @unchecked Sendable {
+    static let shared = InstanceCache()
+
+    /// A map of active instances, grouped by app. Keys are FirebaseApp names and values are
+    /// instances of Storage associated with the given app.
+    private var instances: [String: Storage] = [:]
+
+    /// Lock to manage access to the instances array to avoid race conditions.
+    private var instancesLock: os_unfair_lock = .init()
+
+    private init() {}
+
+    func storage(app: FirebaseApp, bucket: String) -> Storage {
+      os_unfair_lock_lock(&instancesLock)
+      defer { os_unfair_lock_unlock(&instancesLock) }
+
+      if let instance = instances[bucket] {
+        return instance
+      }
+      let newInstance = FirebaseStorage.Storage(app: app, bucket: bucket)
+      instances[bucket] = newInstance
+      return newInstance
+    }
+  }
+
   let fetcherService = StorageFetcherService()
 
   let dispatchQueue: DispatchQueue
@@ -281,13 +298,6 @@ import FirebaseCore
   /// Once `configured` is true, the emulator can no longer be enabled.
   var configured = false
 
-  /// A map of active instances, grouped by app. Keys are FirebaseApp names and values are
-  /// instances of Storage associated with the given app.
-  private static var instances: [String: Storage] = [:]
-
-  /// Lock to manage access to the instances array to avoid race conditions.
-  private static var instancesLock: os_unfair_lock = .init()
-
   var host: String
   var scheme: String
   var port: Int