Fix concurrency-safety of _ExecutionContext

Sources/CollectionsBenchmark/Benchmark/Benchmark.swift

@@ -15,10 +15,10 @@ public struct Benchmark {
   public let title: String
   internal var _tasks: _SimpleOrderedDictionary<String, AnyTask> = [:]
   internal var _inputGenerators: [_TypeBox: (Int) -> Any] = [:]
-  private let _executionContext = _ExecutionContext.shared
 
   public init(title: String = "") {
     self.title = title
+    _setUpExecutionCheck()
     registerInputGenerator(for: Int.self) { size in
       size
     }
@@ -56,10 +56,6 @@ public struct Benchmark {
     }
   }
 
-  internal func _markAsExecuted() {
-    _executionContext._hasExecuted = true
-  }
-
   public func allTaskNames() -> [String] {
     _tasks.map { $0.key }
   }

Sources/CollectionsBenchmark/Benchmark/_ExecutionContext.swift

@@ -2,28 +2,38 @@
 //
 // This source file is part of the Swift Collections open source project
 //
-// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Copyright (c) 2021 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
 //
 //===----------------------------------------------------------------------===//
 
-import Foundation
+import Foundation // for atexit
+import Synchronization
+
+let _hasDoneSetup: Atomic<Bool> = .init(false)
+let _hasExecuted: Atomic<Bool> = .init(false)
 
 private func _exitHandler() {
-  if !_ExecutionContext.shared._hasExecuted {
+  if !_hasExecuted.load(ordering: .acquiring) {
     fatalError("Did you forget to call Benchmark.start()?")
   }
 }
 
-internal class _ExecutionContext {
-  var _hasExecuted = false
-
-  private init() {
+internal func _setUpExecutionCheck() {
+  let (exchanged, _) = _hasDoneSetup.compareExchange(
+    expected: false, desired: true, ordering: .acquiringAndReleasing)
+  if exchanged {
     // Check that Benchmark.start() was executed before exiting the process.
+    // FIXME: Calling convention mismatch
     atexit(_exitHandler)
   }
-
-  static let shared: _ExecutionContext = { _ExecutionContext() }()
+}
+
+internal func _markAsExecuted() {
+  guard _hasDoneSetup.load(ordering: .acquiring) else {
+    fatalError("Internal error: _markAsExecuted called before _setUpExecutionCheck")
+  }
+  _hasExecuted.store(true, ordering: .releasing)
 }