Avoid reassigning task-local runtime state in a Runner's event handler unless it's nested

Sources/Testing/Running/Runner.RuntimeState.swift

@@ -25,9 +25,10 @@ extension Runner {
     /// The test case that is running on the current task, if any.
     var testCase: Test.Case?
 
-    /// The runtime state related to the runner running on the current task.
+    /// The runtime state related to the runner running on the current task,
+    /// if any.
     @TaskLocal
-    static var current = Self()
+    static var current: Self?
   }
 }
 
@@ -42,7 +43,10 @@ extension Runner {
   /// In practice, the primary scenario where this is important is when running
   /// the testing library's own tests.
   mutating func configureEventHandlerRuntimeState() {
-    let existingRuntimeState = RuntimeState.current
+    guard let existingRuntimeState = RuntimeState.current else {
+      return
+    }
+
     configuration.eventHandler = { [eventHandler = configuration.eventHandler] event, context in
       RuntimeState.$current.withValue(existingRuntimeState) {
         eventHandler(event, context)
@@ -56,7 +60,7 @@ extension Runner {
 extension Configuration {
   /// The configuration for the current task, if any.
   public static var current: Self? {
-    Runner.RuntimeState.current.configuration
+    Runner.RuntimeState.current?.configuration
   }
 
   /// Call a function while the value of ``Configuration/current`` is set.
@@ -74,7 +78,7 @@ extension Configuration {
       configuration._removeFromAll(identifiedBy: id)
     }
 
-    var runtimeState = Runner.RuntimeState.current
+    var runtimeState = Runner.RuntimeState.current ?? .init()
     runtimeState.configuration = configuration
     return try await Runner.RuntimeState.$current.withValue(runtimeState, operation: body)
   }
@@ -144,7 +148,7 @@ extension Test {
   /// or [`DispatchQueue.async(execute:)`](https://developer.apple.com/documentation/dispatch/dispatchqueue/2016103-async)),
   /// the value of this property may be `nil`.
   public static var current: Self? {
-    Runner.RuntimeState.current.test
+    Runner.RuntimeState.current?.test
   }
 
   /// Call a function while the value of ``Test/current`` is set.
@@ -157,7 +161,7 @@ extension Test {
   ///
   /// - Throws: Whatever is thrown by `body`.
   static func withCurrent<R>(_ test: Self, perform body: () async throws -> R) async rethrows -> R {
-    var runtimeState = Runner.RuntimeState.current
+    var runtimeState = Runner.RuntimeState.current ?? .init()
     runtimeState.test = test
     return try await Runner.RuntimeState.$current.withValue(runtimeState, operation: body)
   }
@@ -177,7 +181,7 @@ extension Test.Case {
   /// or [`DispatchQueue.async(execute:)`](https://developer.apple.com/documentation/dispatch/dispatchqueue/2016103-async)),
   /// the value of this property may be `nil`.
   public static var current: Self? {
-    Runner.RuntimeState.current.testCase
+    Runner.RuntimeState.current?.testCase
   }
 
   /// Call a function while the value of ``Test/Case/current`` is set.
@@ -190,7 +194,7 @@ extension Test.Case {
   ///
   /// - Throws: Whatever is thrown by `body`.
   static func withCurrent<R>(_ testCase: Self, perform body: () async throws -> R) async rethrows -> R {
-    var runtimeState = Runner.RuntimeState.current
+    var runtimeState = Runner.RuntimeState.current ?? .init()
     runtimeState.testCase = testCase
     return try await Runner.RuntimeState.$current.withValue(runtimeState, operation: body)
   }

Tests/TestingTests/Runner.RuntimeStateTests.swift

@@ -25,4 +25,21 @@ struct Runner_RuntimeStateTests {
 
     await Test(name: "foo") {}.run(configuration: configuration)
   }
+
+  // This confirms that an event posted within the closure of `withTaskGroup`
+  // (and similar APIs) in regular tests (those which are not testing the
+  // testing library itself) are handled appropriately and do not crash.
+  @Test func eventPostingInTaskGroup() async {
+    // Enable delivery of "expectation checked" events, merely as a way to allow
+    // an event to be posted during the test below without causing any real
+    // issues to be recorded or otherwise confuse the testing harness.
+    var configuration = Configuration.current ?? .init()
+    configuration.deliverExpectationCheckedEvents = true
+
+    await Configuration.withCurrent(configuration) {
+      await withTaskGroup(of: Void.self) { group in
+        #expect(Bool(true))
+      }
+    }
+  }
 }

Tests/TestingTests/RunnerTests.swift

@@ -49,6 +49,13 @@ struct NeverRunTests {
 }
 
 final class RunnerTests: XCTestCase {
+  func testInitialTaskLocalState() {
+    // These are expected to be `nil` since this is an XCTest.
+    XCTAssertNil(Test.current)
+    XCTAssertNil(Test.Case.current)
+    XCTAssertNil(Configuration.current)
+  }
+
   func testDefaultInit() async throws {
     let runner = await Runner()
     XCTAssertFalse(runner.tests.contains(where: \.isHidden))