Add an upper bound on the number of test cases we run in parallel.

Sources/Testing/ABI/EntryPoints/EntryPoint.swift

@@ -210,6 +210,9 @@ public struct __CommandLineArguments_v0: Sendable {
   /// The value of the `--parallel` or `--no-parallel` argument.
   public var parallel: Bool?
 
+  /// The maximum number of test tasks to run in parallel.
+  public var experimentalMaximumParallelizationWidth: Int?
+
   /// The value of the `--symbolicate-backtraces` argument.
   public var symbolicateBacktraces: String?
 
@@ -336,6 +339,7 @@ extension __CommandLineArguments_v0: Codable {
   enum CodingKeys: String, CodingKey {
     case listTests
     case parallel
+    case experimentalMaximumParallelizationWidth
     case symbolicateBacktraces
     case verbose
     case veryVerbose
@@ -485,6 +489,11 @@ func parseCommandLineArguments(from args: [String]) throws -> __CommandLineArgum
   if args.contains("--no-parallel") {
     result.parallel = false
   }
+  if let maximumParallelizationWidth = args.argumentValue(forLabel: "--experimental-maximum-parallelization-width").flatMap(Int.init)
+    ?? Environment.variable(named: "SWT_EXPERIMENTAL_MAXIMUM_PARALLELIZATION_WIDTH").flatMap(Int.init) {
+    // TODO: decide if we want to repurpose --num-workers for this use case?
+    result.experimentalMaximumParallelizationWidth = maximumParallelizationWidth
+  }
 
   // Whether or not to symbolicate backtraces in the event stream.
   if let symbolicateBacktraces = args.argumentValue(forLabel: "--symbolicate-backtraces") {
@@ -546,6 +555,10 @@ public func configurationForEntryPoint(from args: __CommandLineArguments_v0) thr
 
   // Parallelization (on by default)
   configuration.isParallelizationEnabled = args.parallel ?? true
+  if let maximumParallelizationWidth = args.experimentalMaximumParallelizationWidth {
+    try! FileHandle.stderr.write("MAX WIDTH: \(maximumParallelizationWidth)\n")
+    configuration.maximumParallelizationWidth = maximumParallelizationWidth
+  }
 
   // Whether or not to symbolicate backtraces in the event stream.
   if let symbolicateBacktraces = args.symbolicateBacktraces {

Sources/Testing/CMakeLists.txt

@@ -90,6 +90,7 @@ add_library(Testing
   Support/Graph.swift
   Support/JSON.swift
   Support/Locked.swift
+  Support/Serializer.swift
   Support/VersionNumber.swift
   Support/Versions.swift
   Discovery+Macro.swift

Sources/Testing/Running/Configuration.swift

@@ -8,6 +8,8 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
+private import _TestingInternals
+
 /// A type containing settings for preparing and running tests.
 @_spi(ForToolsIntegrationOnly)
 public struct Configuration: Sendable {
@@ -20,6 +22,18 @@ public struct Configuration: Sendable {
   /// Whether or not to parallelize the execution of tests and test cases.
   public var isParallelizationEnabled: Bool = true
 
+  /// The maximum width of parallelization.
+  ///
+  /// The value of this property determines how many tests (or rather, test
+  /// cases) will run in parallel. The default value of this property is equal
+  /// to twice the number of CPU cores reported by the operating system, or
+  /// `Int.max` if that value is not available.
+  ///
+  /// If the value of ``isParallelizationEnabled`` is `false`, this property has
+  /// no effect.
+  @_spi(Experimental)
+  public var maximumParallelizationWidth: Int = cpuCoreCount.map { max(1, $0) * 2 } ?? .max
+
   /// How to symbolicate backtraces captured during a test run.
   ///
   /// If the value of this property is not `nil`, symbolication will be

Sources/Testing/Running/Runner.swift

@@ -66,6 +66,12 @@ extension Runner {
     .current ?? .init()
   }
 
+  /// Context to apply to a test run.
+  struct Context: Sendable {
+    /// A serializer used to reduce parallelism among test cases.
+    var testCaseSerializer: Serializer
+  }
+
   /// Apply the custom scope for any test scope providers of the traits
   /// associated with a specified test by calling their
   /// ``TestScoping/provideScope(for:testCase:performing:)`` function.
@@ -179,6 +185,7 @@ extension Runner {
   ///
   /// - Parameters:
   ///   - stepGraph: The subgraph whose root value, a step, is to be run.
+  ///   - context: Context for the test run.
   ///
   /// - Throws: Whatever is thrown from the test body. Thrown errors are
   ///   normally reported as test failures.
@@ -193,7 +200,7 @@ extension Runner {
   /// ## See Also
   ///
   /// - ``Runner/run()``
-  private static func _runStep(atRootOf stepGraph: Graph<String, Plan.Step?>) async throws {
+  private static func _runStep(atRootOf stepGraph: Graph<String, Plan.Step?>, context: Context) async throws {
     // Whether to send a `.testEnded` event at the end of running this step.
     // Some steps' actions may not require a final event to be sent — for
     // example, a skip event only sends `.testSkipped`.
@@ -250,18 +257,18 @@ extension Runner {
           try await _applyScopingTraits(for: step.test, testCase: nil) {
             // Run the test function at this step (if one is present.)
             if let testCases = step.test.testCases {
-              await _runTestCases(testCases, within: step)
+              await _runTestCases(testCases, within: step, context: context)
             }
 
             // Run the children of this test (i.e. the tests in this suite.)
-            try await _runChildren(of: stepGraph)
+            try await _runChildren(of: stepGraph, context: context)
           }
         }
       }
     } else {
       // There is no test at this node in the graph, so just skip down to the
       // child nodes.
-      try await _runChildren(of: stepGraph)
+      try await _runChildren(of: stepGraph, context: context)
     }
   }
 
@@ -286,10 +293,11 @@ extension Runner {
   /// - Parameters:
   ///   - stepGraph: The subgraph whose root value, a step, will be used to
   ///     find children to run.
+  ///   - context: Context for the test run.
   ///
   /// - Throws: Whatever is thrown from the test body. Thrown errors are
   ///   normally reported as test failures.
-  private static func _runChildren(of stepGraph: Graph<String, Plan.Step?>) async throws {
+  private static func _runChildren(of stepGraph: Graph<String, Plan.Step?>, context: Context) async throws {
     let childGraphs = if _configuration.isParallelizationEnabled {
       // Explicitly shuffle the steps to help detect accidental dependencies
       // between tests due to their ordering.
@@ -331,7 +339,7 @@ extension Runner {
 
     // Run the child nodes.
     try await _forEach(in: childGraphs.lazy.map(\.value), namingTasksWith: taskNamer) { childGraph in
-      try await _runStep(atRootOf: childGraph)
+      try await _runStep(atRootOf: childGraph, context: context)
     }
   }
 
@@ -340,12 +348,15 @@ extension Runner {
   /// - Parameters:
   ///   - testCases: The test cases to be run.
   ///   - step: The runner plan step associated with this test case.
+  ///   - context: Context for the test run.
   ///
   /// If parallelization is supported and enabled, the generated test cases will
   /// be run in parallel using a task group.
-  private static func _runTestCases(_ testCases: some Sequence<Test.Case>, within step: Plan.Step) async {
+  private static func _runTestCases(_ testCases: some Sequence<Test.Case>, within step: Plan.Step, context: Context) async {
+    let configuration = _configuration
+
     // Apply the configuration's test case filter.
-    let testCaseFilter = _configuration.testCaseFilter
+    let testCaseFilter = configuration.testCaseFilter
     let testCases = testCases.lazy.filter { testCase in
       testCaseFilter(testCase, step.test)
     }
@@ -359,7 +370,11 @@ extension Runner {
     }
 
     await _forEach(in: testCases.enumerated(), namingTasksWith: taskNamer) { _, testCase in
-      await _runTestCase(testCase, within: step)
+      if configuration.isParallelizationEnabled {
+        await context.testCaseSerializer.run { await _runTestCase(testCase, within: step, context: context) }
+      } else {
+        await _runTestCase(testCase, within: step, context: context)
+      }
     }
   }
 
@@ -368,10 +383,11 @@ extension Runner {
   /// - Parameters:
   ///   - testCase: The test case to run.
   ///   - step: The runner plan step associated with this test case.
+  ///   - context: Context for the test run.
   ///
   /// This function sets ``Test/Case/current``, then invokes the test case's
   /// body closure.
-  private static func _runTestCase(_ testCase: Test.Case, within step: Plan.Step) async {
+  private static func _runTestCase(_ testCase: Test.Case, within step: Plan.Step, context: Context) async {
     let configuration = _configuration
 
     Event.post(.testCaseStarted, for: (step.test, testCase), configuration: configuration)
@@ -428,6 +444,14 @@ extension Runner {
       eventHandler(event, context)
     }
 
+    // Context to pass into the test run. We intentionally don't pass the Runner
+    // itself (implicitly as `self` nor as an argument) because we don't want to
+    // accidentally depend on e.g. the `configuration` property rather than the
+    // current configuration.
+    let context = Context(
+      testCaseSerializer: Serializer(maximumWidth: runner.configuration.maximumParallelizationWidth)
+    )
+
     await Configuration.withCurrent(runner.configuration) {
       // Post an event for every test in the test plan being run. These events
       // are turned into JSON objects if JSON output is enabled.
@@ -454,7 +478,7 @@ extension Runner {
             taskAction = "running iteration #\(iterationIndex + 1)"
           }
           _ = taskGroup.addTaskUnlessCancelled(name: decorateTaskName("test run", withAction: taskAction)) {
-            try? await _runStep(atRootOf: runner.plan.stepGraph)
+            try? await _runStep(atRootOf: runner.plan.stepGraph, context: context)
           }
           await taskGroup.waitForAll()
         }

Sources/Testing/Support/Serializer.swift

@@ -0,0 +1,86 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024–2025 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
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+private import _TestingInternals
+
+/// The number of CPU cores on the current system, or `nil` if that
+/// information is not available.
+var cpuCoreCount: Int? {
+#if SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD) || os(Android)
+  return Int(sysconf(Int32(_SC_NPROCESSORS_CONF)))
+#elseif os(Windows)
+  var siInfo = SYSTEM_INFO()
+  GetSystemInfo(&siInfo)
+  return Int(siInfo.dwNumberOfProcessors)
+#elseif os(WASI)
+  return 1
+#else
+#warning("Platform-specific implementation missing: CPU core count unavailable")
+  return nil
+#endif
+}
+
+/// A type whose instances can run a series of work items in strict order.
+///
+/// When a work item is scheduled on an instance of this type, it runs after any
+/// previously-scheduled work items. If it suspends, subsequently-scheduled work
+/// items do not start running; they must wait until the suspended work item
+/// either returns or throws an error.
+///
+/// This type is not part of the public interface of the testing library.
+final actor Serializer {
+  /// The maximum number of work items that may run concurrently.
+  nonisolated let maximumWidth: Int
+
+  /// The number of scheduled work items, including any currently running.
+  private var _currentWidth = 0
+
+  /// Continuations for any scheduled work items that haven't started yet.
+  private var _continuations = [CheckedContinuation<Void, Never>]()
+
+  init(maximumWidth: Int = 1) {
+    self.maximumWidth = maximumWidth
+  }
+
+  /// Run a work item serially after any previously-scheduled work items.
+  ///
+  /// - Parameters:
+  ///     - workItem: A closure to run.
+  ///
+  /// - Returns: Whatever is returned from `workItem`.
+  ///
+  /// - Throws: Whatever is thrown by `workItem`.
+  func run<R>(_ workItem: @isolated(any) @Sendable () async throws -> R) async rethrows -> R where R: Sendable {
+    _currentWidth += 1
+    defer {
+      // Resume the next scheduled closure.
+      if !_continuations.isEmpty {
+        let continuation = _continuations.removeFirst()
+        continuation.resume()
+      }
+
+      _currentWidth -= 1
+    }
+
+    await withCheckedContinuation { continuation in
+      if _currentWidth <= maximumWidth {
+        // Nothing else was scheduled, so we can resume immediately.
+        continuation.resume()
+      } else {
+        // Something was scheduled, so add the continuation to the
+        // list. When it resumes, we can run.
+        _continuations.append(continuation)
+      }
+    }
+
+    return try await workItem()
+  }
+}
+