Add SPI-based withMainSerialExecutor

Package.swift

@@ -23,9 +23,11 @@ let package = Package(
     .package(url: "https://github.com/pointfreeco/xctest-dynamic-overlay", from: "0.8.0"),
   ],
   targets: [
+    .systemLibrary(name: "_CAsyncSupport"),
     .target(
       name: "Dependencies",
       dependencies: [
+        "_CAsyncSupport",                              
         .product(name: "CombineSchedulers", package: "combine-schedulers"),
         .product(name: "Clocks", package: "swift-clocks"),
         .product(name: "XCTestDynamicOverlay", package: "xctest-dynamic-overlay"),

Sources/Dependencies/ConcurrencySupport/MainSerialExecutor.swift

@@ -0,0 +1,12 @@
+import _CAsyncSupport
+
+@_spi(Concurrency) public func withMainSerialExecutor<T>(
+  @_implicitSelfCapture operation: () async throws -> T
+) async rethrows -> T {
+  let hook = swift_task_enqueueGlobal_hook
+  defer { swift_task_enqueueGlobal_hook = hook }
+  swift_task_enqueueGlobal_hook = { job, original in
+    MainActor.shared.enqueue(unsafeBitCast(job, to: UnownedJob.self))
+  }
+  return try await operation()
+}

Sources/_CAsyncSupport/_CAsyncSupport.h

@@ -0,0 +1,248 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Async Algorithms open source project
+//
+// Copyright (c) 2022 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+#if !defined(__has_feature)
+#define __has_feature(x) 0
+#endif
+
+#if !defined(__has_attribute)
+#define __has_attribute(x) 0
+#endif
+
+#if !defined(__has_builtin)
+#define __has_builtin(builtin) 0
+#endif
+
+#if !defined(__has_cpp_attribute)
+#define __has_cpp_attribute(attribute) 0
+#endif
+
+// TODO: These macro definitions are duplicated in BridgedSwiftObject.h. Move
+// them to a single file if we find a location that both Visibility.h and
+// BridgedSwiftObject.h can import.
+#if __has_feature(nullability)
+// Provide macros to temporarily suppress warning about the use of
+// _Nullable and _Nonnull.
+# define SWIFT_BEGIN_NULLABILITY_ANNOTATIONS                        \
+  _Pragma("clang diagnostic push")                                  \
+  _Pragma("clang diagnostic ignored \"-Wnullability-extension\"")
+# define SWIFT_END_NULLABILITY_ANNOTATIONS                          \
+  _Pragma("clang diagnostic pop")
+
+#else
+// #define _Nullable and _Nonnull to nothing if we're not being built
+// with a compiler that supports them.
+# define _Nullable
+# define _Nonnull
+# define SWIFT_BEGIN_NULLABILITY_ANNOTATIONS
+# define SWIFT_END_NULLABILITY_ANNOTATIONS
+#endif
+
+#define SWIFT_MACRO_CONCAT(A, B) A ## B
+#define SWIFT_MACRO_IF_0(IF_TRUE, IF_FALSE) IF_FALSE
+#define SWIFT_MACRO_IF_1(IF_TRUE, IF_FALSE) IF_TRUE
+#define SWIFT_MACRO_IF(COND, IF_TRUE, IF_FALSE) \
+  SWIFT_MACRO_CONCAT(SWIFT_MACRO_IF_, COND)(IF_TRUE, IF_FALSE)
+
+#if __has_attribute(pure)
+#define SWIFT_READONLY __attribute__((__pure__))
+#else
+#define SWIFT_READONLY
+#endif
+
+#if __has_attribute(const)
+#define SWIFT_READNONE __attribute__((__const__))
+#else
+#define SWIFT_READNONE
+#endif
+
+#if __has_attribute(always_inline)
+#define SWIFT_ALWAYS_INLINE __attribute__((always_inline))
+#else
+#define SWIFT_ALWAYS_INLINE
+#endif
+
+#if __has_attribute(noinline)
+#define SWIFT_NOINLINE __attribute__((__noinline__))
+#else
+#define SWIFT_NOINLINE
+#endif
+
+#if __has_attribute(noreturn)
+#define SWIFT_NORETURN __attribute__((__noreturn__))
+#else
+#define SWIFT_NORETURN
+#endif
+
+#if __has_attribute(used)
+#define SWIFT_USED __attribute__((__used__))
+#else
+#define SWIFT_USED
+#endif
+
+#if __has_attribute(unavailable)
+#define SWIFT_ATTRIBUTE_UNAVAILABLE __attribute__((__unavailable__))
+#else
+#define SWIFT_ATTRIBUTE_UNAVAILABLE
+#endif
+
+#if (__has_attribute(weak_import))
+#define SWIFT_WEAK_IMPORT __attribute__((weak_import))
+#else
+#define SWIFT_WEAK_IMPORT
+#endif
+
+// Define the appropriate attributes for sharing symbols across
+// image (executable / shared-library) boundaries.
+//
+// SWIFT_ATTRIBUTE_FOR_EXPORTS will be placed on declarations that
+// are known to be exported from the current image.  Typically, they
+// are placed on header declarations and then inherited by the actual
+// definitions.
+//
+// SWIFT_ATTRIBUTE_FOR_IMPORTS will be placed on declarations that
+// are known to be exported from a different image.  This never
+// includes a definition.
+//
+// Getting the right attribute on a declaratioon can be pretty awkward,
+// but it's necessary under the C translation model.  All of this
+// ceremony is familiar to Windows programmers; C/C++ programmers
+// everywhere else usually don't bother, but since we have to get it
+// right for Windows, we have everything set up to get it right on
+// other targets as well, and doing so lets the compiler use more
+// efficient symbol access patterns.
+#if defined(__MACH__) || defined(__wasi__)
+
+// On Mach-O and WebAssembly, we use non-hidden visibility.  We just use
+// default visibility on both imports and exports, both because these
+// targets don't support protected visibility but because they don't
+// need it: symbols are not interposable outside the current image
+// by default.
+# define SWIFT_ATTRIBUTE_FOR_EXPORTS __attribute__((__visibility__("default")))
+# define SWIFT_ATTRIBUTE_FOR_IMPORTS __attribute__((__visibility__("default")))
+
+#elif defined(__ELF__)
+
+// On ELF, we use non-hidden visibility.  For exports, we must use
+// protected visibility to tell the compiler and linker that the symbols
+// can't be interposed outside the current image.  For imports, we must
+// use default visibility because protected visibility guarantees that
+// the symbol is defined in the current library, which isn't true for
+// an import.
+//
+// The compiler does assume that the runtime and standard library can
+// refer to each other's symbols as DSO-local, so it's important that
+// we get this right or we can get linker errors.
+# define SWIFT_ATTRIBUTE_FOR_EXPORTS __attribute__((__visibility__("protected")))
+# define SWIFT_ATTRIBUTE_FOR_IMPORTS __attribute__((__visibility__("default")))
+
+#elif defined(__CYGWIN__)
+
+// For now, we ignore all this on Cygwin.
+# define SWIFT_ATTRIBUTE_FOR_EXPORTS
+# define SWIFT_ATTRIBUTE_FOR_IMPORTS
+
+// FIXME: this #else should be some sort of #elif Windows
+#else // !__MACH__ && !__ELF__
+
+// On PE/COFF, we use dllimport and dllexport.
+# define SWIFT_ATTRIBUTE_FOR_EXPORTS __declspec(dllexport)
+# define SWIFT_ATTRIBUTE_FOR_IMPORTS __declspec(dllimport)
+
+#endif
+
+// CMake conventionally passes -DlibraryName_EXPORTS when building
+// code that goes into libraryName.  This isn't the best macro name,
+// but it's conventional.  We do have to pass it explicitly in a few
+// places in the build system for a variety of reasons.
+//
+// Unfortunately, defined(D) is a special function you can use in
+// preprocessor conditions, not a macro you can use anywhere, so we
+// need to manually check for all the libraries we know about so that
+// we can use them in our condition below.s
+#if defined(swiftCore_EXPORTS)
+#define SWIFT_IMAGE_EXPORTS_swiftCore 1
+#else
+#define SWIFT_IMAGE_EXPORTS_swiftCore 0
+#endif
+#if defined(swift_Concurrency_EXPORTS)
+#define SWIFT_IMAGE_EXPORTS_swift_Concurrency 1
+#else
+#define SWIFT_IMAGE_EXPORTS_swift_Concurrency 0
+#endif
+#if defined(swift_Distributed_EXPORTS)
+#define SWIFT_IMAGE_EXPORTS_swift_Distributed 1
+#else
+#define SWIFT_IMAGE_EXPORTS_swift_Distributed 0
+#endif
+#if defined(swift_Differentiation_EXPORTS)
+#define SWIFT_IMAGE_EXPORTS_swift_Differentiation 1
+#else
+#define SWIFT_IMAGE_EXPORTS_swift_Differentiation 0
+#endif
+
+#define SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)                          \
+  SWIFT_MACRO_IF(SWIFT_IMAGE_EXPORTS_##LIBRARY,                       \
+                 SWIFT_ATTRIBUTE_FOR_EXPORTS,                         \
+                 SWIFT_ATTRIBUTE_FOR_IMPORTS)
+
+// SWIFT_EXPORT_FROM(LIBRARY) declares something to be a C-linkage
+// entity exported by the given library.
+//
+// SWIFT_RUNTIME_EXPORT is just SWIFT_EXPORT_FROM(swiftCore).
+//
+// TODO: use this in shims headers in overlays.
+#if defined(__cplusplus)
+#define SWIFT_EXPORT_FROM(LIBRARY) extern "C" SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)
+#define SWIFT_EXPORT extern "C"
+#else
+#define SWIFT_EXPORT extern
+#define SWIFT_EXPORT_FROM(LIBRARY) SWIFT_EXPORT_FROM_ATTRIBUTE(LIBRARY)
+#endif
+#define SWIFT_RUNTIME_EXPORT SWIFT_EXPORT_FROM(swiftCore)
+
+// Define mappings for calling conventions.
+
+// Annotation for specifying a calling convention of
+// a runtime function. It should be used with declarations
+// of runtime functions like this:
+// void runtime_function_name() SWIFT_CC(swift)
+#define SWIFT_CC(CC) SWIFT_CC_##CC
+
+// SWIFT_CC(c) is the C calling convention.
+#define SWIFT_CC_c
+
+// SWIFT_CC(swift) is the Swift calling convention.
+// FIXME: the next comment is false.
+// Functions outside the stdlib or runtime that include this file may be built
+// with a compiler that doesn't support swiftcall; don't define these macros
+// in that case so any incorrect usage is caught.
+#if __has_attribute(swiftcall)
+#define SWIFT_CC_swift __attribute__((swiftcall))
+#define SWIFT_CONTEXT __attribute__((swift_context))
+#define SWIFT_ERROR_RESULT __attribute__((swift_error_result))
+#define SWIFT_INDIRECT_RESULT __attribute__((swift_indirect_result))
+#else
+#define SWIFT_CC_swift
+#define SWIFT_CONTEXT
+#define SWIFT_ERROR_RESULT
+#define SWIFT_INDIRECT_RESULT
+#endif
+
+typedef struct _Job* JobRef;
+
+typedef SWIFT_CC(swift) void (*swift_task_enqueueGlobal_original)(JobRef _Nonnull job);
+SWIFT_EXPORT_FROM(swift_Concurrency)
+SWIFT_CC(swift) void (* _Nullable swift_task_enqueueGlobal_hook)(
+  JobRef _Nonnull job, swift_task_enqueueGlobal_original _Nonnull original);
+

Sources/_CAsyncSupport/module.modulemap

@@ -0,0 +1,4 @@
+module _CAsyncSupport [system] {
+  header "_CAsyncSupport.h"
+  export *
+}

Tests/DependenciesTests/MainSerialExecutorTests.swift

@@ -0,0 +1,18 @@
+@_spi(Concurrency) import Dependencies
+import XCTest
+
+final class MainSerialExecutorTests: XCTestCase {
+  func testSerializedExecution() async {
+    let xs = LockIsolated<[Int]>([])
+    await withMainSerialExecutor {
+      await withTaskGroup(of: Void.self) { group in
+        for x in 1...1000 {
+          group.addTask {
+            xs.withValue { $0.append(x) }
+          }
+        }
+      }
+    }
+    xs.withValue { XCTAssertEqual(Array(1...1000), $0) }
+  }
+}