Merge pull request #36239 from etcwilde/ewilde/run-async-main-on-main-thread

[Concurrency] Run async main code on main thread

Author: etcwilde

stdlib/public/Concurrency/Task.swift

@@ -579,6 +579,7 @@ public func runAsyncAndBlock(_ asyncFun: @escaping () async -> ())
 public func _asyncMainDrainQueue() -> Never
 
 public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
+#if os(Windows)
   Task.runDetached {
     do {
       try await asyncFun()
@@ -587,6 +588,21 @@ public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
       _errorInMain(error)
     }
   }
+#else
+  @MainActor @concurrent
+  func _doMain(_ asyncFun: @escaping () async throws -> ()) async {
+    do {
+      try await asyncFun()
+    } catch {
+      _errorInMain(error)
+    }
+  }
+
+  Task.runDetached {
+    await _doMain(asyncFun)
+    exit(0)
+  }
+#endif
   _asyncMainDrainQueue()
 }
 

test/Concurrency/Runtime/mainactor.swift

@@ -66,6 +66,8 @@ actor A {
 
 // CHECK: starting
 // CHECK-NOT: ERROR
+// CHECK: Hello from the main function
+// CHECK-NOT: ERROR
 // CHECK: hello from main actor!
 // CHECK-NOT: ERROR
 // CHECK: on actor instance's queue
@@ -77,6 +79,11 @@ actor A {
 @main struct RunIt {
   static func main() async {
     print("starting")
+    if checkIfMainQueue(expectedAnswer: true) {
+      print("Hello from the main function")
+    } else {
+      print("ERROR: not on the main queue")
+    }
     let result = await someFunc()
     print("finished with return counter = \(result)")
   }