[Concurrency] Initial steps for startSynchronously for Task (#79608)

* [Concurrency] Initial steps for startSynchronously for Task

* [Concurrency] Rename to _startSynchronously while in development

* [Concurrency] StartSynchronously special executor to avoid switching

* startSynchronously bring back more info output

* [Concurrency] startSynchronously with more custom executor tests

* add missing ABI additions to test for x86

* [Concurrency] gyb generate _startSynchronously

* [Concurrency] %import dispatch for Linux startSynchronously test

* [Concurrency] Add TaskGroup.startTaskSynchronously funcs

* [Concurrency] DispatchSerialQueue does not exist on linux still

Author: ktoso

Runtimes/Core/Concurrency/CMakeLists.txt

@@ -1,5 +1,7 @@
 add_subdirectory(InternalShims)
 
+gyb_expand(Task+startSynchronously.swift.gyb Task+startSynchronously.swift)
+
 add_library(swift_Concurrency
   Actor.cpp
   AsyncLet.cpp
@@ -80,7 +82,9 @@ add_library(swift_Concurrency
   TaskGroup+TaskExecutor.swift
   TaskLocal.swift
   TaskSleep.swift
-  TaskSleepDuration.swift)
+  TaskSleepDuration.swift
+  "${CMAKE_CURRENT_BINARY_DIR}/Task+startSynchronously.swift")
+
 include(${SwiftCore_CONCURRENCY_GLOBAL_EXECUTOR}.cmake)
 target_compile_definitions(swift_Concurrency PRIVATE
   $<$<COMPILE_LANGUAGE:C,CXX>:-DSWIFT_TARGET_LIBRARY_NAME=swift_Concurrency>

include/swift/ABI/Executor.h

@@ -77,6 +77,10 @@ class SerialExecutorRef {
     /// Executor that may need to participate in complex "same context" checks,
     /// by invoking `isSameExclusiveExecutionContext` when comparing execution contexts.
     ComplexEquality = 0b01,
+    /// Mark this executor as the one used by `Task.startSynchronously`,
+    /// It cannot participate in switching.
+    // TODO: Perhaps make this a generic "cannot switch" rather than start synchronously specific.
+    StartSynchronously = 0b10,
   };
 
   static_assert(static_cast<uintptr_t>(ExecutorKind::Ordinary) == 0);
@@ -101,6 +105,16 @@ class SerialExecutorRef {
     return SerialExecutorRef(actor, 0);
   }
 
+  static SerialExecutorRef forSynchronousStart() {
+    auto wtable = reinterpret_cast<uintptr_t>(nullptr) |
+                  static_cast<uintptr_t>(ExecutorKind::StartSynchronously);
+    return SerialExecutorRef(nullptr, wtable);
+  }
+  bool isForSynchronousStart() const {
+    return getIdentity() == nullptr &&
+           getExecutorKind() == ExecutorKind::StartSynchronously;
+  }
+
   /// Given a pointer to a serial executor and its SerialExecutor
   /// conformance, return an executor reference for it.
   static SerialExecutorRef forOrdinary(HeapObject *identity,
@@ -127,7 +141,7 @@ class SerialExecutorRef {
 
   const char* getIdentityDebugName() const {
     return isMainExecutor() ? " (MainActorExecutor)"
-           : isGeneric()    ? " (GenericExecutor)"
+           : isGeneric()    ? (isForSynchronousStart() ? " (GenericExecutor/SynchronousStart)" : " (GenericExecutor)")
                             : "";
   }
 

include/swift/ABI/MetadataValues.h

@@ -2668,13 +2668,13 @@ class TaskCreateFlags : public FlagSet<size_t> {
     Task_EnqueueJob                               = 12,
     Task_AddPendingGroupTaskUnconditionally       = 13,
     Task_IsDiscardingTask                         = 14,
-
     /// The task function is consumed by calling it (@callee_owned).
     /// The context pointer should be treated as opaque and non-copyable;
     /// in particular, it should not be retained or released.
     ///
     /// Supported starting in Swift 6.1.
-    Task_IsTaskFunctionConsumed                       = 15,
+    Task_IsTaskFunctionConsumed                   = 15,
+    Task_IsStartSynchronouslyTask                 = 16,
   };
 
   explicit constexpr TaskCreateFlags(size_t bits) : FlagSet(bits) {}
@@ -2707,6 +2707,9 @@ class TaskCreateFlags : public FlagSet<size_t> {
   FLAGSET_DEFINE_FLAG_ACCESSORS(Task_IsTaskFunctionConsumed,
                                 isTaskFunctionConsumed,
                                 setIsTaskFunctionConsumed)
+  FLAGSET_DEFINE_FLAG_ACCESSORS(Task_IsStartSynchronouslyTask,
+                                isSynchronousStartTask,
+                                setIsSYnchronousStartTask)
 };
 
 /// Flags for schedulable jobs.

include/swift/Runtime/Concurrency.h

@@ -1025,6 +1025,9 @@ JobPriority swift_task_getCurrentThreadPriority(void);
 SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
 void swift_task_startOnMainActor(AsyncTask* job);
 
+SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
+void swift_task_startSynchronously(AsyncTask* job);
+
 /// Donate this thread to the global executor until either the
 /// given condition returns true or we've run out of cooperative
 /// tasks to run.

stdlib/public/CompatibilityOverride/CompatibilityOverrideConcurrency.def

@@ -438,6 +438,11 @@ OVERRIDE_TASK(task_startOnMainActor, void,
               SWIFT_EXPORT_FROM(swift_Concurrency), SWIFT_CC(swift),
               swift::, (AsyncTask *task), (task))
 
+// In ACTOR since we need ExecutorTracking info
+OVERRIDE_ACTOR(task_startSynchronously, void,
+               SWIFT_EXPORT_FROM(swift_Concurrency), SWIFT_CC(swift),
+               swift::, (AsyncTask *task), (task))
+
 #undef OVERRIDE
 #undef OVERRIDE_ACTOR
 #undef OVERRIDE_TASK

stdlib/public/Concurrency/Actor.cpp

@@ -1683,8 +1683,8 @@ static void defaultActorDrain(DefaultActorImpl *actor) {
         trackingInfo.setTaskExecutor(taskExecutor);
       }
 
-      // This thread is now going to follow the task on this actor. It may hop off
-      // the actor
+      // This thread is now going to follow the task on this actor.
+      // It may hop off the actor
       runJobInEstablishedExecutorContext(job);
 
       // We could have come back from the job on a generic executor and not as
@@ -2027,7 +2027,9 @@ static void swift_job_runImpl(Job *job, SerialExecutorRef executor) {
   // run jobs.  Actor executors won't expect us to switch off them
   // during this operation.  But do allow switching if the executor
   // is generic.
-  if (!executor.isGeneric()) trackingInfo.disallowSwitching();
+  if (!executor.isGeneric()) {
+    trackingInfo.disallowSwitching();
+  }
 
   auto taskExecutor = executor.isGeneric()
                           ? TaskExecutorRef::fromTaskExecutorPreference(job)
@@ -2149,13 +2151,19 @@ static bool canGiveUpThreadForSwitch(ExecutorTrackingInfo *trackingInfo,
   assert(trackingInfo || currentExecutor.isGeneric());
 
   // Some contexts don't allow switching at all.
-  if (trackingInfo && !trackingInfo->allowsSwitching())
+  if (trackingInfo && !trackingInfo->allowsSwitching()) {
     return false;
+  }
 
   // We can certainly "give up" a generic executor to try to run
   // a task for an actor.
-  if (currentExecutor.isGeneric())
+  if (currentExecutor.isGeneric()) {
+    if (currentExecutor.isForSynchronousStart()) {
+      return false;
+    }
+
     return true;
+  }
 
   // If the current executor is a default actor, we know how to make
   // it give up its thread.
@@ -2276,7 +2284,8 @@ static void swift_task_switchImpl(SWIFT_ASYNC_CONTEXT AsyncContext *resumeContex
                                            : TaskExecutorRef::undefined());
   auto newTaskExecutor = task->getPreferredTaskExecutor();
   SWIFT_TASK_DEBUG_LOG("Task %p trying to switch executors: executor %p%s to "
-                       "new serial executor: %p%s; task executor: from %p%s to %p%s",
+                       "new serial executor: %p%s; task executor: from %p%s to %p%s"
+                       "%s",
                        task,
                        currentExecutor.getIdentity(),
                        currentExecutor.getIdentityDebugName(),
@@ -2285,13 +2294,15 @@ static void swift_task_switchImpl(SWIFT_ASYNC_CONTEXT AsyncContext *resumeContex
                        currentTaskExecutor.getIdentity(),
                        currentTaskExecutor.isDefined() ? "" : " (undefined)",
                        newTaskExecutor.getIdentity(),
-                       newTaskExecutor.isDefined() ? "" : " (undefined)");
+                       newTaskExecutor.isDefined() ? "" : " (undefined)",
+                       trackingInfo->isSynchronousStart() ? "[synchronous start]" : "");
 
   // If the current executor is compatible with running the new executor,
   // we can just immediately continue running with the resume function
   // we were passed in.
   if (!mustSwitchToRun(currentExecutor, newExecutor, currentTaskExecutor,
                        newTaskExecutor)) {
+    SWIFT_TASK_DEBUG_LOG("Task %p run inline", task);
     return resumeFunction(resumeContext); // 'return' forces tail call
   }
 
@@ -2303,6 +2314,7 @@ static void swift_task_switchImpl(SWIFT_ASYNC_CONTEXT AsyncContext *resumeContex
   // If the current executor can give up its thread, and the new executor
   // can take over a thread, try to do so; but don't do this if we've
   // been asked to yield the thread.
+  SWIFT_TASK_DEBUG_LOG("Task %p can give up thread?", task);
   if (currentTaskExecutor.isUndefined() &&
       canGiveUpThreadForSwitch(trackingInfo, currentExecutor) &&
       !shouldYieldThread() &&
@@ -2325,6 +2337,27 @@ static void swift_task_switchImpl(SWIFT_ASYNC_CONTEXT AsyncContext *resumeContex
   task->flagAsAndEnqueueOnExecutor(newExecutor);
 }
 
+SWIFT_CC(swift)
+static void swift_task_startSynchronouslyImpl(AsyncTask* task) {
+  swift_retain(task);
+
+  auto currentTracking = ExecutorTrackingInfo::current();
+  if (currentTracking) {
+    auto currentExecutor = currentTracking->getActiveExecutor();
+    AsyncTask * originalTask = _swift_task_clearCurrent();
+
+    swift_job_run(task, currentExecutor);
+    _swift_task_setCurrent(originalTask);
+  } else {
+    auto originalTask = ActiveTask::swap(task);
+    assert(!originalTask);
+
+    SerialExecutorRef executor = SerialExecutorRef::forSynchronousStart();
+     swift_job_run(task, executor);
+    _swift_task_setCurrent(originalTask);
+  }
+}
+
 #if !SWIFT_CONCURRENCY_ACTORS_AS_LOCKS
 namespace {
 /// Job that allows to use executor API to schedule a block of task-less

stdlib/public/Concurrency/CMakeLists.txt

@@ -183,6 +183,9 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
   ${SWIFT_RUNTIME_CONCURRENCY_EXECUTOR_SOURCES}
   ${SWIFT_RUNTIME_CONCURRENCY_SWIFT_SOURCES}
 
+  GYB_SOURCES
+    Task+startSynchronously.swift.gyb
+
   SWIFT_MODULE_DEPENDS_ANDROID Android
   SWIFT_MODULE_DEPENDS_LINUX Glibc
   SWIFT_MODULE_DEPENDS_LINUX_STATIC Musl

stdlib/public/Concurrency/DiscardingTaskGroup.swift

@@ -205,13 +205,15 @@ public struct DiscardingTaskGroup {
     let flags = taskCreateFlags(
       priority: priority, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: false,
-      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 #else
     let flags = taskCreateFlags(
       priority: priority, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
-      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 #endif
 
@@ -252,13 +254,15 @@ public struct DiscardingTaskGroup {
     let flags = taskCreateFlags(
       priority: priority, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: false,
-      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 #else
     let flags = taskCreateFlags(
       priority: priority, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
-      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 #endif
 
@@ -281,7 +285,8 @@ public struct DiscardingTaskGroup {
     let flags = taskCreateFlags(
       priority: nil, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
-      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 
     // Create the task in this group.
@@ -317,7 +322,8 @@ public struct DiscardingTaskGroup {
     let flags = taskCreateFlags(
       priority: nil, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
-      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 
     // Create the task in this group.
@@ -635,7 +641,8 @@ public struct ThrowingDiscardingTaskGroup<Failure: Error> {
     let flags = taskCreateFlags(
       priority: priority, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
-      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: true, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 
     // Create the task in this group.
@@ -666,7 +673,8 @@ public struct ThrowingDiscardingTaskGroup<Failure: Error> {
     let flags = taskCreateFlags(
       priority: priority, isChildTask: true, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
-      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true
+      addPendingGroupTaskUnconditionally: false, isDiscardingTask: true,
+      isSynchronousStart: false
     )
 
     // Create the task in this group.

stdlib/public/Concurrency/Executor.swift

@@ -542,7 +542,6 @@ internal final class DispatchQueueShim: @unchecked Sendable, SerialExecutor {
 }
 #endif // SWIFT_CONCURRENCY_USES_DISPATCH
 
-
 @available(SwiftStdlib 6.1, *)
 @_silgen_name("swift_task_deinitOnExecutor")
 @usableFromInline

stdlib/public/Concurrency/Task+TaskExecutor.swift

@@ -240,7 +240,7 @@ extension Task where Failure == Never {
       priority: priority, isChildTask: false, copyTaskLocals: true,
       inheritContext: true, enqueueJob: true,
       addPendingGroupTaskUnconditionally: false,
-      isDiscardingTask: false)
+      isDiscardingTask: false, isSynchronousStart: false)
 
 #if $BuiltinCreateAsyncTaskOwnedTaskExecutor
     let (task, _) = Builtin.createTask(
@@ -303,7 +303,7 @@ extension Task where Failure == Error {
       priority: priority, isChildTask: false, copyTaskLocals: true,
       inheritContext: true, enqueueJob: true,
       addPendingGroupTaskUnconditionally: false,
-      isDiscardingTask: false)
+      isDiscardingTask: false, isSynchronousStart: false)
 
 #if $BuiltinCreateAsyncTaskOwnedTaskExecutor
     let (task, _) = Builtin.createTask(
@@ -364,7 +364,7 @@ extension Task where Failure == Never {
       priority: priority, isChildTask: false, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
       addPendingGroupTaskUnconditionally: false,
-      isDiscardingTask: false)
+      isDiscardingTask: false, isSynchronousStart: false)
 
 #if $BuiltinCreateAsyncTaskOwnedTaskExecutor
     let (task, _) = Builtin.createTask(
@@ -425,7 +425,7 @@ extension Task where Failure == Error {
       priority: priority, isChildTask: false, copyTaskLocals: false,
       inheritContext: false, enqueueJob: true,
       addPendingGroupTaskUnconditionally: false,
-      isDiscardingTask: false)
+      isDiscardingTask: false, isSynchronousStart: false)
 
 #if $BuiltinCreateAsyncTaskOwnedTaskExecutor
     let (task, _) = Builtin.createTask(