[Concurrency] More cleanups for futures

Author: DougGregor

include/swift/ABI/Task.h

@@ -326,15 +326,16 @@ class AsyncTask : public HeapObject, public Job {
     /// fragment.
     static size_t storageOffset(const Metadata *resultType)  {
       size_t offset = sizeof(FutureFragment);
-      size_t alignment = std::max(resultType->vw_alignment(), alignof(void *));
+      size_t alignment =
+          std::max(resultType->vw_alignment(), alignof(SwiftError *));
       return (offset + alignment - 1) & ~(alignment - 1);
     }
 
     /// Determine the size of the future fragment given a particular future
     /// result type.
     static size_t fragmentSize(const Metadata *resultType) {
       return storageOffset(resultType) +
-          std::max(resultType->vw_size(), sizeof(void *));
+          std::max(resultType->vw_size(), sizeof(SwiftError *));
     }
   };
 
@@ -463,7 +464,8 @@ class YieldingAsyncContext : public AsyncContext {
 /// task.
 ///
 /// This type matches the ABI of a function `<T> () async throws -> T`, which
-/// is used to describe futures.
+/// is the type used by `Task.runDetached` and `Task.group.add` to create
+/// futures.
 class FutureAsyncContext : public AsyncContext {
 public:
   SwiftError *errorResult = nullptr;

stdlib/public/Concurrency/Task.cpp

@@ -103,9 +103,6 @@ void AsyncTask::completeFuture(AsyncContext *context, ExecutorRef executor) {
     // Find the next waiting task.
     auto nextWaitingTask = waitingTask->getNextWaitingTask();
 
-    // Remove this task from the list.
-    waitingTask->getNextWaitingTask() = nullptr;
-
     // TODO: schedule this task on the executor rather than running it
     // directly.
     waitingTask->run(executor);
@@ -198,7 +195,8 @@ AsyncTaskAndContext swift::swift_task_create_future_f(
     JobFlags flags, AsyncTask *parent, const Metadata *futureResultType,
     AsyncFunctionType<void()> *function, size_t initialContextSize) {
   assert((futureResultType != nullptr) == flags.task_isFuture());
-  assert((futureResultType != nullptr) == flags.task_isFuture());
+  assert(!flags.task_isFuture() ||
+         initialContextSize >= sizeof(FutureAsyncContext));
   assert((parent != nullptr) == flags.task_isChildTask());
 
   // Figure out the size of the header.