Shared Memory and Named Pipes through GHC's native I/O manager (WinIO): each operation on a pipe is an overlapped ReadFile, WriteFile or ConnectNamedPipe made with GHC.Event.Windows.withOverlapped, on a handle associated with the manager's completion port. It is used when nativeIO holds, with or without -threaded, and then TCP goes through network's own calls; otherwise the pipes go through Hmssql.Connection.Backend.Windows, whose foreign calls and errors this module shares. On other platforms this module is empty.

The manager does what that module does by hand. An asynchronous exception interrupts a pending operation, which the manager then cancels with CancelIoEx and waits for, uninterruptibly, so the buffer is never freed while the kernel may still write to it. A completion is processed by the manager's thread, or without -threaded by the scheduler, so no operation needs an event, a helper thread or a worker of its own, and a pipe opened without -threaded needs no asyncDoProc.

Closing is as on the other path: a flag that later operations check, then CancelIoEx on the whole handle, then a wait for the operations in progress. An operation that becomes pending checks the flag once more, in its start callback, and cancels itself if the flag is set by then, so a close that comes between an operation's first check and its start is not lost. pioStopReceiving only sets the flag: the manager allocates each operation's OVERLAPPED, so there is no address to cancel the receive alone by, and hmssql kills the reader thread right after it, which cancels the receive inside the manager.