windows: add GetOverlappedResult function for async I/O

We already have all the structs and constants for async I/O, and the
various functions of x/sys/windows take the right parameters. But we're
missing the final step of any ordinary async I/O routine: getting the
result of overlapped I/O. Without this, the rest of the plumbing
supported by this module isn't actually so useful. So add this small
oversight.

Change-Id: I0ce1a71bce06bc81a83f3b0ca10ad9c4b67af726
Reviewed-on: https://go-review.googlesource.com/c/sys/+/168521
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

windows/syscall_windows.go

@@ -137,6 +137,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	CreateFile(name *uint16, access uint32, mode uint32, sa *SecurityAttributes, createmode uint32, attrs uint32, templatefile int32) (handle Handle, err error) [failretval==InvalidHandle] = CreateFileW
 //sys	ReadFile(handle Handle, buf []byte, done *uint32, overlapped *Overlapped) (err error)
 //sys	WriteFile(handle Handle, buf []byte, done *uint32, overlapped *Overlapped) (err error)
+//sys	GetOverlappedResult(handle Handle, overlapped *Overlapped, done *uint32, wait bool) (err error)
 //sys	SetFilePointer(handle Handle, lowoffset int32, highoffsetptr *int32, whence uint32) (newlowoffset uint32, err error) [failretval==0xffffffff]
 //sys	CloseHandle(handle Handle) (err error)
 //sys	GetStdHandle(stdhandle uint32) (handle Handle, err error) [failretval==InvalidHandle]

windows/zsyscall_windows.go

@@ -77,6 +77,7 @@ var (
 	procCreateFileW                        = modkernel32.NewProc("CreateFileW")
 	procReadFile                           = modkernel32.NewProc("ReadFile")
 	procWriteFile                          = modkernel32.NewProc("WriteFile")
+	procGetOverlappedResult                = modkernel32.NewProc("GetOverlappedResult")
 	procSetFilePointer                     = modkernel32.NewProc("SetFilePointer")
 	procCloseHandle                        = modkernel32.NewProc("CloseHandle")
 	procGetStdHandle                       = modkernel32.NewProc("GetStdHandle")
@@ -654,6 +655,24 @@ func WriteFile(handle Handle, buf []byte, done *uint32, overlapped *Overlapped)
 	return
 }
 
+func GetOverlappedResult(handle Handle, overlapped *Overlapped, done *uint32, wait bool) (err error) {
+	var _p0 uint32
+	if wait {
+		_p0 = 1
+	} else {
+		_p0 = 0
+	}
+	r1, _, e1 := syscall.Syscall6(procGetOverlappedResult.Addr(), 4, uintptr(handle), uintptr(unsafe.Pointer(overlapped)), uintptr(unsafe.Pointer(done)), uintptr(_p0), 0, 0)
+	if r1 == 0 {
+		if e1 != 0 {
+			err = errnoErr(e1)
+		} else {
+			err = syscall.EINVAL
+		}
+	}
+	return
+}
+
 func SetFilePointer(handle Handle, lowoffset int32, highoffsetptr *int32, whence uint32) (newlowoffset uint32, err error) {
 	r0, _, e1 := syscall.Syscall6(procSetFilePointer.Addr(), 4, uintptr(handle), uintptr(lowoffset), uintptr(unsafe.Pointer(highoffsetptr)), uintptr(whence), 0, 0)
 	newlowoffset = uint32(r0)