syscall: make Seek use SetFilePointerEx on Windows, allowing large seek offsets

Fixes #21681
Updates #21728

Change-Id: I79cf4564c1355ecab891102d4215cbbffd8eb0ce
Reviewed-on: https://go-review.googlesource.com/82535
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 23 insertions, 7 deletions

diff --git a/src/syscall/syscall_windows.go b/src/syscall/syscall_windows.go
index 21d5ecfcb3..9026fcdacf 100644
--- a/src/syscall/syscall_windows.go
+++ b/src/syscall/syscall_windows.go
@@ -332,6 +332,27 @@ func Write(fd Handle, p []byte) (n int, err error) {
 
 var ioSync int64
 
+var procSetFilePointerEx = modkernel32.NewProc("SetFilePointerEx")
+
+const ptrSize = unsafe.Sizeof(uintptr(0))
+
+// setFilePointerEx calls SetFilePointerEx.
+// See https://msdn.microsoft.com/en-us/library/windows/desktop/aa365542(v=vs.85).aspx
+func setFilePointerEx(handle Handle, distToMove int64, newFilePointer *int64, whence uint32) error {
+	var e1 Errno
+	if ptrSize == 8 {
+		_, _, e1 = Syscall6(procSetFilePointerEx.Addr(), 4, uintptr(handle), uintptr(distToMove), uintptr(unsafe.Pointer(newFilePointer)), uintptr(whence), 0, 0)
+	} else {
+		// distToMove is a LARGE_INTEGER:
+		// https://msdn.microsoft.com/en-us/library/windows/desktop/aa383713(v=vs.85).aspx
+		_, _, e1 = Syscall6(procSetFilePointerEx.Addr(), 5, uintptr(handle), uintptr(distToMove), uintptr(distToMove>>32), uintptr(unsafe.Pointer(newFilePointer)), uintptr(whence), 0)
+	}
+	if e1 != 0 {
+		return errnoErr(e1)
+	}
+	return nil
+}
+
 func Seek(fd Handle, offset int64, whence int) (newoffset int64, err error) {
 	var w uint32
 	switch whence {
@@ -342,18 +363,13 @@ func Seek(fd Handle, offset int64, whence int) (newoffset int64, err error) {
 	case 2:
 		w = FILE_END
 	}
-	hi := int32(offset >> 32)
-	lo := int32(offset)
 	// use GetFileType to check pipe, pipe can't do seek
 	ft, _ := GetFileType(fd)
 	if ft == FILE_TYPE_PIPE {
 		return 0, ESPIPE
 	}
-	rlo, e := SetFilePointer(fd, lo, &hi, w)
-	if e != nil {
-		return 0, e
-	}
-	return int64(hi)<<32 + int64(rlo), nil
+	err = setFilePointerEx(fd, offset, &newoffset, w)
+	return
 }
 
 func Close(fd Handle) (err error) {