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/82535Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/os_test.go

@@ -1362,14 +1362,26 @@ func TestSeek(t *testing.T) {
 		{-1, io.SeekEnd, int64(len(data)) - 1},
 		{1 << 33, io.SeekStart, 1 << 33},
 		{1 << 33, io.SeekEnd, 1<<33 + int64(len(data))},
+
+		// Issue 21681, Windows 4G-1, etc:
+		{1<<32 - 1, io.SeekStart, 1<<32 - 1},
+		{0, io.SeekCurrent, 1<<32 - 1},
+		{2<<32 - 1, io.SeekStart, 2<<32 - 1},
+		{0, io.SeekCurrent, 2<<32 - 1},
 	}
 	for i, tt := range tests {
+		if runtime.GOOS == "nacl" && tt.out > 1<<30 {
+			t.Logf("skipping test case #%d on nacl; https://golang.org/issue/21728", i)
+			continue
+		}
 		off, err := f.Seek(tt.in, tt.whence)
 		if off != tt.out || err != nil {
-			if e, ok := err.(*PathError); ok && e.Err == syscall.EINVAL && tt.out > 1<<32 {
-				// Reiserfs rejects the big seeks.
-				// https://golang.org/issue/91
-				break
+			if e, ok := err.(*PathError); ok && e.Err == syscall.EINVAL && tt.out > 1<<32 && runtime.GOOS == "linux" {
+				mounts, _ := ioutil.ReadFile("/proc/mounts")
+				if strings.Contains(string(mounts), "reiserfs") {
+					// Reiserfs rejects the big seeks.
+					t.Skipf("skipping test known to fail on reiserfs; https://golang.org/issue/91")
+				}
 			}
 			t.Errorf("#%d: Seek(%v, %v) = %v, %v want %v, nil", i, tt.in, tt.whence, off, err, tt.out)
 		}

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) {