src: define fs.constants.S_IWUSR & S_IRUSR for Win

On Windows, most of the POSIX file mode definitions are not available.
However, functionally equivalent read/write definitions exists, and
chmod() can use them. This patch defines two aliases, so that these
definintions are issued in fs.constants.

fixes: nodejs#41591

PR-URL: nodejs#42757
Refs: nodejs#41591
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -6771,7 +6771,11 @@ operations.
 
 The following constants are exported by `fs.constants`.
 
-Not every constant will be available on every operating system.
+Not every constant will be available on every operating system;
+this is especially important for Windows, where many of the POSIX specific
+definitions are not available.
+For portable applications it is recommended to check for their presence
+before use.
 
 To use more than one constant, use the bitwise OR `|` operator.
 
@@ -6824,6 +6828,8 @@ The following constants are meant for use as the `mode` parameter passed to
   </tr>
 </table>
 
+The definitions are also available on Windows.
+
 ##### File copy constants
 
 The following constants are meant for use with [`fs.copyFile()`][].
@@ -6852,6 +6858,8 @@ The following constants are meant for use with [`fs.copyFile()`][].
   </tr>
 </table>
 
+The definitions are also available on Windows.
+
 ##### File open constants
 
 The following constants are meant for use with `fs.open()`.
@@ -6946,6 +6954,9 @@ The following constants are meant for use with `fs.open()`.
   </tr>
 </table>
 
+On Windows, only `O_APPEND`, `O_CREAT`, `O_EXCL`, `O_RDONLY`, `O_RDWR`,
+`O_TRUNC`, `O_WRONLY` and `UV_FS_O_FILEMAP` are available.
+
 ##### File type constants
 
 The following constants are meant for use with the {fs.Stats} object's
@@ -6990,6 +7001,9 @@ The following constants are meant for use with the {fs.Stats} object's
   </tr>
 </table>
 
+On Windows, only `S_IFCHR`, `S_IFDIR`, `S_IFLNK`, `S_IFMT`, and `S_IFREG`,
+are available.
+
 ##### File mode constants
 
 The following constants are meant for use with the {fs.Stats} object's
@@ -7050,6 +7064,8 @@ The following constants are meant for use with the {fs.Stats} object's
   </tr>
 </table>
 
+On Windows, only `S_IRUSR` and `S_IWUSR` are available.
+
 ## Notes
 
 ### Ordering of callback and promise-based operations

src/node_constants.cc

@@ -47,6 +47,16 @@
 #include <dlfcn.h>
 #endif
 
+#if defined(_WIN32)
+#include <io.h>  // _S_IREAD _S_IWRITE
+#ifndef S_IRUSR
+#define S_IRUSR _S_IREAD
+#endif  // S_IRUSR
+#ifndef S_IWUSR
+#define S_IWUSR _S_IWRITE
+#endif  // S_IWUSR
+#endif
+
 #include <cerrno>
 #include <csignal>
 #include <limits>

test/parallel/test-fs-constants.js

@@ -0,0 +1,8 @@
+'use strict';
+require('../common');
+const fs = require('fs');
+const assert = require('assert');
+
+// Check if the two constants accepted by chmod() on Windows are defined.
+assert.notStrictEqual(fs.constants.S_IRUSR, undefined);
+assert.notStrictEqual(fs.constants.S_IWUSR, undefined);