src,lib: expose memory file mapping flag

Support for UV_FS_O_FILEMAP was added in libuv in version 1.31.0.
This exposes the flag in fs.constants.

Refs: libuv/libuv#2295
PR-URL: #29260
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -5031,6 +5031,12 @@ The following constants are meant for use with `fs.open()`.
     <td><code>O_NONBLOCK</code></td>
     <td>Flag indicating to open the file in nonblocking mode when possible.</td>
   </tr>
+  <tr>
+    <td><code>UV_FS_O_FILEMAP</code></td>
+    <td>When set, a memory file mapping is used to access the file. This flag
+    is available on Windows operating systems only. On other operating systems,
+    this flag is ignored.</td>
+  </tr>
 </table>
 
 ### File Type Constants

src/node_constants.cc

@@ -1127,6 +1127,8 @@ void DefineSystemConstants(Local<Object> target) {
   NODE_DEFINE_CONSTANT(target, O_EXCL);
 #endif
 
+NODE_DEFINE_CONSTANT(target, UV_FS_O_FILEMAP);
+
 #ifdef O_NOCTTY
   NODE_DEFINE_CONSTANT(target, O_NOCTTY);
 #endif

test/parallel/test-fs-fmap.js

@@ -0,0 +1,29 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const join = require('path').join;
+
+const {
+  O_CREAT = 0,
+  O_RDONLY = 0,
+  O_TRUNC = 0,
+  O_WRONLY = 0,
+  UV_FS_O_FILEMAP = 0
+} = fs.constants;
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+// Run this test on all platforms. While UV_FS_O_FILEMAP is only available on
+// Windows, it should be silently ignored on other platforms.
+
+const filename = join(tmpdir.path, 'fmap.txt');
+const text = 'Memory File Mapping Test';
+
+const mw = UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY;
+const mr = UV_FS_O_FILEMAP | O_RDONLY;
+
+fs.writeFileSync(filename, text, { flag: mw });
+const r1 = fs.readFileSync(filename, { encoding: 'utf8', flag: mr });
+assert.strictEqual(r1, text);