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 and adds the prefix 'm' for text flags.
nodejs · joaocgreis · Aug 21, 2019 · Aug 22, 2019 · Aug 22, 2019 · Aug 22, 2019
commit 2002dd16d89c3a60be24c213b2439955e10fadf5
diff --git a/doc/api/fs.md b/doc/api/fs.md
@@ -4949,6 +4949,11 @@ 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.</td>
+  </tr>
 </table>
 
 ### File Type Constants
@@ -5141,6 +5146,10 @@ On Windows, opening an existing hidden file using the `'w'` flag (either
 through `fs.open()` or `fs.writeFile()` or `fsPromises.open()`) will fail with
 `EPERM`. Existing hidden files can be opened for writing with the `'r+'` flag.
 
+All of the flags can be prepended with `'m'` to use a memory file mapping for
+accessing the file (for example `'max+'`). This is only available on Windows,
+on other operative systems `'m'` is ignored.
+
 A call to `fs.ftruncate()` or `filehandle.truncate()` can be used to reset
 the file contents.
 

diff --git a/lib/internal/fs/utils.js b/lib/internal/fs/utils.js
@@ -35,6 +35,7 @@ const {
   O_SYNC,
   O_TRUNC,
   O_WRONLY,
+  UV_FS_O_FILEMAP,
   S_IFBLK,
   S_IFCHR,
   S_IFDIR,
@@ -417,6 +418,10 @@ function stringToFlags(flags) {
     case 'sa+': return O_APPEND | O_CREAT | O_RDWR | O_SYNC;
   }
 
+  if (typeof flags === 'string' && flags.charAt(0) === 'm') {
+    return UV_FS_O_FILEMAP | stringToFlags(flags.substring(1));
+  }
+
   throw new ERR_INVALID_OPT_VALUE('flags', flags);
 }
 

diff --git a/src/node_constants.cc b/src/node_constants.cc
@@ -1127,6 +1127,10 @@ void DefineSystemConstants(Local<Object> target) {
   NODE_DEFINE_CONSTANT(target, O_EXCL);
 #endif
 
+#ifdef UV_FS_O_FILEMAP
+  NODE_DEFINE_CONSTANT(target, UV_FS_O_FILEMAP);
+#endif
+
 #ifdef O_NOCTTY
   NODE_DEFINE_CONSTANT(target, O_NOCTTY);
 #endif

diff --git a/test/parallel/test-fs-fmap.js b/test/parallel/test-fs-fmap.js
@@ -0,0 +1,30 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const join = require('path').join;
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+// Run this test on all platforms. While the 'm' flag 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';
+
+fs.writeFileSync(filename, text, { flag: 'mw' });
+const r1 = fs.readFileSync(filename, { encoding: 'utf8', flag: 'mr' });
+assert.strictEqual(r1, text);
+
+fs.writeFileSync(filename, text, { flag: 'mr+' });
+const r2 = fs.readFileSync(filename, { encoding: 'utf8', flag: 'ma+' });
+assert.strictEqual(r2, text);
+
+fs.writeFileSync(filename, text, { flag: 'ma' });
+const numericFlag = fs.constants.UV_FS_O_FILEMAP | fs.constants.O_RDONLY;
+const r3 = fs.readFileSync(filename, { encoding: 'utf8', flag: numericFlag });
+assert.strictEqual(r3, text + text);
+
+const r4 = fs.readFileSync(filename, { encoding: 'utf8', flag: 'mw+' });
+assert.strictEqual(r4, '');
diff --git a/test/parallel/test-fs-open-flags.js b/test/parallel/test-fs-open-flags.js
@@ -38,7 +38,8 @@ const { O_APPEND = 0,
         O_SYNC = 0,
         O_DSYNC = 0,
         O_TRUNC = 0,
-        O_WRONLY = 0
+        O_WRONLY = 0,
+        UV_FS_O_FILEMAP = 0
 } = fs.constants;
 
 const { stringToFlags } = require('internal/fs/utils');
@@ -65,6 +66,12 @@ assert.strictEqual(stringToFlags('xa+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
 assert.strictEqual(stringToFlags('as+'), O_APPEND | O_CREAT | O_RDWR | O_SYNC);
 assert.strictEqual(stringToFlags('sa+'), O_APPEND | O_CREAT | O_RDWR | O_SYNC);
 
+assert.strictEqual(stringToFlags('mr'), UV_FS_O_FILEMAP | O_RDONLY);
+assert.strictEqual(stringToFlags('mw'),
+                   UV_FS_O_FILEMAP | O_TRUNC | O_CREAT | O_WRONLY);
+assert.strictEqual(stringToFlags('max+'),
+                   UV_FS_O_FILEMAP | O_APPEND | O_CREAT | O_RDWR | O_EXCL);
+
 ('+ +a +r +w rw wa war raw r++ a++ w++ x +x x+ rx rx+ wxx wax xwx xxx')
   .split(' ')
   .forEach(function(flags) {