src: define O_SYNC, O_DSYNC and O_DIRECT on Windows

doc/api/fs.md

@@ -8706,12 +8706,14 @@ The following constants are meant for use with `fs.open()`.
   <tr>
     <td><code>O_SYNC</code></td>
     <td>Flag indicating that the file is opened for synchronized I/O with write
-    operations waiting for file integrity.</td>
+    operations waiting for file integrity. On Windows, this maps to
+    <code>FILE_FLAG_WRITE_THROUGH</code>.</td>
   </tr>
   <tr>
     <td><code>O_DSYNC</code></td>
     <td>Flag indicating that the file is opened for synchronized I/O with write
-    operations waiting for data integrity.</td>
+    operations waiting for data integrity. On Windows, this maps to
+    <code>FILE_FLAG_WRITE_THROUGH</code>.</td>
   </tr>
   <tr>
     <td><code>O_SYMLINK</code></td>
@@ -8721,7 +8723,7 @@ The following constants are meant for use with `fs.open()`.
   <tr>
     <td><code>O_DIRECT</code></td>
     <td>When set, an attempt will be made to minimize caching effects of file
-    I/O.</td>
+    I/O. On Windows, this maps to <code>FILE_FLAG_NO_BUFFERING</code>.</td>
   </tr>
   <tr>
     <td><code>O_NONBLOCK</code></td>

src/node_constants.cc

@@ -1165,10 +1165,22 @@ NODE_DEFINE_CONSTANT(target, UV_FS_O_FILEMAP);
 
 #ifdef O_SYNC
   NODE_DEFINE_CONSTANT(target, O_SYNC);
+#elif UV_FS_O_SYNC != 0
+// Windows lacks the POSIX O_SYNC macro, but libuv maps UV_FS_O_SYNC to
+// FILE_FLAG_WRITE_THROUGH, so expose it under the portable name.
+#define O_SYNC UV_FS_O_SYNC
+  NODE_DEFINE_CONSTANT(target, O_SYNC);
+#undef O_SYNC
 #endif
 
 #ifdef O_DSYNC
   NODE_DEFINE_CONSTANT(target, O_DSYNC);
+#elif UV_FS_O_DSYNC != 0
+// Windows lacks the POSIX O_DSYNC macro, but libuv maps UV_FS_O_DSYNC to
+// FILE_FLAG_WRITE_THROUGH, so expose it under the portable name.
+#define O_DSYNC UV_FS_O_DSYNC
+  NODE_DEFINE_CONSTANT(target, O_DSYNC);
+#undef O_DSYNC
 #endif
 
 
@@ -1178,6 +1190,12 @@ NODE_DEFINE_CONSTANT(target, UV_FS_O_FILEMAP);
 
 #ifdef O_DIRECT
   NODE_DEFINE_CONSTANT(target, O_DIRECT);
+#elif UV_FS_O_DIRECT != 0
+// Windows lacks the POSIX O_DIRECT macro, but libuv maps UV_FS_O_DIRECT to
+// FILE_FLAG_NO_BUFFERING, so expose it under the portable name.
+#define O_DIRECT UV_FS_O_DIRECT
+  NODE_DEFINE_CONSTANT(target, O_DIRECT);
+#undef O_DIRECT
 #endif
 
 #ifdef O_NONBLOCK

test/parallel/test-fs-constants.js

@@ -1,12 +1,21 @@
 'use strict';
-require('../common');
+const common = 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);
 
+// O_SYNC, O_DSYNC and O_DIRECT have no POSIX macro on Windows, but libuv
+// honors them via FILE_FLAG_WRITE_THROUGH / FILE_FLAG_NO_BUFFERING, so they
+// are exposed under their portable names with libuv's flag values.
+if (common.isWindows) {
+  assert.strictEqual(fs.constants.O_SYNC, 0x08000000);
+  assert.strictEqual(fs.constants.O_DSYNC, 0x04000000);
+  assert.strictEqual(fs.constants.O_DIRECT, 0x02000000);
+}
+
 // Check null prototype.
 assert.strictEqual(Object.getPrototypeOf(fs.constants), null);