fs: support util.promisify for fs.readv

PR-URL: #33590
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/fs.md

@@ -3209,6 +3209,9 @@ from the current position.
 The callback will be given three arguments: `err`, `bytesRead`, and
 `buffers`. `bytesRead` is how many bytes were read from the file.
 
+If this method is invoked as its [`util.promisify()`][]ed version, it returns
+a `Promise` for an `Object` with `bytesRead` and `buffers` properties.
+
 ## `fs.readvSync(fd, buffers[, position])`
 <!-- YAML
 added:

lib/fs.js

@@ -597,6 +597,9 @@ function readv(fd, buffers, position, callback) {
   return binding.readBuffers(fd, buffers, position, req);
 }
 
+ObjectDefineProperty(readv, internalUtil.customPromisifyArgs,
+                     { value: ['bytesRead', 'buffers'], enumerable: false });
+
 function readvSync(fd, buffers, position) {
   validateInt32(fd, 'fd', 0);
   validateBufferArray(buffers);

test/parallel/test-fs-readv-promisify.js

@@ -0,0 +1,18 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const fs = require('fs');
+const readv = require('util').promisify(fs.readv);
+const assert = require('assert');
+const filepath = fixtures.path('x.txt');
+const fd = fs.openSync(filepath, 'r');
+
+const expected = [Buffer.from('xyz\n')];
+
+readv(fd, expected)
+  .then(function({ bytesRead, buffers }) {
+    assert.deepStrictEqual(bytesRead, expected[0].length);
+    assert.deepStrictEqual(buffers, expected);
+  })
+  .then(common.mustCall());