fs: make writeFile consistent with readFile wrt fd

As it is, `readFile` always reads from the current position of the file,
if a file descriptor is used. But `writeFile` always writes from the
beginning of the file.

This patch fixes this inconsistency by making `writeFile` also to write
from the current position of the file when used with a file descriptor.

PR-URL: https://github.com/nodejs/node/pull/23709
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

1 files changed, 35 insertions, 0 deletions

diff --git a/test/parallel/test-fs-promises-readfile-with-fd.js b/test/parallel/test-fs-promises-readfile-with-fd.js
new file mode 100644
index 0000000000..9bf4c18536
--- /dev/null
+++ b/test/parallel/test-fs-promises-readfile-with-fd.js
@@ -0,0 +1,35 @@
+'use strict';
+
+/*
+ * This test makes sure that `readFile()` always reads from the current
+ * position of the file, instead of reading from the beginning of the file.
+ */
+
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const { writeFileSync } = require('fs');
+const { open } = require('fs').promises;
+
+const tmpdir = require('../common/tmpdir');
+tmpdir.refresh();
+
+const fn = path.join(tmpdir.path, 'test.txt');
+writeFileSync(fn, 'Hello World');
+
+async function readFileTest() {
+  const handle = await open(fn, 'r');
+
+  /* Read only five bytes, so that the position moves to five. */
+  const buf = Buffer.alloc(5);
+  const { bytesRead } = await handle.read(buf, 0, 5, null);
+  assert.strictEqual(bytesRead, 5);
+  assert.deepStrictEqual(buf.toString(), 'Hello');
+
+  /* readFile() should read from position five, instead of zero. */
+  assert.deepStrictEqual((await handle.readFile()).toString(), ' World');
+}
+
+
+readFileTest()
+  .then(common.mustCall());