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, 2 insertions, 2 deletions

diff --git a/lib/fs.js b/lib/fs.js
index 335b164483..873e93d9cd 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -1229,7 +1229,7 @@ function writeFile(path, data, options, callback) {
   function writeFd(fd, isUserFd) {
     const buffer = isArrayBufferView(data) ?
       data : Buffer.from('' + data, options.encoding || 'utf8');
-    const position = /a/.test(flag) ? null : 0;
+    const position = (/a/.test(flag) || isUserFd) ? null : 0;
 
     writeAll(fd, isUserFd, buffer, 0, buffer.byteLength, position, callback);
   }
@@ -1247,7 +1247,7 @@ function writeFileSync(path, data, options) {
   }
   let offset = 0;
   let length = data.byteLength;
-  let position = /a/.test(flag) ? null : 0;
+  let position = (/a/.test(flag) || isUserFd) ? null : 0;
   try {
     while (length > 0) {
       const written = fs.writeSync(fd, data, offset, length, position);