From 9fc598b7c28e80a18e1eff106c2f6bfd0d4f3e20 Mon Sep 17 00:00:00 2001
From: Olov Lassus <olov.lassus@gmail.com>
Date: Thu, 5 Mar 2015 01:03:34 +0100
Subject: [PATCH] fs: fix partial write corruption in writeFile and
 writeFileSync

1. writeFileSync bumps position incorrectly, causing it to
drift in iteration three and onwards.

2. Append mode files will get corrupted in the middle if
writeFile or writeFileSync iterates multiple times, unless
running on Linux. position starts out as null so first write is
OK, but then position will refer to a location inside an
existing file, corrupting that data. Linux ignores position for
append mode files so it doesn't happen there.

This commit fixes these two related issues by bumping position
correctly and by always using null as the position argument
to write/writeSync for append mode files.
---
 lib/fs.js | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/lib/fs.js b/lib/fs.js
index b2aba37f561be8..0b534c14fe0f47 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -1097,7 +1097,9 @@ function writeAll(fd, buffer, offset, length, position, callback) {
       } else {
         offset += written;
         length -= written;
-        position += written;
+        if (position !== null) {
+          position += written;
+        }
         writeAll(fd, buffer, offset, length, position, callback);
       }
     }
@@ -1146,13 +1148,17 @@ fs.writeFileSync = function(path, data, options) {
   if (!(data instanceof Buffer)) {
     data = new Buffer('' + data, options.encoding || 'utf8');
   }
-  var written = 0;
+  var offset = 0;
   var length = data.length;
   var position = /a/.test(flag) ? null : 0;
   try {
-    while (written < length) {
-      written += fs.writeSync(fd, data, written, length - written, position);
-      position += written;
+    while (length > 0) {
+      var written = fs.writeSync(fd, data, offset, length, position);
+      offset += written;
+      length -= written;
+      if (position !== null) {
+        position += written;
+      }
     }
   } finally {
     fs.closeSync(fd);