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fs: fix partial write corruption in writeFile and writeFileSync by olov · Pull Request #1063 · nodejs/node · GitHub

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fs: fix partial write corruption in writeFile and writeFileSync #1063

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fs: fix partial write corruption in writeFile and writeFileSync #1063

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16 changes: 11 additions & 5 deletions lib/fs.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1097,7 +1097,9 @@ function writeAll(fd, buffer, offset, length, position, callback) {
} else {
offset += written;
length -= written;
position += written;
if (position !== null) {

piscisaureus Mar 5, 2015

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Why the null check here?

olov Mar 5, 2015

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

This is to fix 2. position may be null (see fs.writeFile caller) and without the check position will become a number instead, pointing to the middle of an existing file => corrupting data.

piscisaureus Mar 5, 2015

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Ah, got it, makes sense.

bnoordhuis Mar 6, 2015

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I think the check needs to be if (typeof position === 'number') {, else undefined or false will still trigger the bad behavior. It's also the condition that the documentation for fs.write() mentions.

olov Mar 7, 2015

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I don't see how position can become undefined or false in any of the two code paths.

olov Mar 7, 2015

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To elaborate, position is internal to writeFileSync and writeFile. In the latter case via an internal writeAll function that's only called from writeFile.
position += written;
}
writeAll(fd, buffer, offset, length, position, callback);
}
}
Expand Down Expand Up @@ -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);
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