uploaded files are not flushed to disk

[uhthomas]

It looks like multer doesn't call sync or datasync when files are closed, which can lead to data corruption.

multer/storage/disk.js

Line 37 in 368c8a1

var outStream = fs.createWriteStream(finalPath)

NodeJS does support a flush option which will ensure data is flushed to disk before closing the file. It would be great if multer could add an option to set this.

https://nodejs.org/api/fs.html#filehandlecreatewritestreamoptions

Environment information

Version:

v2.1.1

Platform:

Linux

Node.js version:

v25.8.1

Any other relevant information:

What steps will reproduce the bug?

Upload a file, and observe that in certain conditions the handler may say that the upload has completed, but in reality the data has not been written to disk and so the file is either corrupt or lost.

[mahmoodhamdi]

Good catch — this is a real issue for reliability. Without fsync/fdatasync, data can sit in the OS page cache and be lost on power failure or crash.

Node.js v22+ added the flush: true option to createWriteStream, which calls fdatasync before close. For multer's disk storage, the fix would be straightforward in storage/disk.js:

var outStream = fs.createWriteStream(finalPath, { flush: true })

For older Node versions, the alternative is to manually call fs.fdatasync(fd, cb) before closing:

outStream.on('finish', function() {
  fs.fdatasync(outStream.fd, function(err) {
    // then close
  })
})

An opt-in flag like flushToDisk: true in the disk storage options would make sense — flushing adds latency, so it shouldn't be the default for users who don't need durability guarantees (e.g., temp file processing).

[uhthomas]

Thanks ChatGPT.