limits.fileSize not working properly with different files

[yaneony]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.x.x

Plugin version

8.3.0

Node.js version

LTS

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

Debian 12

Description

I have very different behavion on different files. Currently only tested with 2 image files, one is JPG ~2.3 Mb, another one is upscaled PNG ~22 Mb. Using following code:

const fastify = require('fastify')();

fastify.register(require('fastify-multipart'));

fastify.post('/upload', async function (req, res) {
  let file = await req.file({ limits: { fileSize: 1024 * 1024 }, throwFileSizeLimit: true });
  if (file.file.truncated) {
    console.log('File too large');
    res.send('File too large');
    return res;
  }
  res.send('OK');
});

fastify.listen(3000, (err) => {
  if (err) throw err;
  console.log(`server listening on ${fastify.server.address().port}`);
});

Problem is, while uploading one file, file limit is working fine and when uploading another one, it isn't working at all. If i read it right, limits are in bytes, so, using (1024*1024) should be limited to 1 Mb. Here are two responses I get:

<ref *1> {
  type: 'file',
  fieldname: 'files[]',
  filename: 'def_upscayl_4x_ultrasharp.png',
  encoding: '7bit',
  mimetype: 'image/png',
  file: FileStream {
    _events: {
      close: undefined,
      error: undefined,
      data: undefined,
      end: [Function (anonymous)],
      readable: undefined,
      limit: [Function (anonymous)]
    },
    _readableState: ReadableState {
      highWaterMark: 16384,
      buffer: [Array],
      bufferIndex: 0,
      length: 64840,
      pipes: [],
      awaitDrainWriters: null,
      [Symbol(kState)]: 1052940
    },
    _maxListeners: undefined,
    bytesRead: 64840,
    truncated: false,
    _eventsCount: 2,
    _read: [Function (anonymous)],
    [Symbol(shapeMode)]: true,
    [Symbol(kCapture)]: false
  },
  fields: { 'files[]': [Circular *1] },
  _buf: null,
  toBuffer: [AsyncFunction: toBuffer]
}

<ref *1> {
  type: 'file',
  fieldname: 'files[]',
  filename: 'SSL27917.JPG',
  encoding: '7bit',
  mimetype: 'image/jpeg',
  file: FileStream {
    _events: {
      close: undefined,
      error: undefined,
      data: undefined,
      end: [Function (anonymous)],
      readable: undefined,
      limit: [Function (anonymous)]
    },
    _readableState: ReadableState {
      highWaterMark: 16384,
      buffer: [Array],
      bufferIndex: 0,
      length: 64851,
      pipes: [],
      awaitDrainWriters: null,
      [Symbol(kState)]: 1052940
    },
    _maxListeners: undefined,
    bytesRead: 64851,
    truncated: false,
    _eventsCount: 2,
    _read: [Function (anonymous)],
    [Symbol(shapeMode)]: true,
    [Symbol(kCapture)]: false
  },
  fields: { 'files[]': [Circular *1] },
  _buf: null,
  toBuffer: [AsyncFunction: toBuffer]
}

But, when you change the limit to only 1024, which would be 1Kb everything is working fine. So, what could be the problem?!
Here are files I'm uploading, so you can see the're have matching file size:

Link to code that reproduces the bug

No response

Expected Behavior

No response

[mcollina]

I'm a bit lost. You are reading only one file in your example. Can you add a complete example with scripts to reproduce?