"msg":"BSONObj::_validateUnownedSize() - size {size} of unowned BSONObj is invalid and differs from previously validated size.","attr":{"size":0} 

We hit this assertion during testing of mongosh. This only happens on macOS, with the latest server alpha (4.9.0-alpha4-271-g7d5cf02, downloaded through http://downloads.10gen.com/osx/mongodb-macos-x86_64-enterprise-latest.tgz).

The full mongod log file is attached. Steps to reproduce are fairly straightforward:

• Install Node.js 14
• Spin up a mongod using the version mentioned above, e.g. mkdir -p db && ./mongodb-darwin-none-x64-latest-alpha/bin/mongod --dbpath db
• Put the following script in a folder and run npm install mongodb@4.0.0-beta.1, then run it repeatedly using node scriptname.js:

const { MongoClient } = require('mongodb');

(async() => {
  const c = await MongoClient.connect(
        'mongodb://localhost/',
        {
          auth: { username: 'anna', password: 'pwd' },
          authSource: 'test',
          connectTimeoutMS: 1000
        }
      );
  await c.close();
})()

This usually crashes the server during the first 3 attempts.

Let me know if there's more data that we can gather for you. This is not particularly urgent for us.