fix(connection): avoid returning readyState = connected if connection state is stale

[vkarpov15]

Fix #14727

cc @alexbevi

Summary

The problem here is that readyState can return connected immediately after an AWS Lambda container thaws after some time. readyState only looks at the last heartbeat, but not how long ago the last heartbeat occurred - with this PR, if the last heartbeat is stale (more than 2 heartbeat intervals) then we assume we're disconnected.

This PR still needs significant testing, but I wanted to put it out for review to solicit early feedback.

Examples

[hasezoey]

Code looks good to me, though i agree this should need some testing

[vkarpov15]

Going to move this one back because I've been having trouble reproducing this issue in AWS lambda

[vkarpov15]

It took some time, but I was able to repro this in Lambda. The following function is adapted from Mongoose's Lambda docs:

const mongoose = require('mongoose');

let conn = null;

const uri = 'mongodb+srv://chris2022:[email protected]/chris2022?retryWrites=true&w=majority';

exports.handler = async function(event, context) {
  // Make sure to add this so you can re-use `conn` between function calls.
  // See https://www.mongodb.com/blog/post/serverless-development-with-nodejs-aws-lambda-mongodb-atlas
  context.callbackWaitsForEmptyEventLoop = false;

  // Because `conn` is in the global scope, Lambda may retain it between
  // function calls thanks to `callbackWaitsForEmptyEventLoop`.
  // This means your Lambda function doesn't have to go through the
  // potentially expensive process of connecting to MongoDB every time.
  if (conn == null) {
    conn = mongoose.createConnection(uri, {
      // and tell the MongoDB driver to not wait more than 5 seconds
      // before erroring out if it isn't connected
      serverSelectionTimeoutMS: 5000
    });

    // `await`ing connection after assigning to the `conn` variable
    // to avoid multiple function calls creating new connections
    await conn.asPromise();
    conn.model('Test', new mongoose.Schema({ name: String }));
  }

  console.log('Start', conn.readyState, new Date(conn._lastHeartbeatAt), Date.now() - conn._lastHeartbeatAt);

  const M = conn.model('Test');

  const doc = await M.findOne();
  console.log('After find', conn.readyState, new Date(conn._lastHeartbeatAt), Date.now() - conn._lastHeartbeatAt);
  console.log(doc);

  return doc;
};

Output:

Response
{
  "_id": "641b1cb82525faca0c0be001",
  "name": "Run Hook",
  "__v": 0
}

Function Logs
START RequestId: ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa Version: $LATEST
2024-09-17T16:36:30.288Z	ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa	INFO	Start 0 2024-09-17T16:36:08.749Z 21539
2024-09-17T16:36:30.408Z	ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa	INFO	After find 1 2024-09-17T16:36:30.371Z 37
2024-09-17T16:36:30.408Z	ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa	INFO	{
  _id: new ObjectId('641b1cb82525faca0c0be001'),
  name: 'Run Hook',
  __v: 0
}
END RequestId: ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa
REPORT RequestId: ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa	Duration: 204.32 ms	Billed Duration: 205 ms	Memory Size: 128 MB	Max Memory Used: 104 MB

Request ID
ec6e7091-f2dd-47f8-8a4a-8cee7cccc1fa

So readyState is correctly 0 at the beginning, but then changes to 1 when Mongoose gets a successful heartbeat back.