fix(NODE-6630): read all messages in buffer when chunk arrives

[nbbeeken]

Description

What is changing?

• Empty the current pool of chunks into the SizedMessageStream if more than one message can be read from it

Is there new documentation needed for these changes?

No

What is the motivation for this change?

Addresses an issue where streaming hellos would take approximately the maxAwaitTimeMS amount of time to push out the next update from the buffer.

Release Highlight

Fix processing of multiple messages within one network data chunk

During elections, or other scenarios where the server is pushing multiple topology updates to the driver in a short period of time, a bug in the driver's socket code led to backlog of topology updates that would remain in the buffer until another heartbeat arrived from the server. This could lead to delays in the driver recovering from an election and/or an increase in MongoServerSelectionErrors.

Now, all messages in the current buffer are returned to the driver leading to faster processing times.

Huge thank you to @andreim-brd for sharing a self-contained reproduction that proved to be instrumental in the identification of the underlying issue!

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

re-requesting review