Rewrite the quit debouncer to drop quits early #1091
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This was referenced Aug 14, 2020
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For the uninitiated: Debouncing quits is something we do when a IRC server netsplits, to avoid the matrix server being flooded with /leave requests and/or killing the bridge.
Currently the quit debouncer code operates at the IrcHandler level, which means it's already gone through some hot paths of the code such as identifying which rooms to talk into. Furthermore, the debouncer operates by storing a deferred promise to work on later when bridging delayed QUITs.
In the field, we've noticed that the bridge will crash and burn long before it gets to this queue, so this code attempts to drop QUITs very early on, and store the work in a simple data structure to try and limit the amount of work involved when debouncing.
Please review this PR with a mindset of trying to achieve the above with the smallest per-request footprint on CPU/memory.