Add totalWaiters to PoolStats

[steve-chavez]

Hello @avanov ,

As mentioned in bos#42 (comment), I'd like to limit the waiter queue to do backpressure. So I've added totalWaiters to PoolStats.

Related: PostgREST/postgrest#2042 (comment)

LMWYT, thanks!

[avanov]

This looks good and useful semantically. What I'm not sure of is whether the length of the queue should be calculated here or inside WaiterQueue module. Logically it's a property of the queue entity that could be read from it, but never modified directly.

[steve-chavez]

@avanov Thanks for the review. I did it like this because I also plan to add maxWaiters and I think that wouldn't fit inside the WaiterQueue module. Seemed more consistent to have them both outside.

[avanov]

@steve-chavez yeah I get that from the linked discussion, though aren't they two different concepts? One has to do with a "peekable" stat value (property of the queue), another has to do with utilising all available underlying stats for higher level interface of back-pressuring (back-pressuring being a property of the queue). For instance, the following sequence:

removeSelf <- push waiters var
modifyTVar_ waitersSizeVar (+ 1)

it assumes that push always leads to (+1) for the stat value, and that pop results in (subtract 1).

[steve-chavez]

@avanov Sorry for the late reply here. Since the newest hasql-pool doesn't use resource-pool(ref), I no longer need this change(we're transitioning to the newest hasql-pool PostgREST/postgrest#2391).