smartconnpool: do not allow connections to starve

[vmg]

Description

This is a potential fix for the issue described in #17662. The bug report has a very comprehensive explanation of the bug (thank you!!), so I'll stick to explaining the fix here.

I've considered introducing stronger synchronization to the pool to prevent the race but decided against it because the race does not affect correctness, as the connections in the pool would only starve in situations where the service just stops receiving new requests over time. As long as the pool periodically receives connection requests, no connection can starve because the clients who raced with other clients and are stuck in the waitlist will wake up right away when the new connections come in. I don't think persistently slowing down the pool to handle for this corner case is worth it.

Hence, my proposed solution reuses the expiry loop (the periodic loop that checks that the connections in the wailist haven't expired naturally by the client) to detect connections that could potentially be starving. This will not be a common occurrence, but when it happens, we can simply force a cycle of the pool (pull from the stacks and hand over to the waiting connections) and that will always awake any starving connections.

The pros of this approach is that it adds zero overhead to the normal get/put path for the pool.
The cons of this approach is that a starving connection can starve for up to 100ms (which is the frequency I've set for the starving loop).

Open to alternative takes for this fix, ideally ones without much overhead to the get/put path.

cc @arthurschreiber @mhamza15 @harshit-gangal @deepthi

Related Issue(s)

• Fixes Bug Report: connection pool timed out errors when there is a spike in borrowed/waiting connections due to race condition #17662

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.77%. Comparing base (cb7d61a) to head (e0538b7).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/pools/smartconnpool/pool.go	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17675      +/-   ##
==========================================
+ Coverage   67.75%   67.77%   +0.01%     
==========================================
  Files        1587     1587              
  Lines      255780   255806      +26     
==========================================
+ Hits       173310   173369      +59     
+ Misses      82470    82437      -33     

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[mhamza15]

Can confirm we're not seeing it in our CI either 👍. Thanks @vmg!

[deepthi]

Was -1 being used as some kind of sentinel value? Or was this completely unnecessary?

[vmg]

It was a sentinel value to mark when no Setting connection had been returned to the pool yet, but it was not an interesting optimization, so I removed it.