Replace the pool when reopening.

[arthurschreiber]

Description

It's proven quite difficult to fix some shortcomings in the connection pool like the blocking Close() call (which is quite problematic as this call happens during PlannedReparentShard and makes the operation take longer than required) and race conditions around concurrent Get()/put()/Close() operations.

Instead, I want to pick a previous idea that we disallow reopening a pool after Close() has been called and replace the whole pool object.

The Close() function becomes non-blocking - we close any connections that have been returned to the pool before the Close() call was made, and return. Any in-flight connections will be closed once Recycle() gets called on the connection.

SetCapacity() probably should also be changed in a followup PR to disallow setting the capacity to 0 or lower, and to be non-blocking as well (we can just start closing connections once they get returned to the pool).

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

❌ Patch coverage is 61.27451% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.48%. Comparing base (7dd3c6b) to head (c2adae1).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/pools/smartconnpool/pool.go	44.44%	70 Missing ⚠️
go/vt/vttablet/tabletserver/connpool/pool.go	91.07%	5 Missing ⚠️
go/vt/dbconnpool/connection_pool.go	81.81%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18530      +/-   ##
==========================================
- Coverage   67.50%   67.48%   -0.02%     
==========================================
  Files        1607     1607              
  Lines      263093   263255     +162     
==========================================
+ Hits       177593   177663      +70     
- Misses      85500    85592      +92     

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[arthurschreiber]

Open probably should be changed to return an error when it's called on a pool object that has been closed previously. Unfortunately, that'd require changing the signature and call sites and would bloat up this PRs size.

Maybe that can be done as a followup step.

[harshit-gangal]

I am all in to support that Close should lead to pool not being re-openable but it should still ensure that all the connections are returned back. Otherwise we might end up leaking connections.
This gives us the option to take right steps over capacity zero cases. like block any new operation to happen expect put so that existing connections are accounted in the pool.