VTTablet: smartconnpool: notify all expired waiters

[brendar]

Description

Fixes a bug that causes waitlist.expire to only remove the first expired waiter on each invocation. This is due to calling wl.list.Remove(e) while traversing the list. This sets e.Next to nil, which terminates the loop. The fix is to remove waiters from the list after traversing it.

I think this should be backported to v19+. With the right combination of client behavior, query timeouts, and transaction timeouts, this bug can lead to tablets locking up indefinitely with all attempts to get a connection timing out.

Related Issue(s)

Fixes #16896

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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Deployment Notes

n/a

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.42%. Comparing base (2e47aba) to head (a9f1a6e).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16897      +/-   ##
==========================================
- Coverage   69.43%   69.42%   -0.02%     
==========================================
  Files        1571     1571              
  Lines      203195   203434     +239     
==========================================
+ Hits       141095   141236     +141     
- Misses      62100    62198      +98     

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[mattlord]

Thanks, @brendar ! ❤️

@vmg if you have a moment could you also look please?

[mattlord]

I'm assuming that this test fails on main. Just double checking 🙂

[brendar]

Yup, looks like this:

=== RUN   TestWaitlistExpireWithMultipleWaiters
    waitlist_test.go:61:
        	Error Trace:	/Users/brendan/src/github.com/Shopify/vitess/go/pools/smartconnpool/waitlist_test.go:61
        	Error:      	Timed out waiting for all waiters to expire
        	Test:       	TestWaitlistExpireWithMultipleWaiters
        	Messages:   	Wanted 2, got 1

[vmg]

Yeah this is correct. I think this is the cause of the reports we've seen of connections being stuck when shutting down the tablets and that we've never reproduced. Good catch!

[harshit-gangal]

Good find!