go/vt/vtgate/engine: use sync.Once to resolve nilness issue

[mdlayher]

Description

This is a tricky spot and the linter here doesn't seem to realize the outer loop may alter the value of routedKs in a prior iteration.

✔ ~/src/github.com/planetscale/vitess/go/vt/vtgate/engine [main|✔] 
16:12 $ nilness .
/home/matt/src/github.com/planetscale/vitess/go/vt/vtgate/engine/routing.go:276:16: tautological condition: nil == nil

Even if this is a false positive, the current way this logic is written seems a bit subtle and the intent was not obvious to me at first read.

Using a sync.Once here clearly conveys the intent that the first keyspace name detected is the one that should be used for the remainder of loop iterations.

Related Issue(s)

Assuming #14812 is also merged, this fixes #14684.

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
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

n/a

[vitess-bot]

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[dbussink]

@vitessio/query-serving Is this path (very) performance sensitive? Since afaik this is a more expensive way to express the same thing there and it might not be worth it if it's very performance critical?

[mdlayher]

Good question. FWIW the implementation of once is quite small and I would expect it to be basically free (just an atomic load) following the first iteration that stores a result.

func (o *Once) Do(f func()) {
	// Note: Here is an incorrect implementation of Do:
	//
	//	if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
	//		f()
	//	}
	//
	// Do guarantees that when it returns, f has finished.
	// This implementation would not implement that guarantee:
	// given two simultaneous calls, the winner of the cas would
	// call f, and the second would return immediately, without
	// waiting for the first's call to f to complete.
	// This is why the slow path falls back to a mutex, and why
	// the atomic.StoreUint32 must be delayed until after f returns.

	if atomic.LoadUint32(&o.done) == 0 {
		// Outlined slow-path to allow inlining of the fast-path.
		o.doSlow(f)
	}
}

func (o *Once) doSlow(f func()) {
	o.m.Lock()
	defer o.m.Unlock()
	if o.done == 0 {
		defer atomic.StoreUint32(&o.done, 1)
		f()
	}
}