VTGate: fix warming reads timeout context

[timvaillancourt]

Description

Fixes the warming reads timeout context so it actually works as intended.

CloneForReplicaWarming() was creating a context.WithTimeout using the --warming-reads-query-timeout flag value, but:

1. The cancel func was discarded (suppressed with //nolint), leaking the timeout goroutine
2. The timeout started from clone creation, not goroutine execution — the warming read could time out before it even started if the goroutine was delayed
3. The warming goroutine passed the original request ctx to findRoute() and ExecuteMultiShard(), so when the primary query completed and its context was cancelled, the warming read got a KILL QUERY

The dedicated --warming-reads-query-timeout was effectively unused since the introduction of warming reads in v16.

This PR introduces a WarmingReadsContext() method on the VCursor interface that creates the timeout context inside the warming goroutine using context.WithoutCancel(ctx) as the base (preserves tracing spans and caller IDs while detaching from parent cancellation). The clone is also moved inside the select case to avoid unnecessary work when the channel is full.

Test coverage includes:

• Warming read context has its own deadline, independent of parent
• Parent context cancellation does not cancel the warming read
• findRoute() receives the warming context (via ResolveDestinations hook)
• Warming read is dropped when the channel is full
• Warming goroutine exits on context timeout with DeadlineExceeded

The justification for backports is this timeout is not working as designed, since the introduction of warming reads in v16. cc @olyazavr

Related Issue(s)

Resolves: #19673
Related: #19656

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• 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

AI Disclosure

Claude Code assisted with development and testing. Claude Code prepared this PR summary

[vitess-bot]

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[timvaillancourt]

@copilot please provide a review of this draft

[Copilot]

Pull request overview

Fixes VTGate replica warming reads so --warming-reads-query-timeout actually bounds the warming queries by propagating the timeout context into the warming execution path.

Changes:

• Updated VCursor.CloneForReplicaWarming() to return both a cloned VCursor and the context to use for the warming query.
• Switched the warming read goroutine to use the returned context for routing and execution.
• Added/updated unit coverage to assert the warming read executes with a deadline-bearing context.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
go/vt/vtgate/executorcontext/vcursor_impl.go	Changes CloneForReplicaWarming to return the timeout context alongside the cloned VCursor.
go/vt/vtgate/engine/route.go	Uses the returned warming context for findRoute and ExecuteMultiShard.
go/vt/vtgate/engine/primitive.go	Updates VCursor interface signature for CloneForReplicaWarming.
go/vt/vtgate/engine/route_warming_reads_test.go	Extends test VCursor to return a timeout context and asserts a deadline is present.
go/vt/vtgate/engine/fake_vcursor_test.go	Adjusts test VCursor implementations to match the new interface signature.

Comments suppressed due to low confidence (1)

go/vt/vtgate/executorcontext/vcursor_impl.go:306

• CloneForReplicaWarming creates a context.WithTimeout but discards the returned cancel func. Now that this context is used to run the actual warming query, failing to call cancel will keep the timer/resources around until the timeout fires (and if the warming read is dropped, it still allocates a timer). Consider returning a context.CancelFunc (or encapsulating cancellation in the returned VCursor) and ensuring callers defer cancel() when done; also guard WarmingReadsTimeout <= 0 if "no timeout" is intended (since WithTimeout(..., 0) is immediately expired).

func (vc *VCursorImpl) CloneForReplicaWarming(ctx context.Context) (engine.VCursor, context.Context) {
	callerId := callerid.EffectiveCallerIDFromContext(ctx)
	immediateCallerId := callerid.ImmediateCallerIDFromContext(ctx)

	timedCtx, _ := context.WithTimeout(context.Background(), vc.config.WarmingReadsTimeout) //nolint
	clonedCtx := callerid.NewContext(timedCtx, callerId, immediateCallerId)

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Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

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