fix(localdns): reduce startup polling interval

[jingwenw15]

Summary

• reduce localdns PID-file polling from 1s to 0.1s while preserving the existing timeout semantics
• reduce localdns readiness polling from 1s to 0.1s and scale the production call-site attempts accordingly
• add focused ShellSpec coverage for the 0.1-second polling interval in both helper loops

Testing

• shellspec --shell bash spec/parts/linux/cloud-init/artifacts/localdns_spec.sh:668
• shellspec --shell bash spec/parts/linux/cloud-init/artifacts/localdns_spec.sh:733

Reduces startup time by around 0.6s on average.

[Copilot]

Pull request overview

Reduces localdns startup polling intervals from 1s to 0.1s so the node bootstrap path waits less when CoreDNS comes up quickly, while preserving the existing wall-clock timeout semantics. The PID-file wait now counts in tenths of a second (10× the timeout), and the readiness call-site bumps maxattempts from 60 to 600 to match the 60s timeout at the finer interval.

Changes:

• Introduce LOCALDNS_POLL_INTERVAL_SECONDS=0.1 and rework start_localdns to use tenth-second counters so START_LOCALDNS_TIMEOUT=10 still yields a ~10s ceiling.
• Switch wait_for_localdns_ready's sleep to 0.1s and update the production call-site to wait_for_localdns_ready 600 60.
• Add ShellSpec tests asserting both loops sleep with 0.1.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
parts/linux/cloud-init/artifacts/localdns.sh	Faster polling in start_localdns/wait_for_localdns_ready plus updated max-attempt arg at the call-site.
spec/parts/linux/cloud-init/artifacts/localdns_spec.sh	New ShellSpec cases asserting the 0.1s sleep interval in both helper loops.

[yewmsft]

Two things on the polling-interval drop:

1. Hardcoded constant in a shared file. LOCALDNS_POLL_INTERVAL_SECONDS=0.1 sits at the top of localdns.sh and is used by both start_localdns and wait_for_localdns_ready. If anyone later tunes one site, they'll silently retune the other. Consider giving each call site its own constant (LOCALDNS_PID_POLL_INTERVAL, LOCALDNS_READY_POLL_INTERVAL) so future changes are explicit about scope.

2. wait_for_localdns_ready 600 60 is now coupled to the interval. The first arg is MAX_ATTEMPTS, not seconds — at 0.1s/attempt × 600 attempts you get the same 60s wall clock the old 60 60 invocation produced, but only by coincidence. If LOCALDNS_POLL_INTERVAL_SECONDS changes again, this call site silently breaks the timeout. Either compute attempts from a wall-clock budget (attempts = budget_seconds / interval) or rename the arg / refactor to take a duration.

Spec tests look good for verifying the interval value gets passed; consider adding one that asserts the resulting wall-clock timeout (mocked clock) so a future regression of the interval/attempts coupling fails the test rather than slipping through.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[jingwenw15]

@yewmsft Addressed in the follow-up commits on this branch. The polling constants are now split by purpose (LOCALDNS_PID_POLL_INTERVAL_SECONDS and LOCALDNS_READY_POLL_INTERVAL_SECONDS), readiness now takes a real duration via LOCALDNS_READY_TIMEOUT_SECONDS instead of a hand-derived 600 attempt count, and the ShellSpec coverage now includes mocked-clock timeout behavior. I also added a derived max-attempt bound for readiness so the loop still terminates if the system clock does not advance as expected during early boot.

[Copilot]

Pull request overview

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

Comments suppressed due to low confidence (1)

parts/linux/cloud-init/artifacts/localdns.sh:479

• This error message says the readiness poll interval is invalid, but calculate_max_poll_attempts can also fail due to an invalid timeout_duration value (or missing awk). Consider broadening the wording and/or logging the timeout + interval values to aid diagnosis.

    max_attempts=$(calculate_max_poll_attempts "${timeout_duration}" "${LOCALDNS_READY_POLL_INTERVAL_SECONDS}") || {
        echo "Invalid localdns readiness poll interval configuration."
        return 1