feat(dhcp): suppress DHCP for ignored BMC MAC addresses

[kdhulipala-wq]

When a BMC MAC has suppress_dhcp = true in ignored_bmc_macs, the DiscoverDhcp RPC returns FailedPrecondition and atomically records dhcp_discover_suppressed_at. The decommission workflow polls this timestamp to confirm the BMC DHCP client has returned to INIT state.

No changes to the DHCP server binary; suppression is handled entirely within the existing DiscoverDhcp RPC in NICo Core.

Closes #3816

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Summary by CodeRabbit

• Bug Fixes
  • DHCP discovery now detects when requests from a BMC MAC address are suppressed.
  • Suppressed requests are stopped with a clear failure message, while unsuppressed requests continue normally.

Walkthrough

discover_dhcp checks BMC DHCP suppression before discovery. Suppressed requests commit the transaction and return a failed-precondition error. Other requests continue unchanged.

Changes

DHCP suppression handling

Layer / File(s)	Summary
Acknowledge suppressed BMC requests crates/api-core/src/dhcp/discover.rs	The discovery path checks the requesting BMC MAC. Suppressed requests commit the transaction and return a failed-precondition error. Unsuppressed requests continue through the existing flow.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• NVIDIA/infra-controller#3853: Introduces the BMC suppression persistence and models used by DHCP discovery.
• NVIDIA/infra-controller#4412: Adds related DHCP suppression APIs and server behavior.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The change implements DHCPDISCOVER suppression but does not address DHCPREQUEST NAK responses or DHCP record cache invalidation required by #3816.	Implement DHCPREQUEST NAK handling and provide DHCP record cache invalidation, or update the linked issue scope to match this change.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Out of Scope Changes check	✅ Passed	The changes remain within the stated scope of implementing DHCP suppression in the existing DiscoverDhcp RPC.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly and concisely describes the main change: suppressing DHCP for ignored BMC MAC addresses.
Description check	✅ Passed	The description accurately explains the DHCP suppression behavior, timestamp recording, workflow usage, and implementation scope.

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/api-core/src/dhcp/discover.rs`:
- Around line 502-504: Update the error message in the DHCP suppression path to
use lowercase wording while preserving the interpolated parsed_mac value and
omitting any trailing period. Keep the existing CarbideError::FailedPrecondition
construction unchanged.
- Around line 494-499: Update the cached-response path in
Controller::discover_dhcp to check BMC suppression before returning an unexpired
cached record, ensuring suppressed MAC addresses reach the suppression
acknowledgment flow and do not receive stale cached DHCP responses.
Alternatively, invalidate every cache key for parsed_mac when
db::bmc_suppression::acknowledge begins suppression, while preserving normal
caching for unsuppressed MAC addresses.

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📥 Commits

Reviewing files that changed from the base of the PR and between 3e1b818 and 750d4b2.

📒 Files selected for processing (1)

• crates/api-core/src/dhcp/discover.rs

[ericpretzel]

For decommissioning, we will reset the BMC which should cause it to go back to dhcpdiscover and not request. So NAK'ing requests for this purpose isn't strictly necessary. The resets will be timed so as to not refresh the 60-second cache in case it was hit.

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