[neighorch]: Avoid inserting missing bulk neighbors

[Xichen96]

What I did

Prevented NeighOrch::enableNeighbors() from inserting a default cache entry while checking whether a bulk-enable neighbor exists.

Root cause

The old code read the map before checking it:

ctx->mac = m_syncdNeighbors[neighborEntry].mac;

if (m_syncdNeighbors.find(neighborEntry) == m_syncdNeighbors.end())
{
    continue;
}

std::map::operator[] inserts a default value when the key is missing. The intended absence check was therefore unreachable:

missing neighbor
    -> operator[] inserts a default entry
    -> find() succeeds
    -> default MAC/state enters the bulk-enable path

The fix uses one non-mutating lookup and reads the MAC only after the entry is confirmed:

auto neighborIt = m_syncdNeighbors.find(neighborEntry);
if (neighborIt == m_syncdNeighbors.end())
{
    continue;
}
ctx->mac = neighborIt->second.mac;

Why I did it

MuxNbrHandler::enable() can hand NeighOrch a context for a neighbor that disappeared from m_syncdNeighbors before bulk processing. A read-side guard must not manufacture the state it is trying to detect.

This defect was found while reviewing PR #4746, but it predates that change and belongs in a separate PR because it affects NeighOrch cache correctness and MUX bulk-enable behavior, not the IntfsOrch removal fence.

The fix has no standalone master merge dependency and remains independently reviewable.

How I verified it

Added EnableNeighborsDoesNotInsertMissingNeighbor in tests/mock_tests/neighorch_ut.cpp.

The regression:

1. creates a bulk context for a neighbor absent from m_syncdNeighbors;
2. verifies the cache is empty;
3. calls enableNeighbors();
4. verifies the intended missing-neighbor no-op succeeds;
5. verifies the cache remains empty.

Current exact-head master PR build

• Head: faa3e6df99028137237ea70c9fa99f5a8b8c097c
• Azure PR build: 1171074
• All architecture, ASAN, Docker, Trixie, Test vstest, and TestAsan vstest lanes passed.
• CodeQL, Semgrep, DCO, EasyCLA, and the exact-head Copilot review are clean.
• No local build was used; validation is PR-build only.

202605 PR-build validation

• Validation-only PR: #4747
• The same operator[]-before-find() defect exists on 202605.
• This PR is included after [neighorch]: Track remote next hops on inband RIF #4769 and before batch-2 PRs [intfsorch]: Retry explicit router interface VRF rehome #4764 and [neighorch]: Retry until inband port is available #4770 in the complete validation stack.
• Exact validation head: d4b0502ecdac76d56fdf063a633045c0fb0de977
• Azure PR build: 1171672 — in progress.
• The Tested for 202605 branch label must remain absent until that exact result is complete.

Details if related

• Changed files:
  • orchagent/neighorch.cpp
  • tests/mock_tests/neighorch_ut.cpp
• Originating review finding: [intfsorch]: Fence new RIF dependencies during interface removal #4746 (comment)
• Merge batch 1. PR [neighorch]: Retry until inband port is available #4770 must be rechecked after this PR merges because both touch NeighOrch removal paths.

[mssonicbld]

/azp run

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[Copilot]

Pull request overview

This PR fixes a subtle cache-mutation bug in NeighOrch::enableNeighbors() where reading m_syncdNeighbors[neighborEntry] could insert a default-constructed neighbor entry, making subsequent “not found” handling ineffective. The change ensures missing neighbors are skipped without altering the cache and adds a mock regression test to prevent reintroduction.

Changes:

• Replace operator[] access with a find()-based lookup before reading a neighbor’s cached MAC in enableNeighbors().
• Skip processing for missing neighbor entries without mutating m_syncdNeighbors.
• Add a mock unit test verifying enableNeighbors() does not insert a missing neighbor entry.

Reviewed changes

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

File	Description
orchagent/neighorch.cpp	Avoids unintended insertion into m_syncdNeighbors by using find() before reading cached neighbor data in enableNeighbors().
tests/mock_tests/neighorch_ut.cpp	Adds regression coverage ensuring a missing neighbor in a bulk enable request does not create a cache entry.

[Xichen96]

/azpw run

[mssonicbld]

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Retrying failed(or canceled) jobs...

[mssonicbld]

Retrying failed(or canceled) stages in build 1170080:

✅Stage TestAsan:

• Job vstest: retried.

✅Stage Test:

• Job vstest: retried.
• Job vstest: retried.

[mssonicbld]

/azp run

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[mssonicbld]

This PR has backport request label(s) for branch(es): 202605, but is missing required test information. Please make sure you tick the tested branch(es) in the Tested branch section and provide test evidence (e.g., 202605: <test result>) in the Test result section as well in your PR description.

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