[test][snmp] Retry physical-entity MIB walk on torn/incomplete read in test_transceiver_info

[bingwang-ms]

Description of PR

Summary:
Fixes an intermittent KeyError: 'entPhysName' (or 'entPhysDescr') failure in snmp/test_snmp_phy_entity.py::test_transceiver_info by making the underlying SNMP physical-entity MIB fetch resilient to torn/mid-walk reads.
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605

Approach

What is the motivation for this PR?

test_transceiver_info (and other tests in snmp/test_snmp_phy_entity.py that share the snmp_physical_entity_and_sensor_info fixture) intermittently fail (roughly every 1-2 weeks on some platforms) with:

name_to_snmp_facts[values['entPhysName']] = values
KeyError: 'entPhysName'

or the equivalent for entPhysDescr, on a specific entity OID whose returned dict is missing required fields (e.g. {'entPhysModelName': '', 'entPhysIsFRU': 2}).

This was previously masked by a conditional xfail tied to sonic-net/sonic-buildimage#22213, but that issue has since been closed, so the conditional_mark plugin no longer applies the xfail and the intermittent race now surfaces as a hard failure.

How did you do it?

Root-caused via the snmp_facts ansible module (ansible/library/snmp_facts.py): _collect_physical_entities performs a single SNMP walk over the entPhysicalTable subtree, which - because of how SNMP table OIDs are structured (<table>.<column>.<instance>) - effectively walks one MIB column at a time across all rows (all entPhysDescr rows, then all entPhysContainedIn rows, ..., then entPhysName, etc.), rather than taking one atomic snapshot of the whole table. If a physical entity (e.g. a transceiver) is added, removed, or re-indexed by the SNMP subagent while this walk is still in progress, some columns for that entity can end up populated while others are missing entirely - a torn/inconsistent read for that particular OID.

Fix (test-side, in tests/snmp/test_snmp_phy_entity.py): in get_entity_and_sensor_mib(), after fetching the physical-entity MIB, validate that every returned row has both entPhysDescr and entPhysName (mandatory columns per RFC 2737 for any valid table row). If any row is incomplete, log a warning and retry the whole SNMP fetch (bounded: 60s timeout / 10s interval) before handing the result to the tests, rather than crashing with a raw KeyError. This converts the transient torn-read race into a resilient retry, while still surfacing a real, persistent incompleteness as a failure if it doesn't resolve after retries.

How did you verify/test it?

• Reproduced the original KeyError: 'entPhysName' failure history on the affected testbed (Nokia TH6 platform).

• Applied the fix and ran snmp/test_snmp_phy_entity.py::test_transceiver_info live against the same testbed/DUT 6 times total (1 initial run + 5 repeated back-to-back runs to check for stability):

  Run	Result	Duration
  1 (initial)	PASSED	~6m16s
  2	PASSED	~6m34s
  3	PASSED	~6m35s
  4	PASSED	~6m22s
  5	PASSED	~6m31s
  6	PASSED	~6m27s

  6/6 passed.

• Confirmed via py_compile and flake8 that the change is syntactically valid and lint-clean.

Any platform specific information?

Observed on Nokia TH6 platform (Nokia-IXR7220-H6-O256), but the root cause (a non-atomic, multi-column SNMP table walk racing with subagent-side reindexing) is generic and not platform-specific; the fix applies to all platforms using this fixture.

Supported testbed topology if it's a new test case?

N/A (not a new test case).

Documentation

N/A

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[mssonicbld]

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[mssonicbld]

Cherry-pick PR to msft-202512: Azure/sonic-mgmt.msft#1419

[mssonicbld]

This PR has backport request label(s) for branch(es): 202511,msft-202512,202605, 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., 202511: <test result>) in the Test result section as well in your PR description.

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[StormLiangMS]

@bingwang-ms

Pls update 202605 test result for cherrypick ask.