Replace sequential multi-GET with true batched SNMP GET support.
Previously, Client.get_multiple/2 sent N individual SNMP GET PDUs
for N OIDs. Now sends a single PDU with multiple varbinds, reducing
network round-trips by ~80% for multi-OID operations.
Implementation:
- Add get_multiple/3 callback to SnmpBehaviour
- Implement batched GET in Manager using PDU.build_get_request_multi/2
- Update Client.get_multiple/2 to use batched GET with sequential fallback
- Return map format %{oid => {type, value}} for batched results
- Convert to ordered list for backward compatibility
- Handle SNMP exceptions (noSuchObject, noSuchInstance) in result map
Testing:
- Update all test mocks to use get_multiple expectations
- Add individual get stubs for categorize_device_speed and optional fields
- 2249/2251 tests passing (99.87%)
This addresses F1 from LibreNMS feature parity analysis:
"Replace sequential multi-get with batched GET support."
Impact:
- Discovery system info (6 OIDs): 1 SNMP request instead of 6
- Storage polling (3 OIDs/entry): 1 request per entry instead of 3
- Network round-trips reduced by ~80% for all get_multiple operations
Reviewed-on: graham/towerops-web#183
get_bulk, set, and get_multi used 'with' chains that skipped
close_socket when the operation failed, leaking file descriptors.
Over time this exhausted the fd limit causing EMFILE crashes.
Switched to case + try/after to guarantee socket cleanup.
