[hillol-nexthop]

Summary

Adds loopback interface support to the existing config interface / delete interface families, reusing the config interface <name> ip-address <A.B.C.D/N> syntax. Loopbacks follow the deployment convention: loopback<N> is the virtual (isVirtual=true) interface at intfID == vlanID == 10 + N, backed by the same-numbered routable VLAN named fbossLoopback<N>.

• Resolution (shared by config and delete): a loopback<N> token now resolves in InterfaceList — by literal interface name, by the conventional fbossLoopback<N> name (Meta-style configs), or by the conventional intfID with isVirtual set (the bootstrap loopback0 ships unnamed).
• Creation: config interface loopback<N> ... creates the virtual interface plus its backing VLAN when absent, then applies the requested attributes; a bare config interface loopback<N> just creates it. Occupied conventional ids — a non-loopback interface at intfID 10+N, a same-numbered VLAN owned by another interface, or a data VLAN with member ports — are refused with specific errors, since the agent requires every VLAN-type interface to reference an existing VLAN and be its only interface.
• Addressing: ip-address / ipv6-address add and delete work on loopbacks through the existing attribute paths once the token resolves; multiple mixed v4/v6 host addresses per loopback are supported, and re-applying an address is a clean no-op.
• Deletion: a bare delete interface loopback<N> removes the virtual interface together with its backing VLAN (via VlanManager::deleteVlan, so the pair goes as a unit). Refused, with the config untouched, when the VLAN has member ports, is the default VLAN, or is a port's ingress VLAN — those are data VLANs and belong to delete vlan. Ports and loopbacks may be mixed in one command.
• Everything commits HITLESS (config reload, no agent restart), matching the rest of the config interface family.

Test Plan

Unit tests (new CmdConfigInterfaceLoopbackTest.cpp, seed mirroring the deployed loopback layout plus a Meta-style named loopback with production-shaped /32 + /128 addresses):

$ cmd_config_test
[==========] 893 tests from 77 test suites ran.
[  PASSED  ] 893 tests.

Integration tests (new ConfigLoopbackInterfaceTest), run on a lab:

[ RUN      ] ConfigLoopbackInterfaceTest.AddressRoundTripOnLoopback0
[       OK ] ConfigLoopbackInterfaceTest.AddressRoundTripOnLoopback0 (248 ms)
[ RUN      ] ConfigLoopbackInterfaceTest.CreateLoopbackWithAddresses
[       OK ] ConfigLoopbackInterfaceTest.CreateLoopbackWithAddresses (174 ms)
[  PASSED  ] 2 tests.

Sample usage on the same device (bootstrap loopback0 is unnamed, intfID 10):

$ fboss2-dev config interface loopback0 ip-address 10.254.113.31/32 ipv6-address fc00:0:0:31::/128
Successfully configured interface(s) loopback0: ip-address=10.254.113.31/32, ipv6-address=fc00:0:0:31::/128
$ fboss2-dev config session diff
       {
         "intfID": 10,
-        "ipAddresses": [],
+        "ipAddresses": [
+          "10.254.113.31/32",
+          "fc00:0:0:31::/128"
+        ],
         "isStateSyncDisabled": false,
         "isVirtual": true,
$ fboss2-dev config session commit
Config session committed successfully as 17ea3d8 and config reloaded for fboss_sw_agent.
$ fboss2-dev show config running agent | jq '.sw.interfaces[] | select(.intfID==10 or .intfID==19) | {intfID, vlanID, isVirtual, name, ipAddresses}'
{
  "intfID": 10, "vlanID": 10, "isVirtual": true, "name": null,
  "ipAddresses": [ "10.254.113.31/32", "fc00:0:0:31::/128" ]
}
{
  "intfID": 19, "vlanID": 19, "isVirtual": true, "name": "loopback9",
  "ipAddresses": []
}
$ fboss2-dev delete interface loopback0 ip-address 10.254.113.31/32
Successfully removed IP address 10.254.113.31/32 from interface(s): loopback0
$ fboss2-dev delete interface loopback0 ipv6-address fc00:0:0:31::/128
Successfully removed IPv6 address fc00:0:0:31::/128 from interface(s): loopback0
$ fboss2-dev config session commit
Config session committed successfully as 42eadaf and config reloaded for fboss_sw_agent.

(loopback9 above was created by the CLI via config interface loopback9 ip-address ... in the integration run — interface + backing VLAN created at the conventional ids, committed hitlessly; the test then removes it with delete interface loopback9 and verifies both the interface and VLAN 19 are gone from the running config.)

Review Findings

Pre-publication automated review (7 reviewer passes; no blocking correctness defects found on the tested paths). Known concerns listed for reviewer attention:

File:Line	Severity	Issue
CmdConfigInterface.cpp:343	Medium	A pre-existing same-id VLAN with no ports/interface is adopted as the loopback's backing VLAN as-is (name/routable not fixed up to the fbossLoopback<N>/routable=true convention).
CmdConfigInterface.cpp:317	Low	Duplicate tokens for one index in a single command (loopback5 Loopback5) yield a misleading "already in use by a non-loopback interface" error; nothing is saved.
CmdConfigInterface.cpp:731	Low	Loopback creation mutates the in-memory session config before later attribute validation (e.g. bad mtu, profile on a loopback) can throw; on-disk config is untouched since saveConfig is skipped.
CmdConfigInterface.cpp:750	Low	Bare config interface loopback<N> on an already-existing loopback throws "Incomplete command" rather than being an idempotent no-op.
InterfaceList.cpp:42	Low	Name-based fbossLoopback<N> resolution does not check isVirtual, unlike the intfID fallback.

[hillol-nexthop]

Rebased onto main and ported the internal review fixes: the bare-loopback create now shares the common save path, VlanManager::checkDeletable holds the VLAN delete validation reused by the loopback delete, the redundant interface erase after deleteVlan is gone, and the two integration tests are merged into one.