[Nexthop][fboss2-dev] Add config and delete interface sflow sample-dest

[vybhav-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

Background: what sFlow sampling is and what sample-dest chooses

A switch forwards far more traffic than it could ever copy for analysis, so sFlow samples instead: the ASIC picks one in N packets on a port (N is Port.sFlowIngressRate/sFlowEgressRate) and exports just those. Statistically that is enough to see top talkers, flow mix, and anomalies without touching line-rate forwarding.

Port.sampleDest chooses where each sampled copy goes, and the two options trade CPU cost against flexibility:

• cpu: the ASIC punts the sample to the switch CPU, and the agent wraps it into sFlow datagrams for the configured collectors. Flexible, but each sample costs CPU and punt-path bandwidth.
• mirror: the ASIC forwards the sample out the port's ingress mirror session straight to a remote collector box, encapsulated, with no CPU involvement. Line rate, but the collector does all the work, and it only exists for ingress sampling.

The field is a modifier: it takes effect once a port's sampling rate is nonzero, and an unset field leaves the ASIC default behavior.

What: Adds fboss2-dev config interface <ports> sflow sample-dest <cpu|mirror> and its delete counterpart delete interface <ports> sflow sample-dest.

• config sets Port.sampleDest: sFlow samples are processed on-box (cpu) or sent to the port's ingress mirror (mirror).
• delete clears the optional field, returning the port to its unset default.

Why: Port.sampleDest had no CLI coverage in either direction.

How: New sflow subcommand branch under config interface / delete interface, in the same tree shape as ipv6/switchport. The CLI refuses mirror when the port has a non-zero sFlowEgressRate. That is the constraint the agent enforces (ApplyThriftConfig: "Egress sampling to mirror destination is unsupported"), but the CLI catches it first with a targeted message, before the config is touched. Hitless: sampling is (re)programmed from the port delta at runtime (SaiPortManager::addSamplePacket/removeSamplePacket).

Sample usage

$ fboss2-dev config interface eth1/1/1 sflow sample-dest cpu
Successfully set sFlow sample destination for interface(s) eth1/1/1 to cpu
$ fboss2-dev config interface eth1/2/1 sflow sample-dest mirror
Port eth1/2/1: sample-dest mirror requires sFlowEgressRate 0 — egress sampling to a mirror destination is unsupported
$ fboss2-dev delete interface eth1/1/1 sflow sample-dest
Reset sFlow sample destination for interface(s) eth1/1/1

Test Plan

Unit tests (13 new, config + delete):

[==========] 13 tests from 1 test suite ran. (1034 ms total)
[  PASSED  ] 13 tests.

Full config suite: //fboss/cli/fboss2/test/config:cmd_config_test PASSED in 53.1s.

Integration test on an Nexthop DUT: set cpu, verify it lands in the running config, delete, verify the field is unset again.

[       OK ] ConfigSflowSampleDestTest.SetThenDeleteSampleDest (159 ms)
[  PASSED  ] 1 test.

All of the above ran against the internal tree this commit was cherry-picked from. The upstream tree was not built locally; this PR's CI is the build validation for the rebased commit.

Review Findings

Pre-publication review (5 reviewers + verifier) raised three findings, all fixed before push: mixed interface lists now report L3-only names as skipped in both commands (a 13th unit test covers it), and the integration test gained a best-effort TearDown that clears a committed sampleDest on failure.

[joseph5wu]

Let's follow how we handle multiple interface attributes in CmdConfigInterface and do the same thing for CmdConfigInterfaceSflow rather than create three different classes.

[joseph5wu]

Instead of creating a standalone CmdConfigInterfaceSflowSampleDest, can we have CmdConfigInterfaceSflow to handle all sflow related attributes in the same class:

• sample_dest
• ingress_rate
• egress_rate?

[vybhav-nexthop]

will add these 3 here in this PR itself, since they seemed to be simple enough to implement

[vybhav-nexthop]

Added ingress-rate and egress-rate too, in 0b89a7e: config/delete interface <ports> sflow ingress-rate|egress-rate <N>, mapping to Port.sFlowIngressRate/sFlowEgressRate, both handled by the same CmdConfigInterfaceSflow/CmdDeleteInterfaceSflow class as sample-dest. egress-rate carries the same MIRROR/egress-rate constraint sample-dest already enforces, checked from the other direction (refuses a non-zero egress-rate on a port whose sample-dest is already MIRROR).

All three attributes now go through one class each on the config/delete side, per your original comment.