[Nexthop][fboss2-dev] add fboss2-dev config/delete qos default-queue-config commands

[vybhav-nexthop]

Summary

This PR adds two commands for the switch-wide default port queue
settings:

fboss2-dev config qos default-queue-config <queue-id> <attr> <value> [<attr> <value> ...]
fboss2-dev delete qos default-queue-config <queue-id>

What is defaultPortQueues? Each front-panel port has a set of
hardware egress queues, and each queue carries settings for scheduling
(strict-priority vs. weighted round robin), buffer allocation (reserved
and shared bytes, MMU scaling factor) and congestion management (ECN
marking / early drop thresholds). SwitchConfig.defaultPortQueues (a
list<PortQueue> in switch_config.thrift) is the fallback queue
config the agent applies to every port that has no explicit
portQueueConfigName override — one place to define the queue behavior
for the whole switch. The config is sparse: only the queues you define
are changed; queues absent from the list are reset to the ASIC defaults
(WRR, weight 1).

How the commands behave:

• Set/merge semantics (config): if the queue-id already exists in
  defaultPortQueues its entry is updated in place; otherwise a new entry
  is created (with scheduling = WEIGHTED_ROUND_ROBIN, since the thrift
  field is required). Multiple <attr> <value> pairs can be set in one
  invocation.
• Supported attributes: scheduling (full enum names or short WRR
  / SP / DRR), weight, reserved-bytes, shared-bytes,
  scaling-factor, stream-type, buffer-pool-name, and
  active-queue-management (see below).
• Delete removes the whole entry: delete qos default-queue-config <queue-id> erases that queue's entry from defaultPortQueues,
  reverting the queue to ASIC defaults at the next coldboot. Per-attribute
  deletion is intentionally not supported — re-run the config command to
  change individual attributes. Deleting a queue-id that has no entry is
  an error (fails loudly on typos instead of staging a no-op coldboot
  commit), consistent with the copp delete commands.
• Validation: enum attributes are checked against the thrift enums
  (QueueScheduling, MMUScalingFactor, StreamType,
  QueueCongestionBehavior) and the error message lists every valid
  value; byte/weight/threshold values must be non-negative; unknown
  attributes, missing values and malformed queue-ids are rejected before
  anything is staged. Queue-ids are bounded to 0..128 (shared with the
  queuing-policy parser).
• Commit level: AGENT_COLDBOOT for both commands — queue buffer
  carving cannot be applied hitlessly, so committing restarts the agents.

Active Queue Management (AQM)

A queue can act on packets before it fills, to signal congestion early
instead of tail-dropping when full. Each AQM policy pairs a
detection ramp (when to act) with a congestion-behavior (what to
do):

• detection — detection linear minimum-length <bytes> maximum-length <bytes> probability <pct>: a WRED ramp over queue depth.
  Below minimum-length do nothing; between min and max act with a
  probability rising to probability%; above maximum-length act always.
• congestion-behavior — congestion-behavior <ECN|EARLY_DROP>:
  • EARLY_DROP — drop packets early (WRED). Works for any traffic.
• ECN — mark the Congestion-Experienced bit instead of dropping; only
  for ECN-capable flows.

PortQueue.aqms is a list, so a single queue can carry both an
ECN and an EARLY_DROP policy at once (mark ECN-capable traffic, drop the
rest) — the ASIC applies the matching one per packet.
congestion-behavior is the key that identifies which AQM entry an
edit targets, so it is required whenever active-queue-management is
given:

# ECN on queue 6
fboss2-dev config qos default-queue-config 6 active-queue-management \
    congestion-behavior ECN detection linear minimum-length 40000 maximum-length 60000 probability 100

# add EARLY_DROP alongside it (a second entry — does not clobber the ECN one)
fboss2-dev config qos default-queue-config 6 active-queue-management \
    congestion-behavior EARLY_DROP detection linear minimum-length 30000 maximum-length 50000 probability 10

Usage:

$ fboss2-dev config qos default-queue-config 0 scheduling WRR weight 10
Successfully configured default-queue-config queue-id 0
$ fboss2-dev config session commit          # AGENT_COLDBOOT — restarts agents

$ fboss2-dev delete qos default-queue-config 0
Successfully deleted default-queue-config queue-id 0
$ fboss2-dev config session commit          # AGENT_COLDBOOT

Extra things done in this PR

• Shared PortQueue attribute handling. config qos default-queue-config and config qos queuing-policy configure the same
  cfg::PortQueue attributes into different parts of the switch config;
  the shared logic now lives in PortQueueConfigUtils::applyPortQueueConfig
  used by both, and the 0..128 queue-id bound is one shared constant.
• fake-SAI shutdown fix. The agent's SAI graceful-exit sets
  SwitchPreShutdown; the fake SAI didn't handle it, returned
  SAI_STATUS_INVALID_PARAMETER and aborted the agent (a thrown
  SaiApiError inside the noexcept shutdown handler std::terminate()s)
  on every commit-triggered restart under the simulator. The fake SAI now
  accepts it as a no-op, matching real adapters. Real hardware is
  unaffected.

Changed files

• commands/config/qos/PortQueueConfigUtils.{h,cpp} — shared
  applyPortQueueConfig: PortQueue scalar + AQM attribute handling.
• commands/config/qos/default_queue_config/CmdConfigQosDefaultQueueConfig.{h,cpp}
  — new config command (transactional apply, behavior-keyed AQM).
• commands/delete/qos/CmdDeleteQos.{h,cpp},
  commands/delete/qos/default_queue_config/CmdDeleteQosDefaultQueueConfig.{h,cpp}
  — new delete qos subtree + handler.
• commands/config/qos/queuing_policy/CmdConfigQosQueuingPolicyQueueId.cpp
  — migrated onto the shared helper.
• utils/CmdUtilsCommon.h — shared kMaxQueueId queue-id bound.
• agent/hw/sai/fake/FakeSaiSwitch.cpp — accept
  SAI_SWITCH_ATTR_PRE_SHUTDOWN as a no-op.
• CmdListConfig.cpp — register the new commands.
• unit + integration tests; cmake/BUCK wiring.

Test Plan

• Unit: cmd_config_test passes. AQM coverage pins the new behavior
  (each fails on a revert): ECN+EARLY_DROP coexistence (aqms.size()==2,
  both entries preserved), edit-by-behavior preserves the other entry,
  bare congestion-behavior preserves existing detection,
  missing/duplicate behavior rejected, no-attributes rejected; delete
  queue-id bound (>128 rejected, 128 accepted).
• Integration (real hardware, Nexthop device, real-SAI): one end-to-end
  test. NRestarts=0 on the sw agent before and after — both agent
  restarts in the run were the deliberate coldboots, not crashes — and
  the device is left at its pre-test config.

[       OK ] ConfigQosDefaultQueueConfigTest.CreateThenDeleteQueueEntry (45564 ms)
[  PASSED  ] 1 test.

Review Findings

Reviewed with fboss-review (11-reviewer pass) prior to submission;
findings addressed. No open issues above the confidence threshold.