[benoit-nexthop]

Summary

FBOSS tab completion (fboss2 __completion) only walks the CLI11
sub-command tree, so it stops at config arp and cannot offer timeout,
max-probes, ...: the config/delete commands keep the last level of
tokens in a list inside one handler rather than as one .cpp/.h per leaf,
to avoid code bloat.

This gives commands a uniform way to complete those tokens:

• A Traits may declare
  static std::vector<std::string> completeArgs(const std::vector<std::string>& typed)
  returning the tokens that may follow the positionals typed so far.
  argRegistrar<Traits> (now a struct carrying both addCliArg and the
  optional completer) picks it up, and CmdSubcommands remembers it per
  CLI11 command. No command-tree entries change; a Traits without
  completeArgs simply gets no leaf completion.
• utils/ArgCompletion.h holds the three grammars that recur across the
  tree so nearly every command is a one-liner: completeAttrGrammar for
  [<object>...] <attr> <value> ... (per-attr enum values, valueless
  attrs, multi-word attrs such as timers hold-time, used attrs not
  offered again), completeAttrList for the delete-side <attr> [<attr>...]
  and completePositions for fixed positional grammars.
• CmdCompletion.cpp walks the CLI11 tree along the typed words (skipping
  options and their values), then offers sub-commands, the completer's
  candidates, or option names when the word starts with '-'. Sub-commands
  are not offered while an attribute's value is being completed, and
  nothing is offered while an option's value is under the cursor.
• completeArgs added to every config/delete command with a static
  vocabulary (arp, interface and its sub-commands, load-balancing, copp,
  traffic-policy, acl, tunnel, qos, bgp, static routes, l2, ptp, vlan
  tagging, dhcp, srv6). Free-form-only commands (names, ids, prefixes)
  opt out by not declaring it.
• The bash script now passes the word under the cursor, completes
  fboss2-dev as well as fboss2, and asks the binary being completed.
  package.py ships it under share/bash-completion and setup_fboss_env
  sources it in interactive shells (before its idempotency guard, since
  completion specs are not inherited by child shells; bash-completion's
  lazy loader would not look under /opt/fboss anyway). The image now also
  bundles the bash-completion package.

Test Plan

Unit tests: ArgCompletionTest covers the three grammars; CmdCompletionTest
completes against the real fboss2-dev command trees (it lives in cmd_test,
which links the config command tree).

fboss2_framework_test
[  PASSED  ] 42 tests.
fboss2_cmd_test --gtest_filter='CmdCompletionTest.*:CmdListConfigTest.*'
[  PASSED  ] 9 tests.
fboss2_cmd_config_test
[  PASSED  ] 1162 tests.

On an NH-4010-F test switch, installed the new fboss2-dev, the completion
script under /opt/fboss/share/bash-completion/completions/fboss2 and the
new setup_fboss_env, then checked that a login shell registers the
completions:

$ bash -lic 'complete -p fboss2-dev fboss2'
complete -F _fboss2_complete fboss2-dev
complete -F _fboss2_complete fboss2

and pressed TAB in an interactive shell for a range of command lines.

Sample outputs

Captured from an interactive bash login shell on the
switch (80 columns, <TAB> marks where TAB was pressed). Leaf attributes,
attribute values, two-word BGP attributes and options all complete; a
single match is filled in directly:

$ fboss2-dev config arp <TAB>
age-interval    max-probes      stale-interval  timeout

$ fboss2-dev config arp ti<TAB>
$ fboss2-dev config arp timeout

$ fboss2-dev config arp timeout 30 <TAB>
age-interval    max-probes      stale-interval

$ fboss2-dev config interface eth1/1/1 <TAB>
description                lookup-class
dhcp                       loopback-mode
flow-control-rx            mtu
flow-control-tx            name
ip-address                 no-shutdown
ipv6                       pfc-config
ipv6-address               profile
lldp-expected-chassis      queue-config
lldp-expected-port-desc    sflow
lldp-expected-system-desc  shutdown
lldp-expected-system-name  switchport
lldp-expected-ttl          type
lldp-expected-value

$ fboss2-dev config interface eth1/1/1 loopback-mode <TAB>
MAC   NIF   none  PHY

$ fboss2-dev config interface eth1/1/1 sflow sample-dest <TAB>
cpu     mirror

$ fboss2-dev config copp reason <TAB>
arp                 eapol               mpls-unknown-label
arp-response        host-miss           ndp
bgp                 l3-dest-miss        port-mtu-error
bgpv6               l3-mtu-error        samplepacket
bpdu                l3-slow-path        ttl-0
cpu-is-nhop         lacp                ttl-1
dhcp                lldp                unmatched
dhcpv6              mpls-ttl-1

$ fboss2-dev config copp queue 1 scheduling <TAB>
DEFICIT_ROUND_ROBIN   SP                    WRR
DRR                   STRICT_PRIORITY
INTERNAL              WEIGHTED_ROUND_ROBIN

$ fboss2-dev config protocol bgp neighbor 1.2.3.4 timers <TAB>
hold-time              out-delay
keepalive              withdraw-unprog-delay

$ fboss2-dev show interface --<TAB>
--agent-port       --detail           --fsdb-port        --qsfp-port
--aggregate        --file             --help
--aggregate-hosts  --filter           --host
--color            --fmt              --loglevel

[benoit-nexthop]

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