Debugging Wrapper • Guides • developers.urbit.org

The /lib/dbug.hoon agent wrapper adds support to view the state of a Gall agent. It is applied to an existing Gall agent as a single drop-in line, %- agent:dbug.

Before we look at the code, let's consider the functionality it exposes. By supplying %- agent:dbug, an associated +dbug generator can be invoked against the agent state.

For instance, using the %azimuth agent, we can expose the current state of the agent:

> :azimuth +dbug
[ %7
  url=~.
  net=%default
  refresh=~m5
  whos={}
  nas=[%0 points={} operators={} dns=<||>]
  own={}
  spo={}
  logs=~
  sap=[%0 id=[hash=0x0 number=0] nas=[%0 points={} operators={} dns=<||>] owners={} sponsors={}]
]

> :azimuth +dbug %bowl
>   [ [our=~zod src=~zod dap=%azimuth]
  [wex={} sup={}]
  act=3
    eny
  0v1rn.n49dr.2u8t5.h7be5.6dcq7.9hon5.6m3pr.3hcb8.u7tmv.qddpq.kent7.1ftc7.9tao6.hfsht.4i0c3.ak3t7.t8d8j.nn4eb.b7eh3.4d5pr.t8ftg
  now=~2023.2.3..20.03.23..f60e
  byk=[p=~zod q=%base r=[%da p=~2023.1.26..02.41.25..926a]]
]

> :azimuth +dbug [%incoming ~]
>   no matching subscriptions

> :azimuth +dbug [%state '(lent whos)']
>   0

There are four actions exposed by the wrapper via the +dbug generator:

:app +dbug exposes the entire state, just dumping the current agent state.

:app +dbug %bowl shows the agent's bowl. The Gall bowl consists of:

+$  bowl              ::  standard app state
  $:  $:  our=ship    ::  host
          src=ship    ::  guest
          dap=term    ::  agent
      ==              ::
      $:  wex=boat    ::  outgoing subs
          sup=bitt    ::  incoming subs
      ==              ::
      $:  act=@ud     ::  change number
          eny=@uvJ    ::  entropy
          now=@da     ::  current time
          byk=beak    ::  load source
  ==  ==              ::

:app +dbug [%state 'hoon'] exposes data in the state, including evaluated Hoon like (lent values).
:app +dbug [?(%incoming outgoing) specifics] reveals details about the subscriptions.

The Code

/gen/dbug.hoon:

Click to expand

/+  *dbug
:-  %say
|=  $:  ::  environment
        *
        ::  inline arguments
        args=?(~ [what=?(%bowl %state) ~] [=poke ~])
        ::  named arguments
        ~
    ==
:-  %dbug
?-  args
  ~          [%state '']
  [@ ~]      ?-(what.args %bowl [%bowl ~], %state [%state ''])
  [[@ *] ~]  poke.args
==

/lib/dbug.hoon:

Click to expand

::  dbug: agent wrapper for generic debugging tools
::
::    usage: %-(agent:dbug your-agent)
::
|%
+$  poke
  $%  [%bowl ~]
      [%state grab=cord]
      [%incoming =about]
      [%outgoing =about]
  ==
::
+$  about
  $@  ~
  $%  [%ship =ship]
      [%path =path]
      [%wire =wire]
      [%term =term]
  ==
::
++  agent
  |=  =agent:gall
  ^-  agent:gall
  !.
  |_  =bowl:gall
  +*  this  .
      ag    ~(. agent bowl)
  ::
  ++  on-poke
    |=  [=mark =vase]
    ^-  (quip card:agent:gall agent:gall)
    ?.  ?=(%dbug mark)
      =^  cards  agent  (on-poke:ag mark vase)
      [cards this]
    =/  dbug
      !<(poke vase)
    =;  =tang
      ((%*(. slog pri 1) tang) [~ this])
    ?-  -.dbug
      %bowl   [(sell !>(bowl))]~
    ::
        %state
      =?  grab.dbug  =('' grab.dbug)  '-'
      =;  product=^vase
        [(sell product)]~
      =/  state=^vase
        ::  if the underlying app has implemented a /dbug/state scry endpoint,
        ::  use that vase in place of +on-save's.
        ::
        =/  result=(each ^vase tang)
          (mule |.(q:(need (need (on-peek:ag /x/dbug/state)))))
        ?:(?=(%& -.result) p.result on-save:ag)
      %+  slap
        (slop state !>([bowl=bowl ..zuse]))
      (ream grab.dbug)
    ::
        %incoming
      =;  =tang
        ?^  tang  tang
        [%leaf "no matching subscriptions"]~
      %+  murn
        %+  sort  ~(tap by sup.bowl)
        |=  [[* a=[=ship =path]] [* b=[=ship =path]]]
        (aor [path ship]:a [path ship]:b)
      |=  [=duct [=ship =path]]
      ^-  (unit tank)
      =;  relevant=?
        ?.  relevant  ~
        `>[path=path from=ship duct=duct]<
      ?:  ?=(~ about.dbug)  &
      ?-  -.about.dbug
        %ship  =(ship ship.about.dbug)
        %path  ?=(^ (find path.about.dbug path))
        %wire  %+  lien  duct
               |=(=wire ?=(^ (find wire.about.dbug wire)))
        %term  !!
      ==
    ::
        %outgoing
      =;  =tang
        ?^  tang  tang
        [%leaf "no matching subscriptions"]~
      %+  murn
        %+  sort  ~(tap by wex.bowl)
        |=  [[[a=wire *] *] [[b=wire *] *]]
        (aor a b)
      |=  [[=wire =ship =term] [acked=? =path]]
      ^-  (unit tank)
      =;  relevant=?
        ?.  relevant  ~
        `>[wire=wire agnt=[ship term] path=path ackd=acked]<
      ?:  ?=(~ about.dbug)  &
      ?-  -.about.dbug
        %ship  =(ship ship.about.dbug)
        %path  ?=(^ (find path.about.dbug path))
        %wire  ?=(^ (find wire.about.dbug wire))
        %term  =(term term.about.dbug)
      ==
    ==
  ::
  ++  on-peek
    |=  =path
    ^-  (unit (unit cage))
    ?.  ?=([@ %dbug *] path)
      (on-peek:ag path)
    ?+  path  [~ ~]
      [%u %dbug ~]                 ``noun+!>(&)
      [%x %dbug %state ~]          ``noun+!>(on-save:ag)
      [%x %dbug %subscriptions ~]  ``noun+!>([wex sup]:bowl)
    ==
  ::
  ++  on-init
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  on-init:ag
    [cards this]
  ::
  ++  on-save   on-save:ag
  ::
  ++  on-load
    |=  old-state=vase
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  (on-load:ag old-state)
    [cards this]
  ::
  ++  on-watch
    |=  =path
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  (on-watch:ag path)
    [cards this]
  ::
  ++  on-leave
    |=  =path
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  (on-leave:ag path)
    [cards this]
  ::
  ++  on-agent
    |=  [=wire =sign:agent:gall]
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  (on-agent:ag wire sign)
    [cards this]
  ::
  ++  on-arvo
    |=  [=wire =sign-arvo]
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  (on-arvo:ag wire sign-arvo)
    [cards this]
  ::
  ++  on-fail
    |=  [=term =tang]
    ^-  (quip card:agent:gall agent:gall)
    =^  cards  agent  (on-fail:ag term tang)
    [cards this]
  --
--

As we examine this code, there are two particularly interesting aspects:

How /lib/dbug.hoon modifies an agent's arms by adding functionality over the top of them.
How /gen/dbug.hoon utilizes the modified arms with an elegant and simple invocation.

There is also extensive use of tank/tang formatted error messaging.

How the library works

By applying this door builder using %- censig, the ++on-poke and ++on-peek arms can be modified. (In fact, all of the arms can be modified but most of the arms are pass-throughs to the modified agent.)

`++on-poke`

Click to expand

++  on-poke
    |=  [=mark =vase]
    ^-  (quip card:agent:gall agent:gall)
    ?.  ?=(%dbug mark)
      =^  cards  agent  (on-poke:ag mark vase)
      [cards this]
    =/  dbug
      !<(poke vase)
    =;  =tang
      ((%*(. slog pri 1) tang) [~ this])
    ?-  -.dbug
      %bowl   [(sell !>(bowl))]~
    ::
        %state
      =?  grab.dbug  =('' grab.dbug)  '-'
      =;  product=^vase
        [(sell product)]~
      =/  state=^vase
        ::  if the underlying app has implemented a /dbug/state scry endpoint,
        ::  use that vase in place of +on-save's.
        ::
        =/  result=(each ^vase tang)
          (mule |.(q:(need (need (on-peek:ag /x/dbug/state)))))
        ?:(?=(%& -.result) p.result on-save:ag)
      %+  slap
        (slop state !>([bowl=bowl ..zuse]))
      (ream grab.dbug)
    ::
        %incoming
      =;  =tang
        ?^  tang  tang
        [%leaf "no matching subscriptions"]~
      %+  murn
        %+  sort  ~(tap by sup.bowl)
        |=  [[* a=[=ship =path]] [* b=[=ship =path]]]
        (aor [path ship]:a [path ship]:b)
      |=  [=duct [=ship =path]]
      ^-  (unit tank)
      =;  relevant=?
        ?.  relevant  ~
        `>[path=path from=ship duct=duct]<
      ?:  ?=(~ about.dbug)  &
      ?-  -.about.dbug
        %ship  =(ship ship.about.dbug)
        %path  ?=(^ (find path.about.dbug path))
        %wire  %+  lien  duct
               |=(=wire ?=(^ (find wire.about.dbug wire)))
        %term  !!
      ==
    ::
        %outgoing
      =;  =tang
        ?^  tang  tang
        [%leaf "no matching subscriptions"]~
      %+  murn
        %+  sort  ~(tap by wex.bowl)
        |=  [[[a=wire *] *] [[b=wire *] *]]
        (aor a b)
      |=  [[=wire =ship =term] [acked=? =path]]
      ^-  (unit tank)
      =;  relevant=?
        ?.  relevant  ~
        `>[wire=wire agnt=[ship term] path=path ackd=acked]<
      ?:  ?=(~ about.dbug)  &
      ?-  -.about.dbug
        %ship  =(ship ship.about.dbug)
        %path  ?=(^ (find path.about.dbug path))
        %wire  ?=(^ (find wire.about.dbug wire))
        %term  =(term term.about.dbug)
      ==
    ==

The ++on-poke arm has several branches added to it after a check to see whether it is being used through the +dbug generator. If it isn't (as determined by the associated mark), then the poke is passed through to the base agent.

?.  ?=(%dbug mark)
  =^  cards  agent  (on-poke:ag mark vase)
  [cards this]

The following ?- wuthep handles the input arguments: %state is the most interesting code in this library. The code first checks whether the base agent has a /dbug/state peek endpoint already (in which case it passes it through), otherwise it evaluates the requested Hoon expression against the agent's state (obtained via ++on-save:ag).

  %state
=?  grab.dbug  =('' grab.dbug)  '-'
=;  product=^vase
  [(sell product)]~
=/  state=^vase
  ::  if the underlying app has implemented a /dbug/state scry endpoint,
  ::  use that vase in place of +on-save's.
  ::
  =/  result=(each ^vase tang)
    (mule |.(q:(need (need (on-peek:ag /x/dbug/state)))))
  ?:(?=(%& -.result) p.result on-save:ag)
%+  slap
  (slop state !>([bowl=bowl ..zuse]))
(ream grab.dbug)

This branch includes the use of a rare =? tiswut conditional leg change and the reversed =/ tisfas, =; tismic. There is also some direct compilation of cords taking place:

++sell is a vase pretty-printer.
++slop conses two vases together as a cell.
++slap compiles a Hoon expression and produces a vase of the result.
++ream parses a cord to a Hoon expression.

`++on-peek`

Click to expand

  ++  on-peek
    |=  =path
    ^-  (unit (unit cage))
    ?.  ?=([@ %dbug *] path)
      (on-peek:ag path)
    ?+  path  [~ ~]
      [%u %dbug ~]                 ``noun+!>(&)
      [%x %dbug %state ~]          ``noun+!>(on-save:ag)
      [%x %dbug %subscriptions ~]  ``noun+!>([wex sup]:bowl)
    ==

The ++on-peek arm adds several peek endpoints which expose the state (via ++onsave:ag) and the subscriptions.

> .^(noun %gx /(scot %p our)/azimuth/(scot %da now)/dbug/subscriptions/noun)
[0 0]

How the generator works

The generator explicitly injects the %dbug mark in its return cask ([mark noun]). This is a valid if uncommon operation, and it works here because the mark is never used as a transforming gate but only as a marker to see whether the arms need to pass through the values. The no-argument input is routed through the %state with an empty cord.

:-  %dbug
?-  args
  ~          [%state '']
  [@ ~]      ?-(what.args %bowl [%bowl ~], %state [%state ''])
  [[@ *] ~]  poke.args
==

Library authors should consider augmenting developer capabilities by exposing appropriate functionality using a wrapper agent similar to /lib/dbug.