urbit/pkg/arvo/app/link-proxy-hook.hoon

::  link-proxy-hook: make local pages available to foreign ships
::
::    this is a "proxy" style hook, relaying foreign subscriptions into local
::    stores if permission conditions are met.
::    the patterns herein should one day be generalized into a proxy-hook lib.
::
::    this uses metadata-store to discover resources and their associated
::    groups. it sets the permission condition to be that a ship must be in a
::    group associated with the resource it's subscribing to.
::    we check this on-watch, but also subscribe to metadata & groups so that
::    we can kick subscriptions if needed (eg ship removed from group).
::
::    we deduplicate incoming subscriptions on the same path, ensuring we have
::    exactly one local subscription per unique incoming subscription path.
::    this comes at the cost of assuming that the store's initial response is
::    whatever's returned by the scry at that path, but perhaps that should
::    become part of the stores standard anyway.
::
::    when adding support for new paths, the only things you'll likely want
::    to touch are +permitted, +initial-response, & +kick-proxies.
::
/-  *metadata-store, *group
/+  *link, metadata, default-agent, verb, dbug, group-store, grpl=group,
    resource
~%  %link-proxy-hook-top  ..is  ~
|%
+$  state-0
  $:  %0
      ::TODO  we use this to detect "first sub started" and "last sub left",
      ::      but can't we use [wex sup]:bowl for that?
      active=(map path (set ship))
  ==
::
+$  card  card:agent:gall
--
::
=|  state-0
=*  state  -
::
%-  agent:dbug
%+  verb  |
^-  agent:gall
=<
  |_  =bowl:gall
  +*  this  .
      do    ~(. +> bowl)
      def   ~(. (default-agent this %&) bowl)
  ::
  ++  on-init
    ^-  (quip card _this)
    :_  this
    ~[watch-groups:do watch-metadata:do]
  ::
  ++  on-save  !>(state)
  ++  on-load
    |=  old=vase
    ^-  (quip card _this)
    [~ this(state !<(state-0 old))]
  ::
  ++  on-watch
    |=  =path
    ^-  (quip card _this)
    ::  the local ship should just use link-store directly
    ::TODO  do we want to allow this anyway, to avoid client-side target checks?
    ::
    ?<  (team:title [our src]:bowl)
    ?>  (permitted:do src.bowl path)
    =^  cards  state
      (start-proxy:do src.bowl path)
    [cards this]
  ::
  ++  on-leave
    |=  =path
    ^-  (quip card _this)
    =^  cards  state
      (stop-proxy:do src.bowl path)
    [cards this]
  ::
  ++  on-agent
    |=  [=wire =sign:agent:gall]
    ^-  (quip card _this)
    ?:  ?=([%groups ~] wire)
      =^  cards  state
        (take-groups-sign:do sign)
      [cards this]
    ?:  ?=([%proxy ^] wire)
      =^  cards  state
        (handle-proxy-sign t.wire sign)
      [cards this]
    ~|  [dap.bowl %weird-wire wire]
    !!
  ::
  ++  on-poke  on-poke:def
  ++  on-peek  on-peek:def
  ++  on-arvo  on-arvo:def
  ++  on-fail  on-fail:def
  --
::
|_  =bowl:gall
+*  md  ~(. metadata bowl)
   grp   ~(. grpl bowl)
::
::  permissions
::
++  permitted
  |=  [who=ship =path]
  ^-  ?
  ::  we only expose /local-pages and /annotations,
  ::  to ships in the groups associated with the resource.
  ::  (no url-specific annotations subscriptions, either.)
  ::
  =/  target=(unit ^path)
    ?:  ?=([%local-pages ^] path)
      `t.path
    ?:  ?=([%annotations ~ ^] path)
      `t.t.path
    ~
  ?~  target  |
  %+  lien  (groups-from-resource:md %link u.target)
  |=  =group-path
  ^-  ?
  (~(has in (members-from-path:grp group-path)) who)
::
++  kick-revoked-permissions
  |=  [=path who=(list ship)]
  ^-  (list card)
  %+  murn  who
  |=  =ship
  ^-  (unit card)
  ::  no need to remove to ourselves
  ::
  ?:  =(our.bowl ship)  ~
  ?:  (permitted ship path)  ~
  `(kick-proxies ship path)
::
::  metadata subscription
::
++  watch-metadata
  ^-  card
  [%pass /metadata %agent [our.bowl %metadata-store] %watch /app-name/link]
::
++  take-metadata-sign
  |=  =sign:agent:gall
  ^-  (quip card _state)
  ?-  -.sign
    %poke-ack     ~|([dap.bowl %unexpected-poke-ack /metadata] !!)
    %kick         [[watch-metadata]~ state]
  ::
      %watch-ack
    ?~  p.sign  [~ state]
    =/  =tank
      :-  %leaf
      "{(trip dap.bowl)} failed subscribe to metadata store. very wrong!"
    %-  (slog tank u.p.sign)
    [~ state]
  ::
      %fact
    =*  mark  p.cage.sign
    =*  vase  q.cage.sign
    ?.  ?=(%metadata-update mark)
      ~|  [dap.bowl %unexpected-mark mark]
      !!
    %-  handle-metadata-update
    !<(metadata-update vase)
  ==
::
++  handle-metadata-update
  |=  upd=metadata-update
  ^-  (quip card _state)
  :_  state
  ?.  ?=(%remove -.upd)  ~
  ?>  =(%link app-name.resource.upd)
  ::  if a group is no longer associated with a resource,
  ::  we need to re-check permissions for everyone in that group.
  ::
  %+  kick-revoked-permissions
    app-path.resource.upd
  %~  tap  in
  (members-from-path:grp group-path.upd)
::
::  groups subscription
::TODO  largely copied from link-listen-hook. maybe make a store-listener lib?
::
++  watch-groups
  ^-  card
  [%pass /groups %agent [our.bowl %group-store] %watch /groups]
::
++  take-groups-sign
  |=  =sign:agent:gall
  ^-  (quip card _state)
  ?-  -.sign
    %poke-ack   ~|([dap.bowl %unexpected-poke-ack /groups] !!)
    %kick       [[watch-groups]~ state]
  ::
      %watch-ack
    ?~  p.sign  [~ state]
    =/  =tank
      :-  %leaf
      "{(trip dap.bowl)} failed subscribe to group store. very wrong!"
    %-  (slog tank u.p.sign)
    [~ state]
  ::
      %fact
    =*  mark  p.cage.sign
    =*  vase  q.cage.sign
    ?+  mark  ~|([dap.bowl %unexpected-mark mark] !!)
      %group-initial  [~ state]
      %group-update   (handle-group-update !<(update:group-store vase))
    ==
  ==
::
++  handle-group-update
  |=  upd=update:group-store
  ^-  (quip card _state)
  :_  state
  ?.  ?=(%remove-members -.upd)  ~
  ::  if someone was removed from a group, find all link resources associated
  ::  with that group, then kick their subscriptions if they're no longer
  ::
  %-  zing
  %+  turn  (app-paths-from-group:md %link (en-path:resource resource.upd))
  |=  =app-path
  ^-  (list card)
  %+  kick-revoked-permissions
    app-path
  ~(tap in ships.upd)
::
::  proxy subscriptions
::
++  kick-proxies
  |=  [who=ship =path]
  ^-  card
  =-  [%give %kick - `who]
  :~  [%local-pages path]
      [%annotations %$ path]
  ==
::
++  handle-proxy-sign
  |=  [=wire =sign:agent:gall]
  ^-  (quip card _state)
  ?-  -.sign
    %poke-ack     ~|([dap.bowl %unexpected-poke-ack wire] !!)
    %fact         [[%give %fact ~[wire] cage.sign]~ state]
    %kick         [[(proxy-pass-link-store wire %watch wire)]~ state]
  ::
      %watch-ack
    ?~  p.sign  [~ state]
    =/  =tank
      :-  %leaf
      "{(trip dap.bowl)} failed subscribe to link-store. very wrong!"
    %-  (slog tank u.p.sign)
    [~ state]
  ==
::
++  proxy-pass-link-store
  |=  [=path =task:agent:gall]
  ^-  card
  :*  %pass
      [%proxy path]
      %agent
      [our.bowl %link-store]
      task
  ==
::
++  initial-response
  |=  =path
  ^-  card
  =;  =initial
    [%give %fact ~ %link-initial !>(initial)]
  ?+  path  !!
      [%local-pages ^]
    [%local-pages (scry-for (map ^path pages) %link-store path)]
  ::
      [%annotations %$ ^]
    [%annotations (scry-for (per-path-url notes) %link-store path)]
  ==
::
++  start-proxy
  |=  [who=ship =path]
  ^-  (quip card _state)
  :_  state(active (~(put ju active) path who))
  :_  ~
  ::  if we already have a local subscription open,
  ::
  ?.  =(~ (~(get ju active) path))
    ::  gather the initial response ourselves, and send that.
    ::
    (initial-response path)
  ::  else, open a local subscription,
  ::  sending outward its initial response when we hear it.
  ::
  (proxy-pass-link-store path %watch path)
::
++  stop-proxy
  |=  [who=ship =path]
  ^-  (quip card _state)
  =.  active  (~(del ju active) path who)
  :_  state
  ::  if there are still subscriptions remaining, do nothing.
  ::
  ?.  =(~ (~(get ju active) path))  ~
  ::  else, close the local subscription.
  ::
  [(proxy-pass-link-store path %leave ~)]~
::
::  helpers
::
++  scry-for
  |*  [=mold =app-name =path]
  .^  mold
    %gx
    (scot %p our.bowl)
    app-name
    (scot %da now.bowl)
    (snoc `^path`path %noun)
  ==
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