negotiate: support un/match notifications

Depending on the flag at the head of the sample given to the library, it
may poke the underlying agent with a %negotiate-notification whenever we
match or unmatch with a specific gill.

desk/lib/negotiate.hoon

@@ -9,6 +9,8 @@
 ::      usage
 ::
 ::    to use this library, you must supply it with three things:
+::    - a flag specifying whether the inner agent should be notified of version
+::      negotiation events (matching & unmatching of external agents)
 ::    - a map of, per protocol your agent exposes, a version noun
 ::    - a map of, per agent name, a map of, per protocol, the version we expect
 ::    call this library's +agent arm with those three arguments, and then call
@@ -38,6 +40,14 @@
 ::    where we now do have matching versions. in upgrade scenarios, changing
 ::    the library arguments should generally suffice.
 ::
+::    if the flag at the start of the sample is set to true then, whenever we
+::    start to match or stop matching with a specific gill, we send a poke to
+::    the inner agent, marked %negotiate-notification, containing both a flag
+::    indicating whether we now match, and the gill for which the notification
+::    applies.
+::    (the initial state, of not having negotiated at all, counts as "not
+::    matching".)
+::
 ::    if an agent was previously using epic, it can trivially upgrade into
 ::    this library by making the following changes:
 ::    - change its own epic version number
@@ -68,10 +78,8 @@
   |=  [=bowl:gall =gill:gall]
   ?=(%match (read-status bowl gill))
 ::
-::TODO  want to +notify-me to subscribe to self about heed changes
-::
 ++  agent
-  |=  [our-versions=(map protocol version) =our=config]
+  |=  [notify=? our-versions=(map protocol version) =our=config]
   ^-  $-(agent:gall agent:gall)
   |^  agent
   ::
@@ -102,6 +110,7 @@
     ::    negotiation where needed.
     ::
     ++  inflate
+      |=  knew=(unit config)
       ^-  (quip card _state)
       =*  boat=boat:gall  wex.bowl
       ::  establish subs from .want where versions match
@@ -149,6 +158,17 @@
           (~(put in kill) [wire gill])
         $(need t.need)
       ::
+      =/  notes=(list card)
+        ?.  notify  ~
+        ?~  knew    ~
+        %+  murn  ~(tap in `(set gill:gall)`(~(run in ~(key by heed)) head))
+        |=  =gill:gall
+        ^-  (unit card)
+        =/  did=?  (match(know u.knew) gill)
+        =/  now=?  (match gill)
+        ?:  =(did now)  ~
+        `(notify-inner now gill)
+      ::
       =^  inis  state
         =|  caz=(list card)
         =/  inz=(list [gill:gall protocol])  ~(tap in init)
@@ -157,6 +177,7 @@
         =^  car  state  (negotiate i.inz)
         $(caz (weld car caz), inz t.inz)
       :_  state
+      %+  weld  notes
       %+  weld  open
       %+  weld  inis
       %+  turn  ~(tap in kill)
@@ -269,14 +290,29 @@
       `[%give %fact [/~/negotiate/version/[protocol]]~ %noun !>(version)]
     ::
     ++  heed-changed
-      |=  [for=[gill:gall protocol] new=version]
+      |=  [for=[=gill:gall protocol] new=version]
       ^-  (quip card _state)
       =/  hav=(unit version)
         ~|  %unrequested-heed
         (~(got by heed) for)
       ?:  =(`new hav)  [~ state]
-      =.  heed  (~(put by heed) for `new)
-      inflate
+      =/  did=?  &(notify (match gill.for))
+      =.  heed   (~(put by heed) for `new)
+      ::  we may need to notify the inner agent
+      ::
+      =/  nos=(list card)
+        ?.  notify  ~
+        =/  now=?  (match gill.for)
+        ?:  =(did now)  ~
+        [(notify-inner now gill.for)]~
+      =^  caz  state  (inflate ~)
+      [(weld caz nos) state]
+    ::
+    ++  notify-inner
+      |=  event=[match=? =gill:gall]
+      ^-  card
+      :+  %pass  /~/negotiate/notify
+      [%agent [our dap]:bowl %poke %negotiate-notification !>(event)]
     ::
     ++  watch-version
       |=  [=gill:gall =protocol]
@@ -353,7 +389,7 @@
       ?.  ?=([[%negotiate *] *] q.ole)
         =.  ours   our-versions
         =.  know   our-config
-        =^  caz    state  inflate:up
+        =^  caz    state  (inflate:up ~)
         =^  cards  inner  (on-load:og ole)
         =^  cards  state  (play-cards:up cards)
         [(weld caz cards) this]
@@ -365,8 +401,9 @@
             ?:  =(ours our-versions)  ~
             (ours-changed:up ours our-versions)
           =.  ours   our-versions
+          =/  knew   know
           =.  know   our-config
-          =^  caz2   state  inflate:up
+          =^  caz2   state  (inflate:up `knew)
           =^  cards  inner  (on-load:og ile)
           =^  cards  state  (play-cards:up cards)
           [:(weld caz1 caz2 cards) this]
@@ -395,6 +432,8 @@
       ::
       ~|  wire=t.t.wire
       ?+  t.t.wire  ~|([%negotiate %unexpected-wire] !!)
+        [%notify ~]  [~ this]
+      ::
           [%heed @ @ @ ~]
         =/  for=[gill:gall protocol]
           =*  w  t.t.t.wire