Break apart plugin reconciler (#332)

* Start splitting apart reconciler, with tests

* Reify children in reify

* Baseline hydrate implementation

* Remove debug print

* Scaffold out diff implementation, just supporting name changes

* invariant -> error in decodeValue

* Flesh out diff and add getProperty

* Clear out top-level reconciler interface, start updating code that touches it

* Address review feedback

* Add (experimental) Selene configuration

* Add emptiness checks to PatchSet, remove unimplement invert method

* Improve descendant destruction behavior in InstanceMap

* Track instanceId on all reify errors

* Base implementation of applyPatch, returning partial patches on failure

* Change reify to accept InstanceMap and insert instances into it

* Start testing applyPatch for removals

* Add test for applyPatch adding instances successfully and not

* Add , which is just error with formatting

* Correctly use new diff and applyPatch APIs

* Improve applyPatch logging and fix field name typo

* Better debug output when reify fails

* Print out unapplied patch in debug mode

* Don't write properties if their values are not different.

This was exposed trying to sync the Rojo plugin, which
has a gigantic ModuleScript in it with the reflection
database. This workaround was present in some form in
many versions of Rojo, and I guess we still need it.

This time, I actually documented why it's here so that
I don't forget for the umpteenth time...

* Add placeholder test that needs to happen still

* Introduce easier plugin testing, write applyPatch properties test

* Delete legacy get/setCanonicalProperty files

* Fix trying to remove numbers instead of instances

* Change applyPatch to return partial patches instead of binary success

* Work towards being able to decode and apply refs

* Add helpers for PatchSet assertions

* Apply refs in reify, test all cases

* Improve diagnostics when patches fail to apply

* Stop logging when destroying untracked instances, it's ok

* Remove read before setting property in applyPatch

* Fix diff thinking all properties are changed

plugin/src/ServeSession.lua

@@ -5,6 +5,7 @@ local Fmt = require(script.Parent.Parent.Fmt)
 local t = require(script.Parent.Parent.t)
 
 local InstanceMap = require(script.Parent.InstanceMap)
+local PatchSet = require(script.Parent.PatchSet)
 local Reconciler = require(script.Parent.Reconciler)
 local strict = require(script.Parent.strict)
 
@@ -214,22 +215,36 @@ function ServeSession:__initialSync(rootInstanceId)
 			-- the tree defined in this response.
 			self.__apiContext:setMessageCursor(readResponseBody.messageCursor)
 
-			Log.trace("Computing changes that plugin needs to make to catch up to server...")
+			-- For any instances that line up with the Rojo server's view, start
+			-- tracking them in the reconciler.
+			Log.trace("Matching existing Roblox instances to Rojo IDs")
+			self.__reconciler:hydrate(readResponseBody.instances, rootInstanceId, game)
 
 			-- Calculate the initial patch to apply to the DataModel to catch us
 			-- up to what Rojo thinks the place should look like.
-			local hydratePatch = self.__reconciler:hydrate(
+			Log.trace("Computing changes that plugin needs to make to catch up to server...")
+			local success, catchUpPatch = self.__reconciler:diff(
 				readResponseBody.instances,
 				rootInstanceId,
 				game
 			)
 
-			Log.trace("Computed hydration patch: {:#?}", debugPatch(hydratePatch))
+			if not success then
+				Log.error("Could not compute a diff to catch up to the Rojo server: {:#?}", catchUpPatch)
+			end
+
+			Log.trace("Computed hydration patch: {:#?}", debugPatch(catchUpPatch))
 
 			-- TODO: Prompt user to notify them of this patch, since it's
-			-- effectively a conflict between the Rojo server and the client.
+			-- effectively a conflict between the Rojo server and the client. In
+			-- the future, we'll ask which changes the user wants to keep.
 
-			self.__reconciler:applyPatch(hydratePatch)
+			local unappliedPatch = self.__reconciler:applyPatch(catchUpPatch)
+
+			if not PatchSet.isEmpty(unappliedPatch) then
+				Log.warn("Could not apply all changes requested by the Rojo server:\n{}",
+					PatchSet.humanSummary(self.__instanceMap, unappliedPatch))
+			end
 		end)
 end
 
@@ -237,7 +252,12 @@ function ServeSession:__mainSyncLoop()
 	return self.__apiContext:retrieveMessages()
 		:andThen(function(messages)
 			for _, message in ipairs(messages) do
-				self.__reconciler:applyPatch(message)
+				local unappliedPatch = self.__reconciler:applyPatch(message)
+
+				if not PatchSet.isEmpty(unappliedPatch) then
+					Log.warn("Could not apply all changes requested by the Rojo server:\n{}",
+						PatchSet.humanSummary(self.__instanceMap, unappliedPatch))
+				end
 			end
 
 			if self.__status ~= Status.Disconnected then