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/Reconciler/hydrate.lua

@@ -0,0 +1,50 @@
+--[[
+	Defines the process of "hydration" -- matching up a virtual DOM with
+	concrete instances and assigning them IDs.
+]]
+
+local invariant = require(script.Parent.Parent.invariant)
+
+local function hydrate(instanceMap, virtualInstances, rootId, rootInstance)
+	local virtualInstance = virtualInstances[rootId]
+
+	if virtualInstance == nil then
+		invariant("Cannot hydrate an instance not present in virtualInstances\nID: {}", rootId)
+	end
+
+	instanceMap:insert(rootId, rootInstance)
+
+	local existingChildren = rootInstance:GetChildren()
+
+	-- For each existing child, we'll track whether it's been paired with an
+	-- instance that the Rojo server knows about.
+	local isExistingChildVisited = {}
+	for i = 1, #existingChildren do
+		isExistingChildVisited[i] = false
+	end
+
+	for _, childId in ipairs(virtualInstance.Children) do
+		local virtualChild = virtualInstances[childId]
+
+		for childIndex, childInstance in ipairs(existingChildren) do
+			if not isExistingChildVisited[childIndex] then
+				-- We guard accessing Name and ClassName in order to avoid
+				-- tripping over children of DataModel that Rojo won't have
+				-- permissions to access at all.
+				local ok, name, className = pcall(function()
+					return childInstance.Name, childInstance.ClassName
+				end)
+
+				-- This rule is very conservative and could be loosened in the
+				-- future, or more heuristics could be introduced.
+				if ok and name == virtualChild.Name and className == virtualChild.ClassName then
+					isExistingChildVisited[childIndex] = true
+					hydrate(instanceMap, virtualInstances, childId, childInstance)
+					break
+				end
+			end
+		end
+	end
+end
+
+return hydrate