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/applyPatch.lua

@@ -0,0 +1,131 @@
+--[[
+	Apply a patch to the DOM. Returns any portions of the patch that weren't
+	possible to apply.
+
+	Patches can come from the server or be generated by the client.
+]]
+
+local Log = require(script.Parent.Parent.Parent.Log)
+
+local PatchSet = require(script.Parent.Parent.PatchSet)
+local Types = require(script.Parent.Parent.Types)
+local invariant = require(script.Parent.Parent.invariant)
+
+local decodeValue = require(script.Parent.decodeValue)
+local getProperty = require(script.Parent.getProperty)
+local reify = require(script.Parent.reify)
+local setProperty = require(script.Parent.setProperty)
+
+local function applyPatch(instanceMap, patch)
+	-- Tracks any portions of the patch that could not be applied to the DOM.
+	local unappliedPatch = PatchSet.newEmpty()
+
+	for _, removedIdOrInstance in ipairs(patch.removed) do
+		if Types.RbxId(removedIdOrInstance) then
+			instanceMap:destroyId(removedIdOrInstance)
+		else
+			instanceMap:destroyInstance(removedIdOrInstance)
+		end
+	end
+
+	for id, virtualInstance in pairs(patch.added) do
+		if instanceMap.fromIds[id] ~= nil then
+			-- This instance already exists. We might've already added it in a
+			-- previous iteration of this loop, or maybe this patch was not
+			-- supposed to list this instance.
+			--
+			-- It's probably fine, right?
+			continue
+		end
+
+		-- Find the first ancestor of this instance that is marked for an
+		-- addition.
+		--
+		-- This helps us make sure we only reify each instance once, and we
+		-- start from the top.
+		while patch.added[virtualInstance.Parent] ~= nil do
+			id = virtualInstance.Parent
+			virtualInstance = patch.added[id]
+		end
+
+		local parentInstance = instanceMap.fromIds[virtualInstance.Parent]
+
+		if parentInstance == nil then
+			-- This would be peculiar. If you create an instance with no
+			-- parent, were you supposed to create it at all?
+			invariant(
+				"Cannot add an instance from a patch that has no parent.\nInstance {} with parent {}.\nState: {:#?}",
+				id,
+				virtualInstance.Parent,
+				instanceMap
+			)
+		end
+
+		local failedToReify = reify(instanceMap, patch.added, id, parentInstance)
+
+		if not PatchSet.isEmpty(failedToReify) then
+			Log.debug("Failed to reify as part of applying a patch: {}", failedToReify)
+			PatchSet.assign(unappliedPatch, failedToReify)
+		end
+	end
+
+	for _, update in ipairs(patch.updated) do
+		local instance = instanceMap.fromIds[update.id]
+
+		if instance == nil then
+			-- We can't update an instance that doesn't exist.
+			-- TODO: Should this be an invariant?
+			continue
+		end
+
+		-- Track any part of this update that could not be applied.
+		local unappliedUpdate = {
+			id = update.id,
+			changedProperties = {},
+		}
+		local partiallyApplied = false
+
+		if update.changedClassName ~= nil then
+			-- TODO: Support changing class name by destroying + recreating.
+			unappliedUpdate.changedClassName = update.changedClassName
+			partiallyApplied = true
+		end
+
+		if update.changedName ~= nil then
+			instance.Name = update.changedName
+		end
+
+		if update.changedMetadata ~= nil then
+			-- TODO: Support changing metadata. This will become necessary when
+			-- Rojo persistently tracks metadata for each instance in order to
+			-- remove extra instances.
+			unappliedUpdate.changedMetadata = update.changedMetadata
+			partiallyApplied = true
+		end
+
+		if update.changedProperties ~= nil then
+			for propertyName, propertyValue in pairs(update.changedProperties) do
+				local ok, decodedValue = decodeValue(propertyValue, instanceMap)
+				if not ok then
+					unappliedUpdate.changedProperties[propertyName] = propertyValue
+					partiallyApplied = true
+					continue
+				end
+
+				local ok = setProperty(instance, propertyName, decodedValue)
+				if not ok then
+					unappliedUpdate.changedProperties[propertyName] = propertyValue
+					partiallyApplied = true
+				end
+			end
+		end
+
+		if partiallyApplied then
+			table.insert(unappliedPatch.updated, unappliedUpdate)
+		end
+	end
+
+	return unappliedPatch
+end
+
+return applyPatch