Ref property application can fail when pointed-to instances don't yet exist

[kennethloeffler]

This issue is related to the order of addition operations within the Rojo plugin's reconciler:

rojo/plugin/src/Reconciler/applyPatch.lua

Lines 40 to 79 in 5c4260f

	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

There's a problem at line 73. In this loop, we reify each incoming added instance one after the other, thereby creating each one with Instance.new and applying any properties (which includes Refs). It's possible to break this by having two sibling instances, and creating a Ref property on one that points to the other:

{
  "name": "big-bad-refs",
  "tree": {
    "$className": "DataModel",
    "ReplicatedStorage": {
      "MyCoolPart": {
        "$className": "Part",
        "$id": "MyCoolPart"
      },
      "ObjectValue": {
        "$className": "ObjectValue",
        "$attributes": {
          "Rojo_Target_Value": "MyCoolPart"
        }
      }
    }
  }
}

When I sync this project into Roblox Studio, it fails to apply ObjectValue.Value. If I disconnect and reconnect, then the property applies correctly, suggesting that the cause is related to the order of operations (i.e. MyCoolPart doesn't yet exist at the time at the time of the ObjectValue's reification). For correctness, we should be creating all added instances before attempting to apply Ref properties. We'll probably need to decouple instance creation and property application in this case.

Originally posted by @kennethloeffler in #843 (comment)