Iterate on plugin reconciler

- Renamed setProperty to setCanonicalProperty, which is more usefully
  descriptive. Also added a detailed comment.
- Fixed reconciler behavior with regards to removing known instances
  when $ignoreUnknownInstances is set

plugin/src/setCanonicalProperty.lua

@@ -0,0 +1,52 @@
+local Logging = require(script.Parent.Logging)
+
+--[[
+	Attempts to set a property on the given instance.
+
+	This method deals in terms of what Rojo calls 'canonical properties', which
+	don't necessarily exist either in serialization or in Lua-reflected APIs,
+	but may be present in the API dump.
+
+	Ideally, canonical properties map 1:1 with properties we can assign, but in
+	some cases like LocalizationTable contents and CollectionService tags, we
+	have to read/write properties a little differently.
+]]
+local function setCanonicalProperty(instance, key, value)
+	-- The 'Contents' property of LocalizationTable isn't directly exposed, but
+	-- has corresponding (deprecated) getters and setters.
+	if instance.ClassName == "LocalizationTable" and key == "Contents" then
+		instance:SetContents(value)
+		return
+	end
+
+	-- If we don't have permissions to access this value at all, we can skip it.
+	local readSuccess, existingValue = pcall(function()
+		return instance[key]
+	end)
+
+	if not readSuccess then
+		-- An error will be thrown if there was a permission issue or if the
+		-- property doesn't exist. In the latter case, we should tell the user
+		-- because it's probably their fault.
+		if existingValue:find("lacking permission") then
+			Logging.trace("Permission error reading property %s on class %s", tostring(key), instance.ClassName)
+			return
+		else
+			error(("Invalid property %s on class %s: %s"):format(tostring(key), instance.ClassName, existingValue), 2)
+		end
+	end
+
+	local writeSuccess, err = pcall(function()
+		if existingValue ~= value then
+			instance[key] = value
+		end
+	end)
+
+	if not writeSuccess then
+		error(("Cannot set property %s on class %s: %s"):format(tostring(key), instance.ClassName, err), 2)
+	end
+
+	return true
+end
+
+return setCanonicalProperty