Experimental warning to help find connections that are not disconnected

This commit introduces a new configuration option which aims to help Roact users more easily find components which are not properly disconnecting connections when unmounted and are potentially causing memory leaks because of this reason.

A full traversal of the component is performed to find all currently active connections but some keys are excluded to reduce false positives.

If a connection is referenced in a component but it's closure does not close over self as an upvalue then it is not a concern that the component still has a reference after being unmounted.
This commonly occurs through references to component context or the store. Ideally we would be able to inspect the closure associated with each active connection and warn only if the closure uses self as an upvalue.

Internal Roact connections are also ignored for this check, so that connections within EventManager do not cause this warning to be emitted.

src/Component.lua

@@ -2,6 +2,7 @@ local assign = require(script.Parent.assign)
 local ComponentLifecyclePhase = require(script.Parent.ComponentLifecyclePhase)
 local Type = require(script.Parent.Type)
 local Symbol = require(script.Parent.Symbol)
+local getActiveConnections = require(script.Parent.getActiveConnections)
 local invalidSetStateMessages = require(script.Parent.invalidSetStateMessages)
 local internalAssert = require(script.Parent.internalAssert)
 
@@ -367,6 +368,36 @@ function Component:__unmount()
 	for _, childNode in pairs(virtualNode.children) do
 		reconciler.unmountVirtualNode(childNode)
 	end
+
+	if config.experimentalActiveConnectionsWarning then
+		--[[
+			A full traversal of the component is performed but some keys are excluded to reduce false positives.
+
+			If a connection is referenced in a component but it's closure does not close over
+			self as an upvalue then it is not a concern that the component still has a reference
+			after being unmounted. This commonly occurs through references to component context or the store.
+			Ideally we would be able to inspect the closure associated with each active connection
+			and warn only if the closure uses self as an upvalue.
+
+			Internal Roact connections are also ignored for this check, so that connections within EventManager
+			do not cause this warning to be emitted.
+		--]]
+		local ignoreKeys = {
+			InternalData,
+			"_context",
+			"store",
+		}
+
+		local activeConnections = getActiveConnections(self, ignoreKeys, self.__componentName)
+		for name, _ in pairs(activeConnections) do
+			warn("Roact experimental active connection warning:\n" ..
+				self.__componentName.. " did not clear connection " .. name .. "\n" ..
+				"Note: This warning is experimental and may reflect an issue with the implementation of your component\n" ..
+				"If the closure referenced closes self as an upvalue then this connection needs to be disconnected " ..
+				"so that this component will be garbage collected and this won't cause a memory leak."
+			)
+		end
+	end
 end
 
 --[[

src/Config.lua

@@ -22,6 +22,9 @@ local defaultConfig = {
 	["elementTracing"] = false,
 	-- Enables validation of component props in stateful components.
 	["propValidation"] = false,
+
+	-- Enables an experimental warning which can help to track down connections not being disconnected
+	["experimentalActiveConnectionsWarning"] = false,
 }
 
 -- Build a list of valid configuration values up for debug messages.

src/getActiveConnections.lua

@@ -0,0 +1,38 @@
+local function getActiveConnections(obj, ignoreKeys, name)
+	local seen = {}
+	local results = {}
+
+	local function shouldIgnoreKey(key)
+		for _, k in pairs(ignoreKeys) do
+			if k == key then
+				return true
+			end
+		end
+		return false
+	end
+
+	local function collectActiveConnections(child, namespace)
+		if seen[child] then
+			-- Break out of cyclical tables
+			return
+		end
+		seen[child] = true
+
+		for k, v in pairs(child) do
+			if not shouldIgnoreKey(k) then
+				local kStr = tostring(k)
+				local fullNameKey = namespace == "" and kStr or namespace.. "." ..kStr
+				if typeof(v) == "table" then
+					collectActiveConnections(v, fullNameKey)
+				elseif typeof(v) == "RBXScriptConnection" and v.Connected then
+					results[fullNameKey] = v
+				end
+			end
+		end
+	end
+
+	collectActiveConnections(obj, name)
+	return results
+end
+
+return getActiveConnections