Pcall with traceback for Roblox

tpcall.lua

--[[
	Use:
	On success:
		local success, return1, return2, ... = tpcall(func, arg1, arg2, ...)
	On error:
		local success, error, traceback = tpcall(func, arg1, arg2, ...)
--]]

--[[ runner.lua: belongs inside tpcall.lua
return setmetatable({}, {
	__call = function(_, func, ...)
		return func(...)
	end
})
--]]

local PREGENERATE_RUNNERS_COUNT = 10

local runnerBase = script.runner

local runnersFree = {}
local runnersById = {}
local runnersByCoroutine = {}

local count = 0
local function getFreeRunner(running)
	if runnersByCoroutine[running] then
		return runnersByCoroutine[running]
	else
		local freeRunner = next(runnersFree)
		if freeRunner then
			return freeRunner
		else
			count = count + 1
			local uniqueId = "tpcall:"..count
			runnerBase.Name = uniqueId
			local newRunner = require(runnerBase:Clone())
			newRunner.id = uniqueId
			runnersById[uniqueId] = newRunner
			return newRunner
		end
	end
end

for i = 1, PREGENERATE_RUNNERS_COUNT do
	count = count + 1
	local uniqueId = "tpcall:"..count
	runnerBase.Name = uniqueId
	local newRunner = require(runnerBase:Clone())
	newRunner.id = uniqueId
	runnersById[uniqueId] = newRunner
	runnersFree[newRunner] = true
end

local lastErrorId, lastError, lastTrace
game:GetService("ScriptContext").Error:Connect(function(err, trace, scr)
	local tpcallId = trace:match("(tpcall:%d+)")
	if tpcallId then
		-- for some reason, the error message includes the script and
		--  line number. The script can have any name, and the error is
		--  arbitrary, so we can't use pattern matching on the error.
		-- We *can* use it on the trace though, which includes the script
		--  name and error. Using this, we can find length counts and get
		--  a substring of the actual error!
		
		-- This approach isn't perfect. It relies on the ", line %d+ -" format.
		-- If for some reason your script has a name like that then it will
		--  produce improper output
		local scriptName, errorLine = trace:match("^(.-), line (%d+) %- [^\n]*\n")
		if scriptName then
			if err:sub(1, #scriptName) == scriptName then
				lastError = err:sub(#scriptName + #errorLine + 4)
			else
				lastError = err
			end
		else
			lastError = err:match("^"..tpcallId..":%d+: (.*)$") or err
		end
		lastErrorId = tpcallId
		lastTrace = trace:match("^(.*)\n[^\n]+\n[^\n]+\n$") or ""
		runnersById[tpcallId].event:Fire()
	end
end)

return function(func, ...)
	local runner = getFreeRunner(coroutine.running())
	local initialCoroutine, initialEvent = runner.coroutine, runner.event
	-- We have been given the runner *for our coroutine*.
	-- This means it's "stacked" on top of another coroutine and event.
	--  (and we are *guaranteed* to finish first, since we are
	--  stacked on top of the coroutine that called this tpcall)
	-- In fact, initialCoroutine and coroutine.running() should be
	--  the same! We only switch to a "new" coroutine when we move
	--  into the BindableEvent, but it's guaranteed that the
	--  BindableEvent coroutine finishes before we continue. We use
	--  BindableEvents to yield until that new coroutine is finished.
	-- The one exception: when this call is at the top of the tpcall
	--  "stack". In that scenario, initialCoroutine and initialEvent
	--  are nil. This is how tpcall knows when a runner is free again:
	--  it's free when the initialCoroutine and initialEvent are nil.
	
	-- If there is an error, lastErrorId = runnerId
	-- If there is no error, results ~= nil
	
	local results
	
	local args = {...}
	local event = Instance.new("BindableEvent")
	runner.event = event
	local running
	local conn
	conn = event.Event:Connect(function()
		conn:disconnect()
		running = coroutine.running()
		runner.coroutine = running
		runnersByCoroutine[running] = runner
		results = {runner(func, unpack(args))}
		event:Fire()
	end)
	runnersFree[runner] = nil
	event:Fire()
	local runnerId = runner.id
	if not results and lastErrorId ~= runnerId then
		event.Event:Wait()
	end
	runnersByCoroutine[running] = nil
	runner.coroutine, runner.event = initialCoroutine, initialEvent
	if not initialCoroutine then
		runnersFree[runner] = true
	end
	if lastErrorId == runnerId then
		lastErrorId = nil
		return false, lastError, lastTrace
	else
		return true, unpack(results)
	end
end

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