random_snippets/using_xpcall.lua

--- This part is unrelated to pcall/xpcall usage. Scroll to "Usage
--- example" below to see the relevant parts.


-- use efficient deque or implement our own FIFO type of thing
local formspec_log = futil and futil.Deque and futil.Deque()
if not formspec_log then
    -- no deque implementation found
    local deq = {}
    deq.size = function(self)
        return #self
    end
    deq.push_front = function(self, v)
        table.insert(self, v)
    end
    deq.pop_back = function(self)
        local v = self[1]
        table.remove(self, 1)
        return v
    end
    deq.iterate = function(self)
        local i = 0
        return function()
            i = i + 1
            local v = self[i]
            if v then
                return v
            end
        end
    end
    deq.__index = deq
    formspec_log = setmetatable({}, deq)
end

local LOG_SIZE = 100

-- use deque for logging, pushing oldest log item out
local function log_add(msg)
    core.log('error', type(msg) == "string" and msg or dump(msg))
    formspec_log:push_front(msg)
    if formspec_log:size() > LOG_SIZE then
        formspec_log:pop_back()
    end
end


local function formspec_log_clear()
    formspec_log:clear()
end


-- passes to xpcall to be able to capture the call stack
local function error_printer(err)
    log_add(err)
    log_add(debug.traceback("----------------------", 2))
    return "error was logged"
end


-- Takes any function and returns a function that takes and returns
-- same [multiple] values if no error happens. On error, logs it.
local function safe_wrap(func)
    local function wrap_f(...)
        local res = table.pack(xpcall(func, error_printer, ...))
        if res[1] then
            -- no error, just return rest of the values
            return select(2, unpack(res))
        else
            -- error was logged by error_printer already
            local msg = dump(res[2])
            if msg then
                log_add(msg)
            end
            return nil
        end
    end
    return wrap_f
end


local function formspec_log_show(playername)
    local log_items = {}
    for item in formspec_log:iterate() do
        table.insert(log_items, type(item) == "string" and item or dump(item))
    end
    local formspec_template = [[ formspec_version[6] size[16,9] textarea[0.1,0.4;15.8,8.5;log;log;%s] ]]
    local log_text = core.formspec_escape(table.concat(log_items,'\n'))
    core.show_formspec(playername, "yl_commons:formspec_log", string.format(formspec_template, log_text))
end


-- These commands will allow us to see the errors saved in
-- the `formspec_log` after they happen.
core.register_chatcommand(
    "log_show",
    {
        privs = { server = true },
        func = function(playername, _params)
            formspec_log_show(playername)
            return true
        end,
    }
)

core.register_chatcommand(
    "log_clear",
    {
        privs = { server = true },
        func = function(playername, _params)
            formspec_log_clear(playername)
            return true
        end,
    }
)



--------------------------------------------------------------------------------
---          Usage example
--------------------------------------------------------------------------------

local function bugged(playername, params)
    for _=1,tonumber(params) do
        core.chat_send_all("Hello!")
    end
    return true, playername
end


-- this command will crash if you pass it wrong arguments
core.register_chatcommand(
    "bugged",
    {
        privs = { },
        func = function(playername, params)
            return bugged(playername, params)
        end,
    }
)

-- This is the simplest safe version using `pcall`, it will return
-- status and error message on crash.

-- This command definition does not require any of the error handling
-- code above. The limitation is that this will only give us the error
-- message.
core.register_chatcommand(
    "bugged_safe",
    {
        privs = { },
        func = function(playername, params)
            local status, ret1, ret2 = pcall(bugged, playername, params)
            -- `status` will be `false` if function call raised an error
            -- `ret1` will contain the error message on error, or first returned value on no errors
            -- `ret2` will contain second returned value on no errors
            -- etc.
            return status, string.format("%s | %s", ret1, ret2)
        end,
    }
)

-- This version allows us to get the error stack trace. We get the
-- trace by passing error handler function `error_printer` to xpcall.
-- (our helper funciton, `error_printer` will be called on error
-- without unwinding the stack, so we can call `debug` functions to
-- examine the error).
core.register_chatcommand(
    "bugged_safe2",
    {
        privs = { },
        func = function(playername, params)
            -- NOTE: if this does not work, you may need to make sure you're using luajit or newer lua
            local status, ret1, ret2 = xpcall(bugged, error_printer, playername, params)
            -- `status` will be `false` if function call raised an error

            -- `ret1` will contain first returned value in case of no
            -- errors, or, in case of error, the value returned by the
            -- error handler function.

            -- `ret2` will contain second returned value on no errors
            -- etc.
            return status, string.format("%s | %s", ret1, ret2)
        end,
    }
)

-- This is equivalent to `xpcall` safe2 version, but uses generic
-- wrapper function we defined above.
core.register_chatcommand(
    "bugged_safe3",
    {
        privs = { },
        func = function(playername, params)
            local safe_bugged = safe_wrap(bugged)
            return safe_bugged(playername, params)
        end,
    }
)