Question

A very strange error, showing an object is nil.

the code in subject is

while pbs:HasNext() do
    local char = self.DecodeCharacter(pbs)
    ...

One would think, that if pbs:HasNext() is true, it means that, pbs is not nil, whatsoever.

However, the print(pbs) – the first line of HTMLEntityCodec:DecodeCharacter prints nil

function HTMLEntityCodec:DecodeCharacter(pbs)
    print(pbs)
    ...

The entire file dumped below, it was stripped from 1800+ lines to 110 so it can be clear for SO users to get he context. But that stripping took away all logic from the code, so do not get confused by that.

#!/usr/bin/env lua

function Inherits( baseClass )
    local new_class = {}
    local class_mt = { __index = new_class }

    function new_class:create()
        local newinst = {}
        setmetatable( newinst, class_mt )
        return newinst
    end

    if baseClass then
        setmetatable( new_class, { __index = baseClass } )
    end

    return new_class
end


-------------------------------------------
-- PushbackString
-------------------------------------------
PushbackString = Inherits({})

function PushbackString:Init(input)
    self.input = input
    self.pushback = nil
    self.temp = nil
    self.index = 0 
    self.mark = 0
end

-- Mark the current index, so the client can reset() to it if need be.        
function PushbackString:HasNext()
    return true
end

function PushbackString:Mark ()
    self.temp = self.pushback
    self.mark = self.index
end


BaseCodec = Inherits({}) 

function BaseCodec:Decode(input)
    local buff = ''    
    local pbs = PushbackString:create()

    pbs:Init(input)

    while pbs:HasNext() do
        local char = self.DecodeCharacter(pbs)
        if char ~= nil then
            buff = buff .. char
        else
            buff = buff .. pbs:Next()
        end
    end
    return buff
end


HTMLEntityCodec = Inherits(BaseCodec)
-- HTMLEntityCodec.classname = ('HTMLEntityCodec')

function HTMLEntityCodec:DecodeCharacter(pbs)
    print(pbs)
    pbs:Mark()    
end

DefaultEncoder = Inherits({})

function DefaultEncoder:Init(codecs)
    self.html_codec = HTMLEntityCodec:create()
end

function DefaultEncoder:TestInput(input , strict)
    print ("\n----------------8<----------------8<----------------\n")
    print ("Input:\t" .. input)
    -- default value
    if strict == nil then strict = true end

    -- nothing to do
    if input == nil then return nil end

    local working = input
    local codecs_found = {}
    local found_count = 0
    local clean = false

    while not clean do
        clean = true
        old = working
        working = self.html_codec:Decode( working )
        if old ~= working then
            print ("Warning:\tINTRUSION DETECTED")
        end
    end

    print ("Output:\t".. working)
    return working
end


local default_encoder = DefaultEncoder:create()
default_encoder:Init()
default_encoder:TestInput("%25", true)

----------8<-----------8<--------------8<----------------

END OF FILE

Console Output:

tzury@1005:~/devel/lua$ lua problem.lua 

----------------8<----------------8<----------------

Input:  %25
nil
lua: problem.lua:70: attempt to index local 'pbs' (a nil value)
stack traceback:
        problem.lua:70: in function 'DecodeCharacter'
        problem.lua:54: in function 'Decode'
        problem.lua:96: in function 'TestInput'
        problem.lua:109: in main chunk
        [C]: ?

Answer 1

In your code, the crash happens on this line:

local char = self.DecodeCharacter(pbs)

The problem is that you are calling DecodeCharacter with incorrect number of arguments.

Solution: call it like this (notice the colon):

local char = self:DecodeCharacter(pbs)

Explanation:

When you define functions in Lua using the colon (:), you are using a syntax sugar which hides an implicit first argument named self. Definitions like:

function HTMLEntityCodec:DecodeCharacter(pbs) ... end

Are actually ‘translated’ to this:

HTMLEntityCodec.DecodeCharacter = function (self, pbs) ... end

When you call the function, you either need to pass the self argument yourself, or use the colon call to supply it automatically. In your code (self.DecodeCharacter(pbs)), you are passing pbs which ends up as self in HTMLEntityCodec.DecodeCharacter, and pbs ends up being nil. Both following calls are equivalent and should solve the issue:

local char = self.DecodeCharacter(self, pbs)
local char = self:DecodeCharacter(pbs)