Re: Is the registry needed?

["Soni L." <fakedme@...>]

On 30/09/15 05:05 PM, Sean Conner wrote:
> It was thus said that the Great Soni L. once stated:
> > The same way you'd do it in Lua: Your module table. Or the metatable.
>    I went through my existing modules to see if I could forgo the use of the
> registry table.  So maybe you can help me.  One problematic module:
>
> 	local signal = require "org.conman.signal" -- [1]
>
> 	signal.catch('interrupt',function()
> 	  print "You can't interrupt me! Muahahahaha!"
> 	end)
>
>    Seems simple to use, but we need to save a reference to the passed in
> function because, as in this example, it can be an anonymous function not
> otherwise anchored into memory.  Second, there are some 30+ signals we can
> capture (keep that in mind).  Third, our signal handler:
Use a table as an upvalue.
> 	static void signal_handler(int sig)
> 	{
> 	 ...
>
>    Okay.  No context other than the signal that was just generated.  This
> means we need to cache the lua_State in a variable visible to our signal
> handler.  That can be done during the luaopen() call, so now we have:
>
> 	static lua_State *m_L;
> 	static void signal_handler(int sig)
> 	{
> 	 ...
>
>    We're running in a signal handler.  There is not much you can do *safely*
> from a signal handler (because you could be in the middle of allocating
> memory, in the middle of a Lua call, halfway through manipulating a linked
> list, any number of half-finished states exist), and the *only* thing you
> can safely call in Lua is lua_sethook().  Even trying to write the signal
> number to a table is unsafe.  So we have:
>
> 	static lua_State *m_L;
> 	static volatile sig_atomic_t m_sig;
> 	static void signal_handler(int sig)
> 	{
> 	  m_sig = sig;
> 	  lua_sethook(m_L,hook_handler,LUA_MASKCALL | LUA_MASKRET | LUA_MASKCOUNT,1);
> 	}
>
>    We now resume from the signal, and when it's safe for Lua to do so, our
> hook_handler() gets called:
>
> 	static void hook_handler(lua_State *L,lua_Debug *ar)
> 	{
> 	 ...
> 	}
>
>    Okay, where's our function to handle the signal?  What meta table?
> Where's the module table?  I could try looking for
> "package.loaded['org.conman.syslog']" but I can't rely upon that if the
> global environment has been sandboxed (the global environment might have
> 'signal' already loaded, but not 'package' or 'require').  I don't want to
> pollute the global environment.  But I have no context to save or retrieve
> which function to run.
>
>    Do you have any alternatives that don't rely upon package.loaded[]?
> Because I have no ideas ...
>
>    -spc
>
> [1]	https://github.com/spc476/lua-conmanorg/blob/master/src/signal.c
This is VERY unsafe! You're potentially breaking a sandbox by doing a 
sethook. And you don't reset it, either. And you're doing it from a 
separate thread. Use a (light?) userdata and do the polling in Lua. Or 
just set the hook and have it called nonstop.

(Also how do you deal with multiple states? As far as I know, you can't, 
unless you use a lightuserdata!)

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