setmetatable vs debug.setmetatable?

[PA <petite.abeille@...>]

Hello,

For the record:

% lua -v
Lua 5.1.2  Copyright (C) 1994-2007 Lua.org, PUC-Rio

With setmetatable, one can assign a metatable per table instance. One 
the other hand, debug.setmetatable seems to assign a metatable per 
object type, not object instance, e.g.:

local aFunction = function() end

debug.setmetatable( aFunction, { __len = function() return 1 end } )

print( #aFunction )

>       1

Unfortunately, it seems like all function instances share the same 
metatable:

local anotherFunction = function() end

debug.setmetatable( anotherFunction, { __len = function() return 2 end 
} )

print( #aFunction, #anotherFunction )

>       2       2

This even though the documentation states that debug.setmetatable is 
"for the given object", not object type:

"Sets the metatable for the given object to the given table (which can 
be nil)."

Bug or feature?

In any case, how to get around such conundrum and properly simulate one 
metatable per function instance?

Perhaps one could try using a function environment, which can be set 
per function instance:

local aFunction = function() end

debug.setmetatable( aFunction, { __len = function( self ) return 
getfenv( self ):__len() end } )

setfenv( aFunction, { __len = function() return 1 end } )

local anotherFunction = function() end

setfenv( anotherFunction, { __len = function() return 2 end } )

print( #aFunction, #anotherFunction )

>       1       2

Is there a more straightforward way?

Thanks in advance.

Cheers,

PA.