Polymorphic luaL_checkudata?

[Peter Odding <xolox@...>]

Hi all! I'm (still) working on binding APR. Currently busy with pipes, which 
seem to be backwards compatible with files but not the other way around:

 * pipes have a timeout which needs two methods (timeout_get/timeout_set)
 * I need to make sure I'm dealing with a pipe (instead of a file) when using 
them as stdin/stdout/stderr for processes

Right now my io_file module uses luaL_checkudata(L, i, "apr-file") to validate 
arguments. Of course when I give pipes their own metatable my io_file module 
will start rejecting pipes, which is not my intention.

One solution is a function in the vein of luaL_checkudata that checks a userdata 
argument for one *of a given set* of metatable names, like luaL_checkudata_m(L, 
i, "apr-file", "apr-pipe"). Have any of you needed this functionality before? 
Have you used a different approach?

I don't mind copying and extending the source for luaL_checkudata() to handle 
one or more metatable names but if I'm not the only one needing this 
functionality, it might make a good candidate for inclusion in the auxiliary 
library of either Lua 5.1.3 or Lua 6.

 - Peter Odding