[CMake] Re: FindLua50.cmake, FindLua51.cmake, FindOpenThreads.cmake

[E. Wing]

Hi, a few comments on your Find modules you just submitted.

For OpenThreads, I'm fairly close to the OpenThreads project and I've
been working on a formal find script for CMake. (I thought one of my
older scripts was already in the distribution too.) Our users have
found a lot of interesting problems, so the final script will
hopefully be extremely robust.

Also, is there a reason for the CHECK_CXX_SOURCE_COMPILES? It doesn't
seem necessary to me. I personally like to avoid these things because
of both performance and also because I always have an eye towards
cross-compiling and I don't expect this type of thing to be
trustworthy in those situations until CMake offers true
cross-compiling support.


As for Lua, the problem with Lua is there isn't really an official way
to install it. So the first problem is that is not necessarily true
that the #include is
#include <lua50/lua.h>
or
#include <lua5.1/lua.h>

There are a lot of permutations that will break this:
#include <lua5.0/lua.h>
or
#include <lua51/lua.h>

Or simply
#include <lua/lua.h>
which is probably the most common occurrence for people who build and
install Lua themselves.

In my opinion, the most portable thing is:
#include "lua.h" and let CMake test all the common permutations. (This
is what SDL officially recommends because the problem is just as bad.)
I also like providing for environmental variables for CMake to check
in case you have a permutation that is not covered.


Second, Lua 5.0 by default likes to split the standard library
(lualib) and core (lua) into two separate pieces, so there might be
two pieces you need to look for assuming the user is using the
standard library, but is often the case. But it is quite possible they
get rolled into one piece for convenience. And it is possible they
don't have the standard library at all (though I expect it to be rare
on platforms the CMake currently supports as the people to avoid the
standard library are on embedded platforms with extremely tight memory
requirements and can't afford ~50k).

Conversely, in Lua 5.1, the default is to build them as one piece, but
it should be possible to keep them as two pieces and of course not
building the standard library at all.

Again, I'm not a fan of the try compiles (CHECK_LIBRARY_EXISTS), but
in this case, you could make an argument for it. But it should mainly
be targeted at finding the standard library in my opinion if used at
all. I'm also thinking that some optional flags to define before
calling FindPackage(Lua) might be the better way to go to handle some
of these situations (like specifying you're using the standard
library) instead of the try compiles.

One last question, should these modules be spouting MESSAGE calls by
default and at all? I was always under the impression that the best
thing to do is punt to standard CMake behavior here if things are not
found because it will do the error messaging for you.

Thanks,
Eric