Re: [Monotone-devel] 3rd party libraries

[Matthew Nicholson]

Stephen Leake wrote:
> Matthew Nicholson <address@hidden> writes:
>
> > Zack Weinberg wrote:
> > > On Thu, Oct 23, 2008 at 3:38 AM, Markus Wanner <address@hidden> wrote:
> > >
> > > >  * lua: pretty similar to sqlite, except the source inclusion variant is
> > > > a bit more complicated. I'm all for dynamic linking and don't see much
> > > > of a reason for source inclusion.
> > > I think I mentioned this before, but if we're going to stop bundling
> > > lua we have to overhaul the error-handling interface between lua and
> > > our code, because we're currently relying on lua's ability to be
> > > compiled as C++ and use C++ exceptions to report errors.  Of course we
> > > need to overhaul it *anyway* because right now lots of errors just
> > > silently get lost, but remember that this is on the critical path for
> > > unbundling it.
> > Yeah, I recently noticed the same issue.  To properly unwind the stack
> > on error in a c++ app, lua needs to be compiled with a c++ compiler.
> > It uses setjmp and longjmp for error handling when compiled as C and
> > exceptions when compiled as C++.  If we don't require stack unwinding
> > on error (I have not looked at the code), then dynamic linking is
> > fine, but I think this is a candidate for bundling.
>
> I'm confused.
>
> You seem to be implying that a dynamically linked object file cannot
> be compiled with a C++ compiler. Is that correct?
>
> Or is there some limitation on dynamically linked object files, that
> says they can't propagate exceptions? 

It has nothing to do with linking really.  It's all about how lua 
signals errors.  If there is an error in some lua code, or c++ or c code 
called from a lua function, that error is handled at the location of the 
nearest lua_pcall() usually.  If compiled as C code, lua jumps to that 
location using setjmp and longjmp, c++ exceptions are used if compiled 
as c++.  This is a special c++ exception that is caught by lua (not by 
the host application).  When using set/longjmp the stack is not unwound, 
so no c++ destructors are called.  This can cause memory and other 
resource leaks.

So this was not completely clear from the other responses.  To help 
visualize this, draw an imaginary line between lua code and c++ code 
called from lua and monotone's other c++ code.  Errors (C++ exceptions) 
are not supposed to cross this line.  Basically the way to signal errors 
from lua code is by calling the lua_error() function (or just error() 
from lua code or luaL_error()).  This will cause lua_pcall() to return 
the error to the host application.  I'm not sure if throwing a c++ 
exception from lua code will cause any problems or not.

If we are very careful and make sure all clean up is done in the event 
of a lua error then there are no problems using the C version of lua but 
this is generally difficult to do.

--
Matthew Nicholson
matt-land.com