libGDB architecture

[Jim Blandy]

> > A consequence of handing out pointers to internal GDB data structures is
> > that GDB's internals and the external client need to reach a very clear
> > concensus over the life time of those objects.  I understand that tcl
> > has a mechanism that allows it and the low level code to keep a
> > reference count.
> 
> This is indeed a problem. However I think it could be solved pretty
> easily by putting a reference count variable in each structure that
> we export. Stop, stop, don't think that we need to change the memory
> handling policy inside GDB once we've added this instance variable.
> 
> We don't need to do that. This refcount is set to one for each GDB
> internal data structure that is exported by libGDB, at the time of
> its inception. When a client imports such a structure, it increments
> the refcount. We then modify the free() functions slightly, like
> this:

How should we handle objects allocated on obstacks?  Most of the
symbolic info (types; variables) is allocated this way.  I don't think
it's practical to keep the entire obstack around until all its
objects' reference counts have dropped to zero.