Re: execute_control_command may not remove its cleanups

[Dave Allan <da_gdb at egenera dot com>]

Ok.  I see your point.  How about setting old_chain to cleanup_chain
unconditionally at the beginning of the function and doing the cleanups
unconditionally at the end?  That way, we're safe against both
scenarios: against doing cleanups prematurely, but also safe against
getting into the function with cleanup_chain null and then freeing
something random at a later point.  

Dave


On Thu, 2004-02-19 at 13:47, Daniel Jacobowitz wrote:
> On Thu, Feb 19, 2004 at 01:21:52PM -0500, Dave Allan wrote:
> > > > However, it seems from code inspection and the gdb internals
> > > > documentation that the call to do_cleanups ought to be unconditional. 
> > > > Does that seem right?  
> > > 
> > > No, instead, the cleanup chain should always have an item on it.  If
> > > make_cleanup is not called then old_chain will remain NULL, and
> > > do_cleanups (NULL) means "do all cleanups", not "do nothing".  It looks
> > > to me like command_handler is responsible for there always being a
> > > cleanup on the chain:
> > >   old_chain = make_cleanup (null_cleanup, 0);
> > > but maybe I'm mistaken about that; it's a bit far down the tree.
> > 
> > I definitely understand that do_cleanups(NULL) will do all cleanups
> > which is not what's wanted here.  The call is do_cleanups(old_chain),
> > though, so if there are cleanups on the chain already, they are
> > preserved.  The problem isn't the do_cleanups call, it's the fact that
> > the do_cleanups call is conditional.  The solution is to remove the if
> > (old_chain) statement and always do the cleanup.  
> > 
> > Given what's stated in the docs, that a function must always remove the
> > cleanups it creates, it would seem to me that regardless of the state of
> > cleanup_chain at the beginning of execute_control_command, whether it's
> > NULL or contains cleanups, we want to get back to that state before we
> > return.  
> > 
> > Looking at what cleanups execute_control_command puts on cleanup_chain,
> > that is correct.  Either one or two cleanups are put on the chain where
> > arg is an automatic variable and function is free_current_contents.  If
> > these cleanups aren't done before the stack frame is destroyed,
> > something undefined will later be freed when the cleanups are done.  
> 
> Think about this again.  Both of those cleanups are conditionally
> created.  If neither of them is created, old_chain will still be NULL.
> This will lead to running cleanups prematurely.  If the cleanup chain
> is non-empty, things work OK.
> 
> The alternative is null_cleanup.