thoughts about unwind_protect

[Jaroslav Hajek]

hi all,

one problem I see about the unwind_protect mechanism is that there is
no automatic cleanup on function exit.
Usually a function code goes like this

unwind_protect_t frame = unwind_protect::begin_frame ();

unwind_protect::protect_var (my_var);
// do stuff

unwind_protect::run_frame ();

The problem is that the last line is not executed if an exception
occurs in the function body, so the unwind_protect operation would be
delayed until the next run_frame or run_all after catching the
exception. Such a delay could have disastrous consequences, however,
if local variables (which no longer exist) were somehow involved in
the registered cleanup action. To combat this problem, liboctave
introduces the octave_interrupt_hook, which is hooked to a handler
that doesn unwind_protect::run_all (). Hence, in case of interrupt
exception, the *whole* unwind_protect stack is run before the
exception is even thrown.
Usually, this does the right thing, since the exception then
propagates to the top level (main loop).
However, it is not advisable to catch interrupt exceptions (for
possible additional handling), because at that moment cleanup actions
will be performed for the whole call stack, so thing may not be in
feasible state (for instance, the octave_call_stack).

what I suggest is to instead use this scheme:

unwind_protect frame;

frame.protect_var ();

// do_stuff

// frame.run () // can be done explicitly or at destruction time.

This has a number of advantages, being simpler and cleaning up
automatically. The only disadvantage is that exceptions should not
occur in destructors, so if a cleanup action is registered that can
throw exception (which is very rare), that action should be run
explicitly (with possibly handling the exception).

I think this is not too much of a hassle, especially considering that
in fact now each frame is run explicitly. After this change, I think
it may be possible to eliminate the octave_interrupt_hook.

Comments?

-- 
RNDr. Jaroslav Hajek, PhD
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz