[gecode-users] Using LocalHandles and LocalObjects with External Resources

[Christian Schulte]

Hi,

This is hard to debug remotely but here is the catch:
 - you have a shared datastructure that you have put into a local object.
 - you need to make sure that both propagator and brancher destruct the
local handle but only if neither needs it any longer. If you make use of the
local handle in both propagator as well as brancher then everything should
be fine.
 - for the propagator it means you have to use the AP_DISPOSE thing as you
did. In its dispose method you have to add an explicit call to the
destructor of the local handle, say that the local object is names lo and of
type Foo. Then lo.~Foo() will do the trick.
 - for the brancher you also have to call in its dispose method the
destructor.

My guess is that the error comes from the fact that the shared object has
already been destroyed by either propagator or brancher and the other still
tries to access it.

Cheers
Christian

--
Christian Schulte, www.gecode.org/~schulte
Professor of Computer Science, KTH, cschulte at kth.se
Expert Researcher, SICS, cschulte at sics.se
-----Original Message-----
From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
Of Peter Backeman
Sent: Saturday, January 7, 2017 02:02
To: users at gecode.org
Subject: [gecode-users] Using LocalHandles and LocalObjects with External
Resources

Hello, 

I've implemented a custom propagator and brancher which are sharing a
data-structure (a matrix of values), To share the structure in an efficient
(and convenient) way I have looked at using LocalHandle and LocalObject. In
MPG (Sec 31.4, page 391) this is described how to be implemented, however I
am doing something wrong because I keep on getting some kind of memory
error.

My main problem is with the notice for disposal (since external resources
are used). If I do not use home.notice(*this, AP_DISPOSE) (line 17 in
attached file), my program does run without errors, but I suspect it might
lead to memory leaks. I've tried figuring out how this should work, but I
have yet to find a full example using LocalHandle/LocalObject (i.e. together
with a model) and no previous issues around this.

I'm attaching a minimum failing example (main.cpp), I'm compiling it on
ubuntu using g++ version 5.4.0. and Gecode version 5.0.0 (I'm fairly sure,
do not know how to exactly check this) using the following command:
	g++ -g -o main src/main.cpp -lgecodedriver -lgecodegist
-lgecodesearch -lgecodeminimodel -lgecodeint -lgecodekernel -lgecodesupport
and when running it I receive the following error (gdb output):
----------------------------------------------------------------------------
-
Program received signal SIGSEGV, Segmentation fault.
Gecode::VarImp<Gecode::Int::IntVarImpConf>::update (sub=<synthetic pointer>,
home=...) at ./gecode/kernel/core.hpp:4438
4438	./gecode/kernel/core.hpp: No such file or directory.
(gdb) where
#0  Gecode::VarImp<Gecode::Int::IntVarImpConf>::update (sub=<synthetic
pointer>, home=...) at ./gecode/kernel/core.hpp:4438
#1  Gecode::Space::update (sub=0x64e380, this=0x64e220) at
./gecode/kernel/var-imp.hpp:484
#2  Gecode::Space::_clone (this=0x63c6a0, share_data=share_data at entry=true,
share_info=share_info at entry=true) at gecode/kernel/core.cpp:584
#3  0x00007ffff776b813 in Gecode::Space::clone (share_info=true,
share_data=true, this=<optimized out>) at ./gecode/kernel/core.hpp:3262
#4  Gecode::Search::Sequential::BAB::next (this=0x64db38) at
./gecode/search/sequential/bab.hh:149
#5  Gecode::Search::WorkerToEngine<Gecode::Search::Sequential::BAB>::next
(this=0x64db30) at ./gecode/search/support.hh:86
#6  0x0000000000411ad9 in Gecode::Search::Base<MyScript>::next
(this=<optimized out>) at /usr/local/include/gecode/search/base.hpp:51
#7
Gecode::Driver::ScriptBase<Gecode::Driver::IgnoreStepOption<Gecode::IntMinim
izeSpace> >::runMeta<MyScript, Gecode::BAB, Gecode::SizeOptions,
Gecode::Driver::EngineToMeta> (o=..., s=0x63c6a0)
    at /usr/local/include/gecode/driver/script.hpp:360
#8  0x0000000000408e15 in
Gecode::Driver::ScriptBase<Gecode::Driver::IgnoreStepOption<Gecode::IntMinim
izeSpace> >::run<MyScript, Gecode::BAB, Gecode::SizeOptions> (o=..., s=0x0)
    at /usr/local/include/gecode/driver/script.hpp:274
#9  0x0000000000405536 in main (argc=1, argv=0x7fffffffe0d8) at
src/main.cpp:63
----------------------------------------------------------------------------
-

Any ideas on what might be the problem is greatly appreciated. My guess is
that perhaps the destructor of the LocalHandle (LH) class have to make sure
dispose is called on the LocalObject (LHO), however my skills in C++  and
Gecode are not enough to know how I can try this. If someone just have an
implementation using LocalHandles/LocalObjects which is working, just
letting me have a look on that would be very helpful!

Best regards,
Peter Backeman 
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