Import box2d-contacts branch from love-experiments

Fix issues with Contact lifetime, based on the work of
Matteo "teomat" Goggi

src/modules/physics/box2d/Contact.cpp

@@ -38,8 +38,22 @@ Contact::Contact(b2Contact *contact)
 }
 
 Contact::~Contact()
+{
+	invalidate();
+}
+
+void Contact::invalidate()
+{
+	if (contact != NULL)
 	{
 		Memoizer::remove(contact);
+		contact = NULL;
+	}
+}
+
+bool Contact::isValid()
+{
+	return contact != NULL ? true : false;
 }
 
 int Contact::getPositions(lua_State *L)

src/modules/physics/box2d/Contact.h

@@ -59,6 +59,18 @@ public:
 
 	virtual ~Contact();
 
+	/**
+	 * Removes the b2Contact pointer from Memoizer and sets it
+	 * to null on the Contact.
+	 **/
+	void invalidate();
+
+	/**
+	 * Returns if the Contact still points to a valid b2Contact.
+	 * @return True if the contact is still valid or false if it has been destroyed.
+	 **/
+	bool isValid();
+
 	/**
 	 * Gets the position of each point of contact.
 	 * @return The position along the x-axis.

src/modules/physics/box2d/World.cpp

@@ -77,9 +77,13 @@ void World::ContactCallback::process(b2Contact *contact, const b2ContactImpulse
 				throw love::Exception("A fixture has escaped Memoizer!");
 		}
 
-		Contact *c = new Contact(contact);
+		Contact *cobj = (Contact *)Memoizer::find(contact);
+		if (!cobj)
+			cobj = new Contact(contact);
+		else
+			cobj->retain();
 
-		luax_newtype(L, "Contact", (PHYSICS_CONTACT_T), (void *)c);
+		luax_newtype(L, "Contact", (PHYSICS_CONTACT_T), (void *)cobj);
 
 		int args = 3;
 		if (impulse)
@@ -290,6 +294,11 @@ void World::BeginContact(b2Contact *contact)
 void World::EndContact(b2Contact *contact)
 {
 	end.process(contact);
+
+	// Letting the Contact know that the b2Contact will be destroyed any second.
+	Contact *c = (Contact *)Memoizer::find(contact);
+	if (c != NULL)
+		c->invalidate();
 }
 
 void World::PreSolve(b2Contact *contact, const b2Manifold *oldManifold)
@@ -468,7 +477,9 @@ int World::getContactList(lua_State *L) const
 	{
 		if (!c) break;
 		Contact *contact = (Contact *)Memoizer::find(c);
-		if (!contact) throw love::Exception("A contact has escaped Memoizer!");
+		if (!contact)
+			contact = new Contact(c);
+		else
 			contact->retain();
 		luax_newtype(L, "Contact", PHYSICS_CONTACT_T, (void *)contact);
 		lua_rawseti(L, -2, i);

src/modules/physics/box2d/wrap_Contact.cpp

@@ -29,7 +29,10 @@ namespace box2d
 
 Contact *luax_checkcontact(lua_State *L, int idx)
 {
-	return luax_checktype<Contact>(L, idx, "Contact", PHYSICS_CONTACT_T);
+	Contact *c = luax_checktype<Contact>(L, idx, "Contact", PHYSICS_CONTACT_T);
+	if (!c->isValid())
+		luaL_error(L, "Attempt to use destroyed contact.");
+	return c;
 }
 
 int w_Contact_getPositions(lua_State *L)