properly clean up @post_step_blocks after step

ext/chipmunk/rb_cpSpace.c

@@ -314,6 +314,10 @@ rb_cpSpaceAddPostStepCallback(int argc, VALUE *argv, VALUE self) {
   VALUE blocks = rb_iv_get(self, "@post_step_blocks");
 
   rb_scan_args(argc, argv, "10&", &obj, &block);
+  if(!RTEST(block)) {
+    rb_raise(rb_eArgError, "Cannot omit the block.");
+  }
+
   rb_hash_aset(blocks, obj, block);
 
   cpSpaceAddPostStepCallback(SPACE(self),
@@ -539,6 +543,7 @@ rb_cpSpaceShapeQuery(int argc, VALUE *argv, VALUE self) {
 static VALUE
 rb_cpSpaceStep(VALUE self, VALUE dt) {
   cpSpaceStep(SPACE(self), NUM2DBL(dt));
+  rb_iv_set(self, "@post_step_blocks", rb_hash_new());
   return Qnil;
 }
 

spec/space_spec.rb

@@ -128,6 +128,23 @@
   end  
 
 
+  it 'can handle junk object for callback' do
+    space = CP::Space.new
+    bod = CP::Body.new 90, 76
+    shapy = CP::Shape::Circle.new bod, 40, CP::ZERO_VEC_2
+    shapy.collision_type = :foo
+
+    bod_one = CP::Body.new 90, 76
+    shapy_one = CP::Shape::Circle.new bod_one, 40, CP::ZERO_VEC_2
+    shapy_one.collision_type = :bar
+    space.add_shape shapy
+    space.add_shape shapy_one
+
+    space.add_collision_func :foo, :bar, 1
+    # just asserting that junk doesn't get registered
+    space.step 1
+  end
+
   it 'can have old style callbacks' do
     space = CP::Space.new
     bod = CP::Body.new 90, 76
@@ -179,6 +196,7 @@
     space.step 1
 
     called.should be_true
+    space.instance_variable_get("@post_step_blocks").should be_empty
   end
 
   it 'can register for post step callbacks' do