collision_detection: enforce pair-bond precondition in GlueToSurface (bug-sweep #40)

src/core/collision_detection/GlueToSurface.cpp

@@ -58,6 +58,20 @@ void GlueToSurface::initialize(System::System &system) {
   // Cache square of cutoff
   distance_sq = Utils::sqr(distance);
 
+  // Check that the bonds have the right number of partners. Both bonds are
+  // stored with a single-partner list in handle_collisions(), so they must
+  // be strict pair bonds (existence is already guaranteed at the seam).
+  assert(system.bonded_ias->contains(bond_centers));
+  if (number_of_partners(*system.bonded_ias->at(bond_centers)) != 1) {
+    throw std::runtime_error("The bond type to be used for binding particle "
+                             "centers needs to be a pair bond");
+  }
+  assert(system.bonded_ias->contains(bond_vs));
+  if (number_of_partners(*system.bonded_ias->at(bond_vs)) != 1) {
+    throw std::runtime_error("The bond type to be used for binding virtual "
+                             "sites needs to be a pair bond");
+  }
+
   if (part_type_vs < 0) {
     throw std::domain_error("Collision detection particle type for virtual "
                             "sites needs to be >=0");

testsuite/python/collision_detection_interface.py

@@ -153,6 +153,10 @@ def test_glue_to_surface(self):
             self.set_coldet("glue_to_surface", part_type_to_attach_vs_to=-1)
         with self.assertRaisesRegex(ValueError, "type after gluing needs to be >=0"):
             self.set_coldet("glue_to_surface", part_type_after_glueing=-1)
+        with self.assertRaisesRegex(RuntimeError, "The bond type to be used for binding particle centers needs to be a pair bond"):
+            self.set_coldet("glue_to_surface", bond_centers=self.bond_angle)
+        with self.assertRaisesRegex(RuntimeError, "The bond type to be used for binding virtual sites needs to be a pair bond"):
+            self.set_coldet("glue_to_surface", bond_vs=self.bond_angle)
         # check if original parameters have been preserved
         self.check_stored_parameters("glue_to_surface", distance=0.5)