Copy arena contents in mj_copyData.

Fixes #1710.

PiperOrigin-RevId: 651351384
Change-Id: I56e3204b30bb18f48278117e85e90464177da973

doc/changelog.rst

@@ -80,6 +80,11 @@ Python bindings
 ^^^^^^^^^^^^^^^
 20. Fixed a memory leak when using ``copy.deepcopy()`` on a ``mujoco.MjData`` instance (:github:issue:`1572`).
 
+Bug fixes
+^^^^^^^^^
+21. Fix an issue where ``mj_copyData`` (or ``copy.copy()`` in the Python bindings) was not copying contact information
+    correctly (:github:issue:`1710`).
+
 Version 3.1.6 (Jun 3, 2024)
 ---------------------------
 

python/mujoco/bindings_test.py

@@ -418,6 +418,9 @@ class MuJoCoBindingsTest(parameterized.TestCase):
     data_copy = copy.copy(self.data)
     self.assertEqual(data_copy.ncon, 2)
 
+    # Make sure contact details are copied.
+    self.assertEqual(data_copy.contact[0].dist, self.data.contact[0].dist)
+
     # Make sure it's a copy.
     mujoco.mj_resetData(self.model, self.data)
     mujoco.mj_forward(self.model, self.data)

src/engine/engine_io.c

@@ -1265,10 +1265,24 @@ mjData* mj_copyData(mjData* dest, const mjModel* m, const mjData* src) {
   }
 
   // copy arena memory
+#undef MJ_D
+#define MJ_D(n) (src->n)
+#undef MJ_M
+#define MJ_M(n) (m->n)
 #define X(type, name, nr, nc)  \
-  dest->name = src->name ? (type*)((char*)dest->arena + PTRDIFF(src->name, src->arena)) : NULL;
+  if (src->name) { \
+    dest->name = (type*)((char*)dest->arena + PTRDIFF(src->name, src->arena)); \
+    memcpy((char*)dest->name, (const char*)src->name, sizeof(type)*nr*nc); \
+  } else { \
+    dest->name = NULL; \
+  }
+
   MJDATA_ARENA_POINTERS
 #undef X
+#undef MJ_M
+#define MJ_M(n) n
+#undef MJ_D
+#define MJ_D(n) n
 
   // restore contact pointer
   dest->contact = dest->arena;