From c3adb50898f66836e7f5b02c8dff848b813f871f Mon Sep 17 00:00:00 2001
From: Taylor Howell <taylorhowell@google.com>
Date: Thu, 2 Jul 2026 04:19:19 -0700
Subject: [PATCH] _realloc_island

PiperOrigin-RevId: 941618362
Change-Id: Idef1deb0ff4976de42aa585993988ae3363cd1fc
---
 python/mujoco/bindings_test.py | 43 +++++++++++++++++++++
 python/mujoco/functions.cc     | 69 ++++++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)

diff --git a/python/mujoco/bindings_test.py b/python/mujoco/bindings_test.py
index 21e048ab..d7cb142f 100644
--- a/python/mujoco/bindings_test.py
+++ b/python/mujoco/bindings_test.py
@@ -699,6 +699,49 @@ class MuJoCoBindingsTest(parameterized.TestCase):
     self.assertEmpty(self.data.contact)
     self.assertEmpty(self.data.efc_id)
 
+  def test_realloc_island(self):
+    # Test allocation on fresh data (on its own)
+    nisland = 2
+    nidof = 4
+    mujoco._functions._realloc_island(self.data, nisland=nisland, nidof=nidof)
+    self.assertEqual(self.data.nisland, nisland)
+    self.assertEqual(self.data.nidof, nidof)
+    self.assertEqual(self.data.island_nv.shape, (nisland,))
+    self.assertEqual(self.data.ifrc_smooth.shape, (nidof,))
+
+    # Test allocation after _realloc_con_efc
+    nefc = 10
+    mujoco._functions._realloc_con_efc(self.data, ncon=0, nefc=nefc)
+
+    nisland = 3
+    nidof = 5
+    mujoco._functions._realloc_island(self.data, nisland=nisland, nidof=nidof)
+
+    self.assertEqual(self.data.nisland, nisland)
+    self.assertEqual(self.data.nidof, nidof)
+    self.assertEqual(self.data.island_nv.shape, (nisland,))
+    self.assertEqual(self.data.ifrc_smooth.shape, (nidof,))
+
+    # Test re-allocation (calling it again with different sizes)
+    nisland2 = 4
+    nidof2 = 6
+    mujoco._functions._realloc_island(self.data, nisland=nisland2, nidof=nidof2)
+
+    self.assertEqual(self.data.nisland, nisland2)
+    self.assertEqual(self.data.nidof, nidof2)
+    self.assertEqual(self.data.island_nv.shape, (nisland2,))
+    self.assertEqual(self.data.ifrc_smooth.shape, (nidof2,))
+
+    # Test insufficient memory handling
+    expected_error = (
+        r'Insufficient arena memory, currently allocated memory=' +
+        r'"[0-9]+[A-Z]?". Increase using <size memory="X"/>.'
+    )
+    with self.assertRaisesRegex(mujoco.FatalError, expected_error):
+      mujoco._functions._realloc_island(self.data, 100000000, 100000000)
+    self.assertEqual(self.data.nisland, 0)
+    self.assertEqual(self.data.nidof, 0)
+
   def test_mj_struct_list_equality(self):
     model2 = mujoco.MjModel.from_xml_string(TEST_XML)
     data2 = mujoco.MjData(model2)
diff --git a/python/mujoco/functions.cc b/python/mujoco/functions.cc
index e56643b0..b8673a0e 100644
--- a/python/mujoco/functions.cc
+++ b/python/mujoco/functions.cc
@@ -1696,6 +1696,7 @@ PYBIND11_MODULE(_functions, pymodule, pybind11::mod_gil_not_used()) {
 #define X(type, name, nr, nc) data->name = nullptr;
           MJDATA_ARENA_POINTERS_SOLVER
           MJDATA_ARENA_POINTERS_DUAL
+          MJDATA_ARENA_POINTERS_ISLAND
 #undef X
         };
 
@@ -1744,6 +1745,74 @@ PYBIND11_MODULE(_functions, pymodule, pybind11::mod_gil_not_used()) {
       },
       py::arg("d"), py::arg("ncon"), py::arg("nefc"), py::arg("nJ") = -1,
       py::call_guard<py::gil_scoped_release>());
+
+  pymodule.def(
+      "_realloc_island",
+      [](MjDataWrapper& d, int nisland, int nidof) {
+        raw::MjData* data = d.get();
+
+        size_t parena_start = data->parena;
+        // Find island block start in arena to reclaim memory on re-allocation.
+        char* min_ptr = nullptr;
+#define X(type, name, nr, nc)                                                   \
+        if (data->name &&                                                       \
+            (!min_ptr || reinterpret_cast<char*>(data->name) < min_ptr)) {      \
+          min_ptr = reinterpret_cast<char*>(data->name);                        \
+        }
+        MJDATA_ARENA_POINTERS_ISLAND
+#undef X
+        if (min_ptr && data->arena) {
+          parena_start = min_ptr - static_cast<char*>(data->arena);
+        }
+
+        auto cleanup = [](raw::MjData* data, size_t target_parena) {
+#define X(type, name, nr, nc) data->name = nullptr;
+          MJDATA_ARENA_POINTERS_ISLAND
+#undef X
+          data->nisland = 0;
+          data->nidof = 0;
+          data->parena = target_parena;
+#ifdef ADDRESS_SANITIZER
+          ASAN_POISON_MEMORY_REGION(
+              static_cast<char*>(data->arena) + target_parena,
+              data->narena - data->pstack - target_parena);
+#endif
+        };
+
+        cleanup(data, parena_start);
+
+        char error_msg[128];
+        error_msg[0] = '\0';
+        const char* error_msg_fmt =
+            "Insufficient arena memory, currently allocated memory=\"%s\". "
+            "Increase using <size memory=\"X\"/>.";
+
+        data->nisland = nisland;
+        data->nidof = nidof;
+
+#undef MJ_M
+#define MJ_M(x) d.model().get()->x
+#undef MJ_D
+#define MJ_D(x) data->x
+#define X(type, name, nr, nc)                                                   \
+        data->name = static_cast<type*>(InterceptMjErrors(::mj_arenaAllocByte)( \
+            data, sizeof(type) * (nr) * (nc), alignof(type)));                  \
+        if (!data->name) {                                                      \
+          cleanup(data, parena_start);                                          \
+          std::snprintf(error_msg, sizeof(error_msg), error_msg_fmt,            \
+                        mju_writeNumBytes(data->narena));                       \
+          throw FatalError(error_msg);                                          \
+        }
+
+        MJDATA_ARENA_POINTERS_ISLAND
+#undef X
+#undef MJ_D
+#define MJ_D(x) x
+#undef MJ_M
+#define MJ_M(x) x
+      },
+      py::arg("d"), py::arg("nisland"), py::arg("nidof"),
+      py::call_guard<py::gil_scoped_release>());
 }  // PYBIND11_MODULE NOLINT(readability/fn_size)
 }  // namespace
 }  // namespace mujoco::python