From e3ded23c6634712bf40dc1147fca1efd99bd6847 Mon Sep 17 00:00:00 2001
From: Saran Tunyasuvunakool <stunya@google.com>
Date: Wed, 10 Aug 2022 05:11:15 -0700
Subject: [PATCH] Fix memory leak in mjCModel::Compile.

This is a very subtle leak caused interaction between longjmp and compiler optimisation. Specifically, at the point where the setjmp takes places, these pointers have never been reassigned from its nullptr initialization. Without the volatile keyword, the compiler is free to assume that these pointers remain nullptr when the setjmp returns, and therefore to pass nullptr directly to the mj_deleteModel and mj_deleteData calls in the subsequent catch block, without ever reading the actual pointer values.

PiperOrigin-RevId: 466649447
Change-Id: I51e82ec8b566deb144221e11a99d5124a8267de4
---
 src/user/user_model.cc | 619 +++++++++++++++++++++--------------------
 src/user/user_model.h  |   2 +
 2 files changed, 316 insertions(+), 305 deletions(-)

diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index 5b79885b..fa6194df 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -2271,9 +2271,16 @@ static void warninghandler(const char* msg) {
 
 // compiler
 mjModel* mjCModel::Compile(const mjVFS* vfs) {
-  unsigned int i;
-  mjModel* m = 0;
-  mjData* data = 0;
+  // The volatile keyword is necessary to prevent a possible memory leak due to
+  // an interaction between longjmp and compiler optimization. Specifically, at
+  // the point where the setjmp takes places, these pointers have never been
+  // reassigned from their nullptr initialization. Without the volatile keyword,
+  // the compiler is free to assume that these pointers remain nullptr when the
+  // setjmp returns, and therefore to pass nullptr directly to the
+  // mj_deleteModel and mj_deleteData calls in the subsequent catch block,
+  // without ever reading the actual pointer values.
+  mjModel* volatile m = nullptr;
+  mjData* volatile data = nullptr;
 
   // save error and warning handlers
   void (*save_error)(const char*) = _mjPRIVATE__get_tls_error_fn();
@@ -2289,312 +2296,13 @@ mjModel* mjCModel::Compile(const mjVFS* vfs) {
   // init random number generator, to make textures reproducible
   srand(123);
 
-  // guard for exceptions
   try {
     if (setjmp(error_jmp_buf) != 0) {
+      // TryCompile resulted in an mju_error which was converted to a longjmp.
       throw mjCError(0, "engine error: %s", errortext);
     }
-
-    // check if nan test works
-    double test = mjNAN;
-    if (mjuu_defined(test)) {
-      throw mjCError(0, "NaN test does not work for present compiler/options");
-    }
-
-    // check for repeated compilation
-    if (compiled) {
-      throw mjCError(0, "model already compiled");
-    }
-
-    // check for joints in world body
-    if (!bodies[0]->joints.empty()) {
-      throw mjCError(0, "joint found in world body");
-    }
-
-    // append directory separator
-    if (!meshdir.empty()) {
-      int n = meshdir.length();
-      if (meshdir[n-1]!='/' && meshdir[n-1]!='\\') {
-        meshdir += '/';
-      }
-    }
-    if (!texturedir.empty()) {
-      int n = texturedir.length();
-      if (texturedir[n-1]!='/' && texturedir[n-1]!='\\') {
-        texturedir += '/';
-      }
-    }
-
-    // add missing keyframes
-    for (i=keys.size(); i<nkey; i++) {
-      AddKey();
-    }
-
-    // make lists of objects created in kinematic tree
-    MakeLists(bodies[0]);
-
-    // set object ids and default names, check for repeated names
-    processlist(bodies, "body");
-    processlist(joints, "joint");
-    processlist(geoms, "geom");
-    processlist(sites, "site");
-    processlist(cameras, "camera");
-    processlist(lights, "light");
-    processlist(meshes, "mesh");
-    processlist(skins, "skin");
-    processlist(hfields, "hfield");
-    processlist(textures, "texture");
-    processlist(materials, "material");
-    processlist(pairs, "pair");
-    processlist(excludes, "exclude");
-    processlist(equalities, "equality");
-    processlist(tendons, "tendon");
-    processlist(actuators, "actuator");
-    processlist(sensors, "sensor");
-    processlist(numerics, "numeric");
-    processlist(texts, "text");
-    processlist(tuples, "tuple");
-    processlist(keys, "key");
-
-    // set default names, convert names into indices
-    SetDefaultNames();
-    IndexAssets();
-
-    // mark meshes that need convex hull
-    for (i=0; i<geoms.size(); i++) {
-      if (geoms[i]->meshid>=0 && geoms[i]->type==mjGEOM_MESH &&
-          (geoms[i]->contype || geoms[i]->conaffinity)) {
-        meshes[geoms[i]->meshid]->needhull = true;
-      }
-    }
-
-    // compile meshes (needed for geom compilation)
-    for (i=0; i<meshes.size(); i++) {
-      meshes[i]->Compile(vfs);
-    }
-
-    // automatically set nuser fields
-    if (nuser_body == -1) {
-      nuser_body = 0;
-      for (i=0; i<bodies.size(); i++) {
-        nuser_body = mjMAX(nuser_body, bodies[i]->userdata.size());
-      }
-    }
-    if (nuser_jnt == -1) {
-      nuser_jnt = 0;
-      for (i=0; i<joints.size(); i++) {
-        nuser_jnt = mjMAX(nuser_jnt, joints[i]->userdata.size());
-      }
-    }
-    if (nuser_geom == -1) {
-      nuser_geom = 0;
-      for (i=0; i<geoms.size(); i++) {
-        nuser_geom = mjMAX(nuser_geom, geoms[i]->userdata.size());
-      }
-    }
-    if (nuser_site == -1) {
-      nuser_site = 0;
-      for (i=0; i<sites.size(); i++) {
-        nuser_site = mjMAX(nuser_site, sites[i]->userdata.size());
-      }
-    }
-    if (nuser_cam == -1) {
-      nuser_cam = 0;
-      for (i=0; i<cameras.size(); i++) {
-        nuser_cam = mjMAX(nuser_cam, cameras[i]->userdata.size());
-      }
-    }
-    if (nuser_tendon == -1) {
-      nuser_tendon = 0;
-      for (i=0; i<tendons.size(); i++) {
-        nuser_tendon = mjMAX(nuser_tendon, tendons[i]->userdata.size());
-      }
-    }
-    if (nuser_actuator == -1) {
-      nuser_actuator = 0;
-      for (i=0; i<actuators.size(); i++) {
-        nuser_actuator = mjMAX(nuser_actuator, actuators[i]->userdata.size());
-      }
-    }
-    if (nuser_sensor == -1) {
-      nuser_sensor = 0;
-      for (i=0; i<sensors.size(); i++) {
-        nuser_sensor = mjMAX(nuser_sensor, sensors[i]->userdata.size());
-      }
-    }
-
-    // compile objects in kinematic tree
-    for (i=0; i<bodies.size(); i++) {
-      bodies[i]->Compile();  // also compiles joints, geoms, sites, cameras, lights
-    }
-
-    // compile all other objects except for keyframes
-    for (i=0; i<skins.size(); i++) skins[i]->Compile(vfs);
-    for (i=0; i<hfields.size(); i++) hfields[i]->Compile(vfs);
-    for (i=0; i<textures.size(); i++) textures[i]->Compile(vfs);
-    for (i=0; i<materials.size(); i++) materials[i]->Compile();
-    for (i=0; i<pairs.size(); i++) pairs[i]->Compile();
-    for (i=0; i<excludes.size(); i++) excludes[i]->Compile();
-    for (i=0; i<equalities.size(); i++) equalities[i]->Compile();
-    for (i=0; i<tendons.size(); i++) tendons[i]->Compile();
-    for (i=0; i<actuators.size(); i++) actuators[i]->Compile();
-    for (i=0; i<sensors.size(); i++) sensors[i]->Compile();
-    for (i=0; i<numerics.size(); i++) numerics[i]->Compile();
-    for (i=0; i<texts.size(); i++) texts[i]->Compile();
-    for (i=0; i<tuples.size(); i++) tuples[i]->Compile();
-
-    // compile defaults: to enforce userdata length for writer
-    for (i=0; i<defaults.size(); i++) {
-      defaults[i]->Compile(this);
-    }
-
-    // sort pair, exclude in increasing signature order; reassign ids
-    sort(pairs.begin(), pairs.end(), comparePair);
-    sort(excludes.begin(), excludes.end(), compareBodyPair);
-    reassignid(pairs);
-    reassignid(excludes);
-
-    // resolve asset references, compute sizes
-    IndexAssets();
-    SetSizes();
-
-    // fuse static if enabled
-    if (fusestatic) {
-      FuseStatic();
-    }
-
-    // set nmocap and body.mocapid
-    for (i=0; i<bodies.size(); i++) {
-      if (bodies[i]->mocap) {
-        bodies[i]->mocapid = nmocap;
-        nmocap++;
-      } else {
-        bodies[i]->mocapid = -1;
-      }
-    }
-
-    // check body mass and inertia
-    for (i=1; i<bodies.size(); i++) {
-      mjCBody* b = bodies[i];
-
-      // find moving body with small mass or inertia
-      if (!b->joints.empty() &&
-          (b->mass<mjMINVAL ||
-           b->inertia[0]<mjMINVAL ||
-           b->inertia[1]<mjMINVAL ||
-           b->inertia[2]<mjMINVAL)) {
-        // does it have static children with mass and inertia
-        bool ok = false;
-        for (size_t j=0; j<b->bodies.size(); j++) {
-          if (b->bodies[j]->joints.empty() &&
-              b->bodies[j]->mass>=mjMINVAL &&
-              b->bodies[j]->inertia[0]>=mjMINVAL &&
-              b->bodies[j]->inertia[1]>=mjMINVAL &&
-              b->bodies[j]->inertia[2]>=mjMINVAL) {
-            ok = true;
-            break;
-          }
-        }
-
-        // error
-        if (!ok) {
-          throw mjCError(b, "mass and inertia of moving bodies must be larger than mjMINVAL");
-        }
-      }
-    }
-
-    // create low-level model
-    m = mj_makeModel(nq, nv, nu, na, nbody, njnt, ngeom, nsite, ncam, nlight,
-                     nmesh, nmeshvert, nmeshtexvert, nmeshface, nmeshgraph,
-                     nskin, nskinvert, nskintexvert, nskinface, nskinbone, nskinbonevert,
-                     nhfield, nhfielddata, ntex, ntexdata, nmat, npair, nexclude,
-                     neq, ntendon, nwrap, nsensor,
-                     nnumeric, nnumericdata, ntext, ntextdata,
-                     ntuple, ntupledata, nkey, nmocap,
-                     nuser_body, nuser_jnt, nuser_geom, nuser_site, nuser_cam,
-                     nuser_tendon, nuser_actuator, nuser_sensor, nnames);
-    if (!m) {
-      throw mjCError(0, "could not create mjModel");
-    }
-
-    // copy everything into low-level model
-    m->opt = option;
-    m->vis = visual;
-    CopyNames(m);
-    CopyTree(m);
-
-    // keyframe compilation needs access to nq, nv, na, nmocap, qpos0
-    for (i=0; i<keys.size(); i++) {
-      keys[i]->Compile(m);
-    }
-
-    // copy objects outsite kinematic tree (including keyframes)
-    CopyObjects(m);
-
-    // scale mass
-    if (settotalmass>0) {
-      mj_setTotalmass(m, settotalmass);
-    }
-
-    // set stack size: user-specified or conservative heuristic
-    if (nstack>0) {
-      m->nstack = nstack;
-    } else {
-      m->nstack = mjMAX(1000,
-                        5*(m->njmax + m->neq + m->nv)*(m->njmax + m->neq + m->nv) +
-                        20*(m->nq + m->nv + m->nu + m->na + m->nbody + m->njnt +
-                            m->ngeom + m->nsite + m->neq + m->ntendon +  m->nwrap));
-    }
-
-    // create data
-    data = mj_makeData(m);
-    if (!data) {
-      throw mjCError(0, "could not create mjData");
-    }
-
-    // normalize keyframe quaternions
-    for (i=0; i<m->nkey; i++) {
-      mj_normalizeQuat(m, m->key_qpos+i*m->nq);
-    }
-
-    // set constant fields
-    mj_setConst(m, data);
-
-    // automatic spring-damper adjustment
-    AutoSpringDamper(m);
-
-    // actuator lengthrange computation
-    LengthRange(m, data);
-
-    // override model statistics if defined by user
-    if (mjuu_defined(extent)) m->stat.extent = (mjtNum)extent;
-    if (mjuu_defined(meaninertia)) m->stat.meaninertia = (mjtNum)meaninertia;
-    if (mjuu_defined(meanmass)) m->stat.meanmass = (mjtNum)meanmass;
-    if (mjuu_defined(meansize)) m->stat.meansize = (mjtNum)meansize;
-    if (mjuu_defined(center[0])) copyvec(m->stat.center, center, 3);
-
-    // assert that model has valid references
-    const char* validationerr = mj_validateReferences(m);
-    if (validationerr) {  // SHOULD NOT OCCUR
-      throw mjCError(0, validationerr);
-    }
-    // test forward simulation
-    mj_resetData(m, data);
-    mj_step(m, data);
-
-    // delete data
-    mj_deleteData(data);
-    data = NULL;
-
-    // pass warning back
-    if (warningtext[0]) {
-      mju::strcpy_arr(errInfo.message, warningtext);
-      errInfo.warning = true;
-    }
-  }
-
-  // handle mjCError exceptions
-  catch (mjCError err) {
+    TryCompile(const_cast<mjModel**>(&m), const_cast<mjData**>(&data), vfs);
+  } catch (mjCError err) {
     // deallocate everything allocated in Compile
     mj_deleteModel(m);
     mj_deleteData(data);
@@ -2619,6 +2327,307 @@ mjModel* mjCModel::Compile(const mjVFS* vfs) {
 }
 
 
+void mjCModel::TryCompile(mjModel** m, mjData** data, const mjVFS* vfs) {
+  // check if nan test works
+  double test = mjNAN;
+  if (mjuu_defined(test)) {
+    throw mjCError(0, "NaN test does not work for present compiler/options");
+  }
+
+  // check for repeated compilation
+  if (compiled) {
+    throw mjCError(0, "model already compiled");
+  }
+
+  // check for joints in world body
+  if (!bodies[0]->joints.empty()) {
+    throw mjCError(0, "joint found in world body");
+  }
+
+  // append directory separator
+  if (!meshdir.empty()) {
+    int n = meshdir.length();
+    if (meshdir[n-1]!='/' && meshdir[n-1]!='\\') {
+      meshdir += '/';
+    }
+  }
+  if (!texturedir.empty()) {
+    int n = texturedir.length();
+    if (texturedir[n-1]!='/' && texturedir[n-1]!='\\') {
+      texturedir += '/';
+    }
+  }
+
+  // add missing keyframes
+  for (int i=keys.size(); i<nkey; i++) {
+    AddKey();
+  }
+
+  // make lists of objects created in kinematic tree
+  MakeLists(bodies[0]);
+
+  // set object ids and default names, check for repeated names
+  processlist(bodies, "body");
+  processlist(joints, "joint");
+  processlist(geoms, "geom");
+  processlist(sites, "site");
+  processlist(cameras, "camera");
+  processlist(lights, "light");
+  processlist(meshes, "mesh");
+  processlist(skins, "skin");
+  processlist(hfields, "hfield");
+  processlist(textures, "texture");
+  processlist(materials, "material");
+  processlist(pairs, "pair");
+  processlist(excludes, "exclude");
+  processlist(equalities, "equality");
+  processlist(tendons, "tendon");
+  processlist(actuators, "actuator");
+  processlist(sensors, "sensor");
+  processlist(numerics, "numeric");
+  processlist(texts, "text");
+  processlist(tuples, "tuple");
+  processlist(keys, "key");
+
+  // set default names, convert names into indices
+  SetDefaultNames();
+  IndexAssets();
+
+  // mark meshes that need convex hull
+  for (int i=0; i<geoms.size(); i++) {
+    if (geoms[i]->meshid>=0 && geoms[i]->type==mjGEOM_MESH &&
+        (geoms[i]->contype || geoms[i]->conaffinity)) {
+      meshes[geoms[i]->meshid]->needhull = true;
+    }
+  }
+
+  // compile meshes (needed for geom compilation)
+  for (int i=0; i<meshes.size(); i++) {
+    meshes[i]->Compile(vfs);
+  }
+
+  // automatically set nuser fields
+  if (nuser_body == -1) {
+    nuser_body = 0;
+    for (int i=0; i<bodies.size(); i++) {
+      nuser_body = mjMAX(nuser_body, bodies[i]->userdata.size());
+    }
+  }
+  if (nuser_jnt == -1) {
+    nuser_jnt = 0;
+    for (int i=0; i<joints.size(); i++) {
+      nuser_jnt = mjMAX(nuser_jnt, joints[i]->userdata.size());
+    }
+  }
+  if (nuser_geom == -1) {
+    nuser_geom = 0;
+    for (int i=0; i<geoms.size(); i++) {
+      nuser_geom = mjMAX(nuser_geom, geoms[i]->userdata.size());
+    }
+  }
+  if (nuser_site == -1) {
+    nuser_site = 0;
+    for (int i=0; i<sites.size(); i++) {
+      nuser_site = mjMAX(nuser_site, sites[i]->userdata.size());
+    }
+  }
+  if (nuser_cam == -1) {
+    nuser_cam = 0;
+    for (int i=0; i<cameras.size(); i++) {
+      nuser_cam = mjMAX(nuser_cam, cameras[i]->userdata.size());
+    }
+  }
+  if (nuser_tendon == -1) {
+    nuser_tendon = 0;
+    for (int i=0; i<tendons.size(); i++) {
+      nuser_tendon = mjMAX(nuser_tendon, tendons[i]->userdata.size());
+    }
+  }
+  if (nuser_actuator == -1) {
+    nuser_actuator = 0;
+    for (int i=0; i<actuators.size(); i++) {
+      nuser_actuator = mjMAX(nuser_actuator, actuators[i]->userdata.size());
+    }
+  }
+  if (nuser_sensor == -1) {
+    nuser_sensor = 0;
+    for (int i=0; i<sensors.size(); i++) {
+      nuser_sensor = mjMAX(nuser_sensor, sensors[i]->userdata.size());
+    }
+  }
+
+  // compile objects in kinematic tree
+  for (int i=0; i<bodies.size(); i++) {
+    bodies[i]->Compile();  // also compiles joints, geoms, sites, cameras, lights
+  }
+
+  // compile all other objects except for keyframes
+  for (int i=0; i<skins.size(); i++) skins[i]->Compile(vfs);
+  for (int i=0; i<hfields.size(); i++) hfields[i]->Compile(vfs);
+  for (int i=0; i<textures.size(); i++) textures[i]->Compile(vfs);
+  for (int i=0; i<materials.size(); i++) materials[i]->Compile();
+  for (int i=0; i<pairs.size(); i++) pairs[i]->Compile();
+  for (int i=0; i<excludes.size(); i++) excludes[i]->Compile();
+  for (int i=0; i<equalities.size(); i++) equalities[i]->Compile();
+  for (int i=0; i<tendons.size(); i++) tendons[i]->Compile();
+  for (int i=0; i<actuators.size(); i++) actuators[i]->Compile();
+  for (int i=0; i<sensors.size(); i++) sensors[i]->Compile();
+  for (int i=0; i<numerics.size(); i++) numerics[i]->Compile();
+  for (int i=0; i<texts.size(); i++) texts[i]->Compile();
+  for (int i=0; i<tuples.size(); i++) tuples[i]->Compile();
+
+  // compile defaults: to enforce userdata length for writer
+  for (int i=0; i<defaults.size(); i++) {
+    defaults[i]->Compile(this);
+  }
+
+  // sort pair, exclude in increasing signature order; reassign ids
+  sort(pairs.begin(), pairs.end(), comparePair);
+  sort(excludes.begin(), excludes.end(), compareBodyPair);
+  reassignid(pairs);
+  reassignid(excludes);
+
+  // resolve asset references, compute sizes
+  IndexAssets();
+  SetSizes();
+
+  // fuse static if enabled
+  if (fusestatic) {
+    FuseStatic();
+  }
+
+  // set nmocap and body.mocapid
+  for (int i=0; i<bodies.size(); i++) {
+    if (bodies[i]->mocap) {
+      bodies[i]->mocapid = nmocap;
+      nmocap++;
+    } else {
+      bodies[i]->mocapid = -1;
+    }
+  }
+
+  // check body mass and inertia
+  for (int i=1; i<bodies.size(); i++) {
+    mjCBody* b = bodies[i];
+
+    // find moving body with small mass or inertia
+    if (!b->joints.empty() &&
+        (b->mass<mjMINVAL ||
+          b->inertia[0]<mjMINVAL ||
+          b->inertia[1]<mjMINVAL ||
+          b->inertia[2]<mjMINVAL)) {
+      // does it have static children with mass and inertia
+      bool ok = false;
+      for (size_t j=0; j<b->bodies.size(); j++) {
+        if (b->bodies[j]->joints.empty() &&
+            b->bodies[j]->mass>=mjMINVAL &&
+            b->bodies[j]->inertia[0]>=mjMINVAL &&
+            b->bodies[j]->inertia[1]>=mjMINVAL &&
+            b->bodies[j]->inertia[2]>=mjMINVAL) {
+          ok = true;
+          break;
+        }
+      }
+
+      // error
+      if (!ok) {
+        throw mjCError(b, "mass and inertia of moving bodies must be larger than mjMINVAL");
+      }
+    }
+  }
+
+  // create low-level model
+  *m = mj_makeModel(nq, nv, nu, na, nbody, njnt, ngeom, nsite, ncam, nlight,
+                    nmesh, nmeshvert, nmeshtexvert, nmeshface, nmeshgraph,
+                    nskin, nskinvert, nskintexvert, nskinface, nskinbone, nskinbonevert,
+                    nhfield, nhfielddata, ntex, ntexdata, nmat, npair, nexclude,
+                    neq, ntendon, nwrap, nsensor,
+                    nnumeric, nnumericdata, ntext, ntextdata,
+                    ntuple, ntupledata, nkey, nmocap,
+                    nuser_body, nuser_jnt, nuser_geom, nuser_site, nuser_cam,
+                    nuser_tendon, nuser_actuator, nuser_sensor, nnames);
+  if (!*m) {
+    throw mjCError(0, "could not create mjModel");
+  }
+
+  // copy everything into low-level model
+  (*m)->opt = option;
+  (*m)->vis = visual;
+  CopyNames(*m);
+  CopyTree(*m);
+
+  // keyframe compilation needs access to nq, nv, na, nmocap, qpos0
+  for (int i=0; i<keys.size(); i++) {
+    keys[i]->Compile(*m);
+  }
+
+  // copy objects outsite kinematic tree (including keyframes)
+  CopyObjects(*m);
+
+  // scale mass
+  if (settotalmass>0) {
+    mj_setTotalmass(*m, settotalmass);
+  }
+
+  // set stack size: user-specified or conservative heuristic
+  if (nstack>0) {
+    (*m)->nstack = nstack;
+  } else {
+    (*m)->nstack = mjMAX(
+        1000,
+        5*((*m)->njmax + (*m)->neq + (*m)->nv)*((*m)->njmax + (*m)->neq + (*m)->nv) +
+        20*((*m)->nq + (*m)->nv + (*m)->nu + (*m)->na + (*m)->nbody + (*m)->njnt +
+            (*m)->ngeom + (*m)->nsite + (*m)->neq + (*m)->ntendon +  (*m)->nwrap));
+  }
+
+  // create data
+  *data = mj_makeData(*m);
+  if (!data) {
+    throw mjCError(0, "could not create mjData");
+  }
+
+  // normalize keyframe quaternions
+  for (int i=0; i<(*m)->nkey; i++) {
+    mj_normalizeQuat(*m, (*m)->key_qpos+i*(*m)->nq);
+  }
+
+  // set constant fields
+  mj_setConst(*m, *data);
+
+  // automatic spring-damper adjustment
+  AutoSpringDamper(*m);
+
+  // actuator lengthrange computation
+  LengthRange(*m, *data);
+
+  // override model statistics if defined by user
+  if (mjuu_defined(extent)) (*m)->stat.extent = (mjtNum)extent;
+  if (mjuu_defined(meaninertia)) (*m)->stat.meaninertia = (mjtNum)meaninertia;
+  if (mjuu_defined(meanmass)) (*m)->stat.meanmass = (mjtNum)meanmass;
+  if (mjuu_defined(meansize)) (*m)->stat.meansize = (mjtNum)meansize;
+  if (mjuu_defined(center[0])) copyvec((*m)->stat.center, center, 3);
+
+  // assert that model has valid references
+  const char* validationerr = mj_validateReferences(*m);
+  if (validationerr) {  // SHOULD NOT OCCUR
+    throw mjCError(0, validationerr);
+  }
+  // test forward simulation
+  mj_resetData(*m, *data);
+  mj_step(*m, *data);
+
+  // delete data
+  mj_deleteData(*data);
+  data = nullptr;
+
+  // pass warning back
+  if (warningtext[0]) {
+    mju::strcpy_arr(errInfo.message, warningtext);
+    errInfo.warning = true;
+  }
+}
+
+
 
 //------------------------------- DECOMPILER -------------------------------------------------------
 
diff --git a/src/user/user_model.h b/src/user/user_model.h
index c1ff5625..45f2e3db 100644
--- a/src/user/user_model.h
+++ b/src/user/user_model.h
@@ -153,6 +153,8 @@ class mjCModel {
   int nuser_sensor;               // number of mjtNums in sensor_user
 
  private:
+  void TryCompile(mjModel** m, mjData** data, const mjVFS* vfs);
+
   void Clear(void);               // clear objects allocated by Compile
 
   template <class T>              // add object of any type