Add option to override the mujoco error callback. (#434)

Mujoco 2.0.2.3 

mujoco_py/cymj.pyx

@@ -53,6 +53,7 @@ cdef extern from "gl/glshim.h":
 # This is the python callback function.  We save it in the global() context
 # so we can access it from a C wrapper function (c_warning_callback)
 cdef object py_warning_callback
+cdef object py_error_callback
 # This is the saved exception.  Because the C callback can not propagate
 # exceptions, this must be set to None before calling into MuJoCo, and then
 # inspected afterwards.
@@ -99,6 +100,36 @@ def get_warning_callback():
     return py_warning_callback
 
 
+cdef void c_error_callback(const char *msg) with gil:
+    '''
+    Wraps the error callback so that we can pass a python function to the callback.
+    MuJoCo error handlers are expected to terminate the program and never return.
+    '''
+    global py_error_callback
+    (<object> py_error_callback)(msg)
+
+
+def set_error_callback(err_callback):
+    '''
+    Set a user-defined error callback.  It should take in a string message
+        (the warning string) and terminate the program.
+    See c_warning_callback, which is the C wrapper to the user defined function
+    '''
+    global py_error_callback
+    global mju_user_error
+    py_error_callback = err_callback
+    mju_user_error = c_error_callback
+
+
+def get_error_callback():
+    '''
+    Returns the user-defined warning callback, for use in e.g. a context
+    manager.
+    '''
+    global py_error_callback
+    return py_error_callback
+
+
 class wrap_mujoco_warning(object):
     '''
     Class to wrap capturing exceptions raised during warning callbacks.

mujoco_py/tests/test_cymj.py

@@ -14,7 +14,7 @@ from mujoco_py import (MjSim, load_model_from_xml,
                        load_model_from_path, MjSimState,
                        ignore_mujoco_warnings,
                        load_model_from_mjb)
-from mujoco_py import const, cymj
+from mujoco_py import const, cymj, functions
 from mujoco_py.tests.utils import compare_imgs
 
 
@@ -334,6 +334,20 @@ def test_mj_warning_raises():
         sim.step()
 
 
+def test_mj_error_called():
+    error_message = None
+
+    def error_callback(msg):
+        nonlocal error_message
+        error_message = msg.decode()
+
+    cymj.set_error_callback(error_callback)
+
+    functions.mju_error("error")
+
+    assert error_message == "error"
+
+
 def test_ignore_mujoco_warnings():
     # Two boxes on a plane need more than 1 contact (nconmax)
     xml = '''

mujoco_py/version.py

 __all__ = ['__version__', 'get_version']
 
-version_info = (2, 0, 2, 2)
+version_info = (2, 0, 2, 3)
 # format:
 # ('mujoco_major', 'mujoco_minor', 'mujoco_py_major', 'mujoco_py_minor')