Just-in-time compiled substep callback

[machinaut]

This introduces a couple things together:

• A way to just-in-time compile small C functions that can be passed to mujoco as callbacks
• A mechanism of naming fields in userdata and accessing them from the function
• A substep callback function that can be set to be called every substep

Differences between this an our current user-defined-dynamics

• Less flexible; cannot access global python state or any python objects or methods
• Faster, parallelizeable; because it doesn't require the GIL, we can do this over MjSimPools

[machinaut]

This test (test_sum_ctrl) is probably the best standalone example of what it looks like to use this.

[machinaut]

This is called "generic" because it's signature matches the mjfGeneric type defined here:

mujoco-py/mujoco_py/pxd/mjdata.pxd

Line 288 in c5f6032

# typedef void (*mjfGeneric)(const mjModel* m, mjData* d)

(and also in the mjdata header)

[wojzaremba]

Windows version is breaking.

It's fine to disable these tests for windows. Something like that:
@pytest.mark.skipif(sys.platform.startswith("win"), reason="This test fails on windows.")

Finished generating code
FAILED
___________________________ TestSubstep.test_hello ____________________________
self = <mujoco_py.tests.test_substep.TestSubstep testMethod=test_hello>
def test_hello(self):
sim = MjSim(load_model_from_xml(XML.format(nuserdata=0)),

              substep_udd_fn=HELLO_FN)

mujoco_py\tests\test_substep.py:42:

---

mujoco_py\mjsim.pyx:92: in mujoco_py.cymj.MjSim.cinit
self.set_substep_udd_fn(substep_udd_fn)
mujoco_py\mjsim.pyx:210: in mujoco_py.cymj.MjSim.set_substep_udd_fn
substep_udd_fn = build_generic_fn(substep_udd_fn,
mujoco_py\builder.py:337: in build_generic_fn
fixed_library_path = manually_link_libraries(mjpro_path, library_path)
mujoco_py\builder.py:129: in manually_link_libraries
tmp_final_cext_dll_path])
c:\miniconda35-x64\envs\test-environment\lib\subprocess.py:266: in check_call
retcode = call(*popenargs, **kwargs)
c:\miniconda35-x64\envs\test-environment\lib\subprocess.py:247: in call
with Popen(*popenargs, **kwargs) as p:
c:\miniconda35-x64\envs\test-environment\lib\subprocess.py:676: in init
restore_signals, start_new_session)

---

self = <subprocess.Popen object at 0x000000A3BECF7C50>
args = 'install_name_tool -change @executable_path/libmujoco150.dylib C:\Users\appveyor\.mujoco\mjpro150\bin/libmujoco150.dylib C:\projects\mujoco-py\_generic_fn_vhxddhpmouwlbxy.cp35-win_amd64_final.pyd~'
executable = None, preexec_fn = None, close_fds = True, pass_fds = ()
cwd = None, env = None
startupinfo = <subprocess.STARTUPINFO object at 0x000000A3BECF7588>
creationflags = 0, shell = False, p2cread = -1, p2cwrite = -1, c2pread = -1
c2pwrite = -1, errread = -1, errwrite = -1, unused_restore_signals = True
unused_start_new_session = False
def _execute_child(self, args, executable, preexec_fn, close_fds,
pass_fds, cwd, env,
startupinfo, creationflags, shell,
p2cread, p2cwrite,
c2pread, c2pwrite,
errread, errwrite,
unused_restore_signals, unused_start_new_session):
"""Execute program (MS Windows version)"""

    assert not pass_fds, "pass_fds not supported on Windows."

    if not isinstance(args, str):
        args = list2cmdline(args)

    # Process startup details
    if startupinfo is None:
        startupinfo = STARTUPINFO()
    if -1 not in (p2cread, c2pwrite, errwrite):
        startupinfo.dwFlags |= _winapi.STARTF_USESTDHANDLES
        startupinfo.hStdInput = p2cread
        startupinfo.hStdOutput = c2pwrite
        startupinfo.hStdError = errwrite

    if shell:
        startupinfo.dwFlags |= _winapi.STARTF_USESHOWWINDOW
        startupinfo.wShowWindow = _winapi.SW_HIDE
        comspec = os.environ.get("COMSPEC", "cmd.exe")
        args = '{} /c "{}"'.format (comspec, args)

    # Start the process
    try:
        hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
                                 # no special security
                                 None, None,
                                 int(not close_fds),
                                 creationflags,
                                 env,
                                 cwd,

                               startupinfo)

E FileNotFoundError: [WinError 2] The system cannot find the file specified
c:\miniconda35-x64\envs\test-environment\lib\subprocess.py:957: FileNotFoundError
mujoco_py/tests/test_substep.py::TestSubstep::test_increment generating ._generic_fn_ucevgvcdpgaisfz.c

[wojzaremba]

should we ensure that "substep_udd_fn" can be only string, or NULL.

[wojzaremba]

It's a bit safer.

[machinaut]

The reason to allow ints is this lets us re-use these functions between sims (in a single python process).

sim1 = MjSim(...., substep_udd_fn=...)
sim2 = MjSim(...., substep_udd_fn=sim1.substep_udd_fn)

[machinaut]

Added test which shows this

[machinaut]

cc: @wojzaremba here's the test you asked for

[bmcgrew]

Given that this is a substantially different implementation from udd, I would prefer to give it a different name. For example, what about substep_userdata_fn?

[machinaut]

Splitting this into several changes with feedback