RE: Incompatibility of pyepics and pvaPy 4.0.0 on Windows

[Mark Rivers via Tech-talk <tech-talk at aps.anl.gov>]

I wrote a very simple test program which I would have expected to fail, since it implements the same logic as the startup error in tomoscan.

 

corvette:~/support/tomoscan/tomoscan>more test_ca.py

import pvaccess

import epics

import threading

import time

 

class test_ca():

    def __init__(self):

        thread = threading.Thread(target=self.test_put, args=(), daemon=True)

        thread.start()

 

    def test_put(self):

         pv = epics.PV('13BMDPG1:cam1:AcquireTime')

         while True:

            print("Setting PV to 0.1")

            pv.put(0.1)

            time.sleep(3)

 

However, I cannot make it fail, even by setting PYEPICS_LIBCA to point to the pvaccess version of ca.dll.

 

Mark

 

 

From: Mark Rivers
Sent: Thursday, September 9, 2021 2:05 PM
To: tech-talk at aps.anl.gov
Cc: 'Veseli, Sinisa' <sveseli at anl.gov>; Matt Newville <newville at cars.uchicago.edu>; Francesco De Carlo <decarlo at aps.anl.gov>; Nikitin, Viktor <vnikitin at anl.gov>
Subject: Incompatibility of pyepics and pvaPy 4.0.0 on Windows

 

Folks,

 

I believe there is an incompatibility of the Python pyepics and pvapy packages on Windows.  I have a Python application called tomoscan that uses both packages.

https://github.com/tomography/tomoscan

 

This works fine on Linux.  It used to work OK on Windows.  However, using the latest versions of pyepics (3.5.0) and pvapy (4.0.0) it crashes as follows on startup:

 

(tomoscan) J:\epics\support\tomoscan\iocBoot\iocTomoScan_13BM_PSO>python -i start_tomoscan.py

Exception in thread Thread-1:

Traceback (most recent call last):

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\threading.py", line 932, in _bootstrap_inner

    self.run()

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\threading.py", line 870, in run

    self._target(*self._args, **self._kwargs)

  File "\\corvette\people_rw\epics\support\tomoscan\tomoscan\tomoscan.py", line 240, in reset_watchdog

    self.epics_pvs['Watchdog'].put(5)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\pv.py", line 54, in wrapped

    return func(self, *args, **kwargs)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\pv.py", line 652, in put

    return ca.put(self.chid, value,

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 610, in wrapper

    return fcn(*args, **kwds)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 1717, in put

    poll()

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 547, in wrapper

    return fcn(*args, **kwds)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 923, in poll

    pend_event(evt)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 910, in pend_event

    ret = libca.ca_pend_event(timeout)

OSError: exception: access violation reading 0x0000000000000048

 

Note that it is crashing with an access violation in the ca_pend_event() in the C library.  This happens when epics.PV.put() is called in a different Python thread.

 

I suspect the problem is that the version of ca.dll that comes with pvapy is somehow incompatible with pyepics.

 

This suspicion comes from the fact that I can modify the tomoscan package so that it does not use pvaccess by commenting out the 2 lines shown below in __init__py:

 

corvette:~/support/tomoscan/tomoscan>more __init__.py

__version__ = '0.1'

 

from tomoscan.tomoscan import *

from tomoscan.tomoscan_pso import *

from tomoscan.tomoscan_step import *

#from tomoscan.tomoscan_stream_pso import *

from tomoscan.tomoscan_13bm_pso import *

from tomoscan.tomoscan_13bm_mcs import *

from tomoscan.tomoscan_2bm import *

from tomoscan.tomoscan_2bm_step import *

from tomoscan.tomoscan_6bm_step import *

from tomoscan.tomoscan_32id import *

from tomoscan.tomoscan_32id_step import *

#from tomoscan.tomoscan_stream_2bm import *

from tomoscan.tomoscan_7bm import *

 

.tomoscan_stream_pso and .tomoscan_stream_2bm are the only files that use pvaccess.

 

With those 2 lines commented out tomoscan starts fine:

(tomoscan) J:\epics\support\tomoscan\iocBoot\iocTomoScan_13BM_PSO>python -i start_tomoscan.py

>>> 

 

However, I can cause it to crash again by telling pyepics to use the version of ca.dll that is part of pvapy using the environment variable PYEPICS_LIBCA.

 

>set PYEPICS_LIBCA=C:\Users\rivers\Anaconda3\envs\tomoscan\Lib\site-packages\pvaccess\ca.dll

 

(tomoscan) J:\epics\support\tomoscan\iocBoot\iocTomoScan_13BM_PSO>python -i start_tomoscan.py

Exception in thread Thread-1:

Traceback (most recent call last):

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\threading.py", line 932, in _bootstrap_inner

    self.run()

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\threading.py", line 870, in run

    self._target(*self._args, **self._kwargs)

  File "\\corvette\people_rw\epics\support\tomoscan\tomoscan\tomoscan.py", line 240, in reset_watchdog

    self.epics_pvs['Watchdog'].put(5)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\pv.py", line 54, in wrapped

    return func(self, *args, **kwargs)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\pv.py", line 652, in put

    return ca.put(self.chid, value,

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 610, in wrapper

    return fcn(*args, **kwds)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 1717, in put

    poll()

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 547, in wrapper

    return fcn(*args, **kwds)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 923, in poll

    pend_event(evt)

  File "C:\Users\rivers\Anaconda3\envs\tomoscan\lib\site-packages\epics\ca.py", line 910, in pend_event

    ret = libca.ca_pend_event(timeout)

OSError: exception: access violation reading 0x0000000000000048

 

This is pretty good evidence that the problem is due to some problem with the version of ca.dll that comes with pvapy 4.0.0.

 

If I downgrade pvapy from 4.0.0 to the previous version, 3.1.0 it works fine, using the original __init__.py that enables pvaccess.  This is true whether or not PYEPICS_LIBCA is defined to point to the pvaccess version of ca.dll.  It seems like there is some problem with the ca.dll that comes with pvapy 4.0.0.

 

Mark