Experimental Physics and
Industrial Control System

It occurred to me later that the situation I was thinking about should only occur if the code was using the CA Provider through the pvaccess API, which is unlikely to be the case here, the code is using pyepics for CA channels and pvapy for PVA channels. However I do think that the access fault you’re seeing is because a thread that is calling libca doesn’t have a CA context.

On Sep 9, 2021, at 5:16 PM, Mark Rivers <rivers at cars.uchicago.edu> wrote:

Ø  That could mean that calling pyepics code from inside a pvaccess callback/notification routine might no longer work if pyepics doesn’t properly set its CA context before calling routines inside libca (unfortunately that’s probably what’s happening, given the access violation that Mark is seeing).

 

The pyepics call that it crashing is not happening inside a pvaccess callback/notification routine, it is happening in the following code:

 

class TomoScan():

…

    def __init__(self, pv_files, macros):

…

        # Start the watchdog timer thread

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

        thread.start()

…

    def reset_watchdog(self):

        """Sets the watchdog timer to 5 every 3 seconds"""

        while True:

            self.epics_pvs['Watchdog'].put(5)

            time.sleep(3)

 

So the tomoscan constructor is creating a new thread with threading.Thread.  That thread is running the simple reset_watchdog function, which simply does an epics.PV.put(5) every 3 seconds.  That call crashes with pvaccess 4.0.0 under the following conditions:

-          tomoscan is built with the functions that include pvaccess

or

-          tomoscan is built without pvaccess but PYEPICS_LIBCA is pointing to the version of ca.dll that comes with pvaccess

Does pyepics give access to the libca routines ca_current_context() and ca_attach_context() ? If so I would recommend having that watchdog thread explicitly attach to the context that was created for the main thread. I think that will probably solve the problem if those APIs are available.