Weird channel access problem within Python softioc process

[Paul Richards via Tech-talk <tech-talk at aps.anl.gov>]

All,

 

At Keck, we have been using the EPICS Python softioc with great success.  The recent 3.1 release has made it even more popular and easier to use.

 

That said, I have found a very odd behavior that I can replicate, relating to the consumption of keywords inside the same process that runs the softioc.  Sometimes it makes sense to do it this way, instead of finding a reference to the channel in the IOC and manipulating that.

 

A simplified example of the problem:

1. Import the pyepics and softioc libraries.
2. Start a softioc with a channel (call it ‘prefix:long’ in my example).
3. Start a thread that periodically updates the channel with a new value.
4. Start another thread that tries to access this channel, either with epics.PV() or epics.caget().
5. Channel access in thread 2 always times out, forever.

 

Today I found a “solution” to this that does not make sense: if I start the softioc (the call to softioc.iotInit()) in the first thread, and not in the main process, everything works as expected.

 

Our engineers are unable to explain this behavior.  I hope there’s someone here on the list that might know why!

 

 

Example code that demonstrates the failure:

--------------------

 

import os

import time

import threading

 

os.environ['EPICS_CA_SERVER_PORT'] = '5100'

os.environ['EPICS_CA_ADDR_LIST'] = 'localhost:5100'

os.environ['EPICS_CA_AUTO_ADDR_LIST'] = 'NO'

 

try:

    import epicscorelibs.path.pyepics

except ImportError:

    pass

import epics

 

from softioc import softioc, builder

 

# Create a long record for this demo

long = builder.longOut('prefix:long', initial_value=1)

builder.LoadDatabase()

softioc.iocInit()

 

def thread1():

    while True:

        val = long.get()

        long.set(val + 1)

        print(f'thread1: {val}')

        time.sleep(1)

 

def thread2():

    pv = epics.PV('prefix:long')

 

    while True:

        val = pv.get()

        print(f'thread2: {val}')

        time.sleep(1)

 

t1 = threading.Thread(target=thread1, daemon=True)

t2 = threading.Thread(target=thread2, daemon=True)

 

t1.start()

t2.start()

 

time.sleep(10)

 

 

 

Example that works; note the only change is the movement of the .iocInit() line down into thread1:

--------------------

 

import os

import time

import threading

 

os.environ['EPICS_CA_SERVER_PORT'] = '5100'

os.environ['EPICS_CA_ADDR_LIST'] = 'localhost:5100'

os.environ['EPICS_CA_AUTO_ADDR_LIST'] = 'NO'

 

try:

    import epicscorelibs.path.pyepics

except ImportError:

    pass

import epics

 

from softioc import softioc, builder

 

# Create a long record for this demo

long = builder.longOut('prefix:long', initial_value=1)

builder.LoadDatabase()

 

def thread1():

    softioc.iocInit()

 

    while True:

        val = long.get()

        long.set(val + 1)

        print(f'thread1: {val}')

        time.sleep(1)

 

def thread2():

   pv = epics.PV('prefix:long')

 

    while True:

        val = pv.get()

        print(f'thread2: {val}')

        time.sleep(1)

 

t1 = threading.Thread(target=thread1, daemon=True)

t2 = threading.Thread(target=thread2, daemon=True)

 

t1.start()

t2.start()

 

time.sleep(10)

 

 

The output when it fails:

 

$ python3 ./demoFailingDaemon.py

Starting iocInit

############################################################################

## EPICS 7.0.6.0

## Rev. 7.0.6.99.1.0

############################################################################

iocRun: All initialization complete

thread1: 1

thread1: 2

thread1: 3

thread2: None

thread1: 4

thread1: 5

thread2: None

thread1: 6

thread1: 7

thread1: 8

thread2: None

thread1: 9

thread1: 10

thread1: 11

 

 

The output when it works:

 

$ python3 ./demoWorkingDaemon.py

Starting iocInit

############################################################################

## EPICS 7.0.6.0

## Rev. 7.0.6.99.1.0

############################################################################

iocRun: All initialization complete

thread1: 1

thread2: 2

thread1: 2

thread2: 3

thread1: 3

thread2: 4

thread1: 4

thread2: 5

thread1: 5

thread2: 6

thread1: 6

thread2: 7

thread1: 7

thread2: 8

thread1: 8

thread2: 9

thread1: 9

thread2: 10

thread1: 10

thread2: 11

 

 

 

 

 

Paul

 

 

Paul Richards Software Engineer W. M. Keck Observatory 65-1120 Mamalahoa Hwy. Kamuela, Hawai'i 96743

Main: (808) 885-7887 Direct: (808) 881-3537 Fax: (808) 885-3535