p4p.client.thread.Context.put does not always work as expected when used inside a handler

[kurup]

I'm trying to replicate similar functionality to how a forward link works in a traditional IOC using p4p.client.thread.Context.put in the handler for a PV however, I've run into an issue with this not always working depending on the order in which PVs are created. A simple example of the problem is:

from p4p.server import Server
from p4p.server.thread import SharedPV
from p4p.client.thread import Context
from p4p.nt import NTScalar

class MyHandler:
    def put(self, pv, op):
        pv.post(op.value())
        op.done()

class MyHandlerWithPut:
    def put(self, pv, op):   
        pv.post(op.value())
        op.done()
        ctxt = Context('pva')
        ctxt.put("DEV:RW:DOUBLE1",{})

pv1 = SharedPV(nt=NTScalar('i'), initial=0.0, handler=MyHandler())
pv2 = SharedPV(nt=NTScalar('i'), initial=0.0, handler=MyHandler())
pv3 = SharedPV(nt=NTScalar('i'), initial=0.0, handler=MyHandler())
pv4 = SharedPV(nt=NTScalar('i'), initial=0.0, handler=MyHandler())
pv5 = SharedPV(nt=NTScalar('i'), initial=0.0, handler=MyHandlerWithPut())

Server.forever(providers=[{
    "DEV:RW:DOUBLE1": pv1, 
    "DEV:RW:DOUBLE2": pv2, 
    "DEV:RW:DOUBLE3": pv3, 
    "DEV:RW:DOUBLE4": pv4,
    "DEV:RW:DOUBLE5": pv5 
    }])

The idea is that if I do a put to DEV:RW:DOUBLE5 this then triggers a put to DEV:RW:DOUBLE1. However, if I do a put to DEV:RW:DOUBLE5 (from another terminal) the server gives this error:

Unexpected
Traceback (most recent call last):
  File "/home/myuser/.local/lib/python3.10/site-packages/p4p/client/thread.py", line 362, in put
    value, i = done.get(timeout=timeout)
  File "/usr/lib/python3.10/queue.py", line 179, in get
    raise Empty
_queue.Empty

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/myuser/.local/lib/python3.10/site-packages/p4p/server/thread.py", line 19, in _on_queue
    M(*args)
  File "/mnt/c/Users/mysuser/Software/p4p_for_isis/examples/basic_server_recipe-testing.py", line 40, in put
    ctxt.put(put_pv,{})
  File "/home/myuser/.local/lib/python3.10/site-packages/p4p/client/thread.py", line 365, in put
    raise TimeoutError()
TimeoutError
Error from WorkQueue w/ functools.partial(<function _on_queue at 0x7f771b1bca60>, <p4p._p4p.ServerOperation object at 0x7f771b1b3be0>, <bound method MyHandlerWithPut.put of <__main__.MyHandlerWithPut object at 0x7f771b1ec7f0>>, SharedPV(value=0), <p4p.server.raw.ServOpWrap object at 0x7f771b1eca30>)
Traceback (most recent call last):
  File "/home/myuser/.local/lib/python3.10/site-packages/p4p/util.py", line 56, in handle
    callable()
  File "/home/myuser/.local/lib/python3.10/site-packages/p4p/server/thread.py", line 27, in _on_queue
    op.done(error=str(err))
  File "src/p4p/_p4p.pyx", line 888, in p4p._p4p.ServerOperation.done
ValueError: Must provide error message

The odd thing is the above code works if I change the link to point to any of the other pvs (e.g. DEV:RW:DOUBLE2). This also works if I create another pv between pv1 and pv5. Diggging about in the code I found that I can fix this by changing the number of workers defined in the __init__ method of the class _DefaultWorkQueue in p4p.util, i.e. if I use def __init__(self, workers=5):

It looks like this problem occurs for the specific configuration of having 5 pvs in a row with the last one linking to the first. I'm not sure if this is a bug or if there is a different way to call a put to a pv from within a handler. This issue may also be related to #144