Keep global registry of SNMP session objects

Before this, every Task instance would create its own SNMP session
instance.  This might not mean much under PySNMP, but is wasteful
using netsnmp-cffi, as each open session takes up a socket resource.
Having multiple simultaneous sessions/sockets per poll device makes
no sense, so `get_snmp_session()` utilizes a `WeakValueDictionary`
to keep track of the existing session for each device and returns
and existing session if there is one.

src/zino/snmp/__init__.py

@@ -1,5 +1,25 @@
 """Zino SNMP back-ends"""
 
+from weakref import WeakValueDictionary
+
+from zino.config.models import PollDevice
+
 # There is only one at the moment:
 # from .pysnmp_backend import *  # noqa
 from .netsnmpy_backend import *  # noqa
+from .netsnmpy_backend import SNMP
+
+_snmp_sessions = WeakValueDictionary()
+
+
+def get_snmp_session(device: PollDevice) -> SNMP:
+    """Factory function for creating an SNMP session.
+
+    This keeps a registry of existing/re-usable SNMP sessions so we avoid duplicate instances and over-use of sockets
+    """
+    # We generate a session key based on every attribute that affects how the SNMP session is set up:
+    key = (device.address, device.community, device.hcounters)
+    session = _snmp_sessions.get(key)
+    if not session:
+        session = _snmp_sessions[key] = SNMP(device)
+    return session

src/zino/tasks/task.py

@@ -4,7 +4,7 @@
 from typing import TYPE_CHECKING
 
 from zino.config.models import PollDevice
-from zino.snmp import SNMP
+from zino.snmp import get_snmp_session
 
 if TYPE_CHECKING:
     from zino.state import ZinoState
@@ -16,7 +16,7 @@ class Task(ABC):
     def __init__(self, device: PollDevice, state: ZinoState):
         self.device = device
         self.state = state
-        self.snmp = SNMP(device=device)
+        self.snmp = get_snmp_session(device=device)
 
     @abstractmethod
     async def run(self):