Handle low-level SnmpTimeoutErrors for SNMPv3

An SNMPv3 conversation may start with a discovery phase, to learn the
remote SNMP entity's authoritative EngineID.  This initial pair of
"discovery" and "report" packages are handled implicitly by NET-SNMP.

However, this can result in a timeout error from the async call to get
a value, rather than a timeout error that we generate "ourselves" when
we have waited too long.  This type of error is raised by pynetsnmp
as "yet another timeout exception class", rather than one of the
existing timeout exceptions.

This adds `SnmpTimeoutError` to the list of handled timeout exceptions
where timeouts are handled explicitly, while in the mibretriever
retrieval methods it is translated to a regular TimeoutError exception.

python/nav/ipdevpoll/plugins/snmpcheck.py

@@ -14,7 +14,7 @@
 # License along with NAV. If not, see <http://www.gnu.org/licenses/>.
 #
 """snmp check plugin"""
-
+from pynetsnmp.netsnmp import SnmpTimeoutError
 from twisted.internet import error, defer
 from twisted.internet.defer import returnValue
 
@@ -69,7 +69,7 @@ def _do_check(self):
         self._logger.debug("checking SNMP v%s availability", self.agent.snmpVersion)
         try:
             result = yield self.agent.walk(SYSTEM_OID)
-        except (defer.TimeoutError, error.TimeoutError):
+        except (defer.TimeoutError, error.TimeoutError, SnmpTimeoutError):
             self._logger.debug("SNMP v%s timed out", self.agent.snmpVersion)
             returnValue(False)
 

python/nav/mibs/mibretriever.py

@@ -33,6 +33,7 @@
 
 import logging
 
+from pynetsnmp.netsnmp import SnmpTimeoutError
 from twisted.internet import defer, reactor
 from twisted.internet.defer import returnValue
 from twisted.internet.error import TimeoutError
@@ -427,6 +428,11 @@ def _result_formatter(result):
 
             return formatted_result
 
+        def _snmp_timeout_handler(failure: defer.failure.Failure):
+            """Transforms SnmpTimeoutErrors into "regular" TimeoutErrors"""
+            failure.trap(SnmpTimeoutError)
+            raise TimeoutError(failure.value)
+
         def _valueerror_handler(failure):
             failure.trap(ValueError)
             self._logger.warning(
@@ -439,6 +445,7 @@ def _valueerror_handler(failure):
             return {}  # alternative is to retry or raise a Timeout exception
 
         deferred = self.agent_proxy.getTable([str(node.oid)])
+        deferred.addErrback(_snmp_timeout_handler)
         deferred.addCallbacks(_result_formatter, _valueerror_handler)
         return deferred