Make LCD reconfigure V3 users when auth data changes.

`CommandGeneratorLcdConfigurator` configures auth data and caches based
on `(username, security engine id)`. If the authentication data is
changed, such as via `usmUserAuthKeyChange`, the cache is not updated
because neither the username nor engine ID changed. Despite new data
being passed in, this results in the old data being used and a "Wrong
SNMP PDU digest" error indication being returned.

Change the LCD to detect a changed authKey, authProtocol, privKey, or
privProtocol and reconfigure the V3 user as appropriate.

pysnmp/hlapi/asyncio/lcd.py

@@ -57,6 +57,16 @@ def configure(
         elif isinstance(authData, UsmUserData):
             authDataKey = authData.userName, authData.securityEngineId
             if authDataKey not in cache["auth"]:
+                add_user = True
+
+            elif self._usm_auth_changed(cache["auth"][authDataKey], authData):
+                config.delV3User(snmpEngine, authData.userName, authData.securityEngineId)
+                add_user = True
+
+            else:
+                add_user = False
+
+            if add_user:
                 config.addV3User(
                     snmpEngine,
                     authData.userName,
@@ -199,6 +209,17 @@ def unconfigure(self, snmpEngine, authData=None, contextName=null, **options):
 
         return addrNames, paramsNames
 
+    @staticmethod
+    def _usm_auth_changed(cachedAuthData, newAuthData):
+        changed = False
+
+        changed |= (cachedAuthData.authKey != newAuthData.authKey)
+        changed |= (cachedAuthData.authProtocol != newAuthData.authProtocol)
+        changed |= (cachedAuthData.privKey != newAuthData.privKey)
+        changed |= (cachedAuthData.privProtocol != newAuthData.privProtocol)
+
+        return changed
+
 
 class NotificationOriginatorLcdConfigurator(AbstractLcdConfigurator):
     cacheKeys = ["auth", "name"]

tests/hlapi/asyncio/manager/cmdgen/test_lcd_configurator.py

@@ -0,0 +1,78 @@
+from unittest import mock
+
+from pysnmp import hlapi
+from pysnmp.hlapi.asyncio.cmdgen import CommandGeneratorLcdConfigurator
+
+
+@mock.patch('pysnmp.entity.config.addV3User')
+@mock.patch('pysnmp.entity.config.delV3User')
+def test_usm_auth_cache_cleared(delV3User, addV3User):
+    """
+    Ensure auth cache is cleared when auth data is changed.
+    """
+    snmpEngine = hlapi.SnmpEngine()
+    transportTarget = hlapi.UdpTransportTarget('198.51.100.1')
+
+    authDataValues = {
+        'userName': 'username',
+        'authKey': 'authkey1',
+        'authProtocol': hlapi.usmHMACMD5AuthProtocol,
+        'privKey': 'privkey1',
+        'privProtocol': hlapi.usmAesCfb128Protocol,
+    }
+
+    lcd = CommandGeneratorLcdConfigurator()
+    initialAuthData = hlapi.UsmUserData(**authDataValues)
+    lcd.configure(snmpEngine, initialAuthData, transportTarget)
+    addV3User.assert_called_with(
+            snmpEngine,
+            initialAuthData.userName,
+            initialAuthData.authProtocol,
+            initialAuthData.authKey,
+            initialAuthData.privProtocol,
+            initialAuthData.privKey,
+            securityEngineId=initialAuthData.securityEngineId,
+            securityName=initialAuthData.securityName,
+            authKeyType=initialAuthData.authKeyType,
+            privKeyType=initialAuthData.privKeyType,
+    )
+
+    # Ensure we do not add/delete if nothing changes
+    addV3User.reset_mock()
+    lcd.configure(snmpEngine, initialAuthData, transportTarget)
+    addV3User.assert_not_called()
+    delV3User.assert_not_called()
+
+    changeAuthValues = {
+        'authKey': 'authKey2',
+        'privProtocol': hlapi.usmDESPrivProtocol,
+        'authProtocol': hlapi.usmHMACSHAAuthProtocol,
+        'privKey': 'privKey2',
+    }
+
+    for field, value in changeAuthValues.items():
+        addV3User.reset_mock()
+        delV3User.reset_mock()
+
+        authDataValues[field] = value
+        authData = hlapi.UsmUserData(**authDataValues)
+        lcd.configure(snmpEngine, authData, transportTarget)
+
+        delV3User.assert_called_with(
+            snmpEngine,
+            authData.userName,
+            authData.securityEngineId,
+        )
+
+        addV3User.assert_called_with(
+                snmpEngine,
+                authData.userName,
+                authData.authProtocol,
+                authData.authKey,
+                authData.privProtocol,
+                authData.privKey,
+                securityEngineId=authData.securityEngineId,
+                securityName=authData.securityName,
+                authKeyType=authData.authKeyType,
+                privKeyType=authData.privKeyType,
+        )