[snmp]: Remove redundant connections to dbs

[SuvarnaMeenakshi]

Signed-off-by: Suvarna Meenakshi sumeenak@microsoft.com

- What I did
init_namespace_dbs() function establishes connections to all required DBs in all namespaces.
There are various connections done in periodic functions like reinit_data() or update_data() which creates many connections and increases the number of open fds for sonic_ax_impl processes eventually leading to error logs like:

RR snmp#snmp-subagent [ax_interface] ERROR: MIBUpdater.start() caught an unexpected exception during update_data()#012Traceback (most recent call last):#012  File "/usr/local/lib/python3.7/dist-packages/ax_interface/mib.py", line 37, in start#012    self.reinit_data()#012  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/mibs/vendor/cisco/ciscoPfcExtMIB.py", line 38, in reinit_data#012    self.oid_name_map = Namespace.get_sync_d_from_all_namespace(mibs.init_sync_d_interface_tables, self.db_conn)#012  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/mibs/__init__.py", line 669, in get_sync_d_from_all_namespace#012    ns_tuple = per_namespace_func(db_conn)#012  File "/usr/local/lib/python3.7/dist-packages/sonic_ax_impl/mibs/__init__.py", line 268, in init_sync_d_interface_tables#012    if_name_map_util, if_id_map_util = port_util.get_interface_oid_map(db_conn, blocking=False)#012  File "/usr/local/lib/python3.7/dist-packages/swsssdk/port_util.py", line 73, in get_interface_oid_map#012    db.connect('COUNTERS_DB')#012  File "/usr/lib/python3/dist-packages/swsscommon/swsscommon.py", line 1690, in connect#012    return _swsscommon.SonicV2Connector_Native_connect(self, db_name, retry_on)#012RuntimeError: Unable to connect to redis (unix-socket): Cannot assign requested address

- How I did it
Remove establishing connections in periodic functions when the connection is already established during init.

- How to verify it

- Description for the changelog

[qiluo-msft]

def __init__(self):

This is not "periodic functions", right?

---

Refers to: src/sonic_ax_impl/mibs/ieee802_1ab.py:152 in d845a95. [](commit_id = d845a95, deletion_comment = False)

[qiluo-msft]

def __init__(self):

Not "periodic functions"?

---

Refers to: src/sonic_ax_impl/mibs/ieee802_1ab.py:500 in d845a95. [](commit_id = d845a95, deletion_comment = False)

[qiluo-msft]

connect

The expectation of SonicV2Connector is that connect to the same DB multiple time will be harmless and no resource leak. We need to dig deeper and check if the expectation is broken.

[prgeor]

Redis DB is not meant to handle too many connections ...there is a limit to number of connections. Each process should establish connection once in its life time and then use the same connection to do any queries ...any number of times. Too many connections request may result in hitting this error

[SuvarnaMeenakshi]

def __init__(self):

This is not "periodic functions", right?

Refers to: src/sonic_ax_impl/mibs/ieee802_1ab.py:152 in d845a95. [](commit_id = d845a95, deletion_comment = False)

Yes it is not periodic, but in places where we do "init_namespace_dbs()", we do not need to connect again as "init_namespace_dbs" also connects to all required DBs in all namespaces.