Fix ConfigDB load config crash with swss-common issue.

[liuh-80]

- What I did
Fix following code issue:
When sonic-py-common change to use sonic-swss-common, sonic_ax_impl will crash because ConfigDB config load multiple times.

    The reason if this issue is because  multi_asic.is_multi_asic() will try load platform information from local environment variable and machine.conf, but if can't load information from local, is_multi_asic() will initialize default sonic DB config and connect to ConfigDB then read platform information.

- How I did it
Code change to load default config before check multi asic.

- How to verify it
Pass all UT and E2E test.

- Description for the changelog

[liuh-80]

Here are more detail about this change:

Step1 to Step4 happened in https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-py-common/sonic_py_common/multi_asic.py

1. is_multi_asic() will check asic count by get_num_asics()
2. get_num_asics() need get asic config file path, so it will call get_asic_conf_file_path()
3. get_asic_conf_file_path() need get current platform:
   platform = get_platform()
   if platform:
   asic_conf_path_candidates.append(os.path.join(HOST_DEVICE_PATH, platform, ASIC_CONF_FILENAME))
4. then get_platform() will try read platfrom from "PLATFORM" environment variable and machine.conf, but if still can't get platform, it will connect to ConfigDB and read from "DEVICE_METADATA" table: get_localhost_info('platform')
5. And when connect to ConfigDB, sonic-swss-common code will check if ConfigDB already initialized, if not it will load default config:

SonicDBInfo& SonicDBConfig::getDbInfo(conststd::string &dbName, conststd::string &netns)
{
std::lock_guardstd::recursive_mutex guard(m_db_info_mutex);

SWSS_LOG_ENTER();

if(!m_init)
    initialize(DEFAULT_SONIC_DB_CONFIG_FILE);

if(!netns.empty())

[qiluo-msft]

Thanks for the stack tracing! The key issue is Step 3 and Step 4. I draft a PR sonic-net/sonic-buildimage#10723.

[liuh-80]

Here is the latest code which still will connect to config DB and initialize it, so the code change in this PR still necessary:

def get_localhost_info(field, config_db=None):
try:
# TODO: enforce caller to provide config_db explicitly and remove its default value
if not config_db:
config_db = ConfigDBConnector()
config_db.connect()

    metadata = config_db.get_table('DEVICE_METADATA')

@@ -81,11 +82,16 @@ def get_machine_info():

return machine_vars

[liuh-80]

Close this PR because the crash issue fixed by other PR.