[sonic-cfggen] Allow cfggen to work on system without ports (sonic-ne…

…t#7999)

#### Why I did it
Allow cfggen to work on system without ports in platform.json or in port_config.ini

#### How I did it
Add json write of PORT section only if the dictionary that contains the ports is not empty.  

#### How to verify it
sonic-cfggen -k ACS-MSN3700 -H -j /etc/sonic/init_cfg.json --print-data

src/sonic-config-engine/portconfig.py

@@ -261,9 +261,9 @@ def parse_platform_json_file(hwsku_json_file, platform_json_file):
  port_dict = readJson(platform_json_file)
  hwsku_dict = readJson(hwsku_json_file)
 
- if not port_dict:
+ if port_dict is None:
  raise Exception("port_dict is none")
- if not hwsku_dict:
+ if hwsku_dict is None:
  raise Exception("hwsku_dict is none")
 
  if INTF_KEY not in port_dict or INTF_KEY not in hwsku_dict:
@@ -285,8 +285,8 @@ def parse_platform_json_file(hwsku_json_file, platform_json_file):
 
  ports.update(child_ports)
 
- if not ports:
- raise Exception("Ports dictionary is empty")
+ if ports is None:
+ raise Exception("Ports dictionary is None")
 
  for i in ports.keys():
  port_alias_map[ports[i]["alias"]]= i

src/sonic-config-engine/sonic-cfggen

@@ -304,7 +304,7 @@ def main():
  if args.port_config is None:
  args.port_config = device_info.get_path_to_port_config_file(hwsku)
  (ports, _, _) = get_port_config(hwsku, platform, args.port_config, asic_id)
- if not ports:
+ if ports is None:
  print('Failed to get port config', file=sys.stderr)
  sys.exit(1)
  deep_update(data, {'PORT': ports})

src/sonic-config-engine/tests/test_cfggen_platformJson.py

@@ -2,11 +2,17 @@
 import json
 import os
 import subprocess
+import sys
 
 import tests.common_utils as utils
 
 from unittest import TestCase
+from portconfig import get_port_config, INTF_KEY
 
+if sys.version_info.major == 3:
+ from unittest import mock
+else:
+ import mock
 
 # Global Variable
 PLATFORM_OUTPUT_FILE = "platform_output.json"
@@ -85,3 +91,10 @@ def test_platform_json_all_ethernet_interfaces(self):
  output_dict = ast.literal_eval(output.strip())
  expected = ast.literal_eval(json.dumps(fh_data))
  self.assertDictEqual(output_dict, expected)
+
+ @mock.patch('portconfig.readJson', mock.MagicMock(return_value={INTF_KEY:{}}))
+ @mock.patch('os.path.isfile', mock.MagicMock(return_value=True))
+ def test_platform_json_no_interfaces(self):
+ (ports, _, _) = get_port_config(port_config_file=self.platform_json)
+ self.assertNotEqual(ports, None)
+ self.assertEqual(ports, {})