Fix mgmt-framework rest server startup script

PR sonic-net#4937 broke rest server startup -- server does not load user
defined configs from db and always comes up with default config.
Fixed mgmt_vars.j2 to load correct db entry and rest-server.sh to
handle optional fields correctly.

Also introduced a new jinja filter 'json' in sonic-cfggen. It formats
the value using json.dumps() function. mgmt_vars.j2 tempalte uses this
filter to transform db values into a syntactically correct json.

Author: sachinholla

dockers/docker-sonic-mgmt-framework/mgmt_vars.j2

@@ -1,4 +1,4 @@
 {
-    "rest_server": {% if REST_SERVER is defined and "default" in RESET_SERVER.keys() %}{{ REST_SERVER['default'] }}{% else %}""{% endif %},
-    "x509" : {% if "x509" in DEVICE_METADATA.keys() %}{{ DEVICE_METADATA["x509"] }}{% else %}""{% endif %}
+    "rest_server": {% if REST_SERVER is defined and "default" in REST_SERVER.keys() %}{{ REST_SERVER['default']|json }}{% else %}""{% endif %},
+    "x509" : {% if "x509" in DEVICE_METADATA.keys() %}{{ DEVICE_METADATA["x509"]|json }}{% else %}""{% endif %}
 }

dockers/docker-sonic-mgmt-framework/rest-server.sh

@@ -11,18 +11,17 @@ fi
 
 # Read basic server settings from mgmt vars entries
 MGMT_VARS=$(sonic-cfggen -d -t $MGMT_VARS_FILE)
-MGMT_VARS=${MGMT_VARS//[\']/\"}
 
 REST_SERVER=$(echo $MGMT_VARS | jq -r '.rest_server')
 
 if [ -n "$REST_SERVER" ]; then
-    SERVER_PORT=$(echo $REST_SERVER | jq -r '.port')
-    CLIENT_AUTH=$(echo $REST_SERVER | jq -r '.client_auth')
-    LOG_LEVEL=$(echo $REST_SERVER | jq -r '.log_level')
+    SERVER_PORT=$(echo $REST_SERVER | jq -r '.port // empty')
+    CLIENT_AUTH=$(echo $REST_SERVER | jq -r '.client_auth // empty')
+    LOG_LEVEL=$(echo $REST_SERVER | jq -r '.log_level // empty')
 
-    SERVER_CRT=$(echo $REST_SERVER | jq -r '.server_crt')
-    SERVER_KEY=$(echo $REST_SERVER | jq -r '.server_key')
-    CA_CRT=$(echo $REST_SERVER | jq -r '.ca_crt')
+    SERVER_CRT=$(echo $REST_SERVER | jq -r '.server_crt // empty')
+    SERVER_KEY=$(echo $REST_SERVER | jq -r '.server_key // empty')
+    CA_CRT=$(echo $REST_SERVER | jq -r '.ca_crt // empty')
 fi
 
 if [[ -z $SERVER_CRT ]] && [[ -z $SERVER_KEY ]] && [[ -z $CA_CRT ]]; then
@@ -31,9 +30,9 @@ fi
 
 # Read certificate file paths from DEVICE_METADATA|x509 entry.
 if [ -n "$X509" ]; then
-    SERVER_CRT=$(echo $X509 | jq -r '.server_crt')
-    SERVER_KEY=$(echo $X509 | jq -r '.server_key')
-    CA_CRT=$(echo $X509 | jq -r '.ca_crt')
+    SERVER_CRT=$(echo $X509 | jq -r '.server_crt // empty')
+    SERVER_KEY=$(echo $X509 | jq -r '.server_key // empty')
+    CA_CRT=$(echo $X509 | jq -r '.ca_crt // empty')
 fi
 
 # Create temporary server certificate if they not configured in ConfigDB

src/sonic-config-engine/sonic-cfggen

@@ -242,6 +242,7 @@ def _get_jinja2_env(paths):
     env.filters['unique_name'] = unique_name
     env.filters['pfx_filter'] = pfx_filter
     env.filters['ip_network'] = ip_network
+    env.filters['json'] = json.dumps
     for attr in ['ip', 'network', 'prefixlen', 'netmask', 'broadcast']:
         env.filters[attr] = partial(prefix_attr, attr)