[bridge] T1156: add missing if statement in config-migration

Fixes:
Traceback (most recent call last):
  File "/opt/vyatta/etc/config-migrate/migrate/interfaces/0-to-1", line 27, in <module>
    for br in config.list_nodes(base):
  File "/usr/lib/python3/dist-packages/vyos/configtree.py", line 255, in list_nodes
    raise ConfigTreeError("Path [{}] doesn't exist".format(path_str))
vyos.configtree.ConfigTreeError: Path [b'interfaces bridge'] doesn't exist

1 files changed, 62 insertions, 58 deletions

diff --git a/src/migration-scripts/interfaces/0-to-1 b/src/migration-scripts/interfaces/0-to-1
index b8e190f2c..38f2bd8f5 100755
--- a/src/migration-scripts/interfaces/0-to-1
+++ b/src/migration-scripts/interfaces/0-to-1
@@ -21,61 +21,65 @@ with open(file_name, 'r') as f:
 config = ConfigTree(config_file)
 base = ['interfaces', 'bridge']
 
-#
-# make stp and igmp-snooping nodes valueless
-#
-for br in config.list_nodes(base):
-    # STP: check if enabled
-    stp_val = config.return_value(base + [br, 'stp'])
-    # STP: delete node with old syntax
-    config.delete(base + [br, 'stp'])
-    # STP: set new node - if enabled
-    if stp_val == "true":
-        config.set(base + [br, 'stp'], value=None)
-
-    # igmp-snooping: check if enabled
-    igmp_val = config.return_value(base + [br, 'igmp-snooping', 'querier'])
-    # igmp-snooping: delete node with old syntax
-    config.delete(base + [br, 'igmp-snooping', 'querier'])
-    # igmp-snooping: set new node - if enabled
-    if igmp_val == "enable":
-        config.set(base + [br, 'igmp', 'querier'], value=None)
-
-#
-# move interface based bridge-group to actual bridge (de-nest)
-#
-bridge_types = ['bonding', 'ethernet', 'l2tpv3', 'openvpn', 'vxlan', 'wireless']
-for type in bridge_types:
-    if not config.exists(['interfaces', type]):
-        continue
-
-    for intf in config.list_nodes(['interfaces', type]):
-        # check if bridge-group exists
-        if config.exists(['interfaces', type, intf, 'bridge-group']):
-            bridge = config.return_value(['interfaces', type, intf, 'bridge-group', 'bridge'])
-
-            # create new bridge member interface
-            config.set(base + [bridge, 'member', 'interface', intf])
-            # format as tag node to avoid loading problems
-            config.set_tag(base + [bridge, 'member', 'interface'])
-
-            # cost: migrate if configured
-            if config.exists(['interfaces', type, intf, 'bridge-group', 'cost']):
-                cost = config.return_value(['interfaces', type, intf, 'bridge-group', 'cost'])
-                # set new node 
-                config.set(base + [bridge, 'member', 'interface', intf, 'cost'], value=cost)
-
-            if config.exists(['interfaces', type, intf, 'bridge-group', 'priority']):
-                priority = config.return_value(['interfaces', type, intf, 'bridge-group', 'priority'])
-                # set new node
-                config.set(base + [bridge, 'member', 'interface', intf, 'priority'], value=priority)
-
-            # Delete the old bridge-group assigned to an interface
-            config.delete(['interfaces', type, intf, 'bridge-group'])
-
-    try:
-        with open(file_name, 'w') as f:
-            f.write(config.to_string())
-    except OSError as e:
-        print("Failed to save the modified config: {}".format(e))
-        sys.exit(1)
+if not config.exists(base):
+    # Nothing to do
+    sys.exit(0)
+else:
+    #
+    # make stp and igmp-snooping nodes valueless
+    #
+    for br in config.list_nodes(base):
+        # STP: check if enabled
+        stp_val = config.return_value(base + [br, 'stp'])
+        # STP: delete node with old syntax
+        config.delete(base + [br, 'stp'])
+        # STP: set new node - if enabled
+        if stp_val == "true":
+            config.set(base + [br, 'stp'], value=None)
+
+        # igmp-snooping: check if enabled
+        igmp_val = config.return_value(base + [br, 'igmp-snooping', 'querier'])
+        # igmp-snooping: delete node with old syntax
+        config.delete(base + [br, 'igmp-snooping', 'querier'])
+        # igmp-snooping: set new node - if enabled
+        if igmp_val == "enable":
+            config.set(base + [br, 'igmp', 'querier'], value=None)
+
+    #
+    # move interface based bridge-group to actual bridge (de-nest)
+    #
+    bridge_types = ['bonding', 'ethernet', 'l2tpv3', 'openvpn', 'vxlan', 'wireless']
+    for type in bridge_types:
+        if not config.exists(['interfaces', type]):
+            continue
+
+        for intf in config.list_nodes(['interfaces', type]):
+            # check if bridge-group exists
+            if config.exists(['interfaces', type, intf, 'bridge-group']):
+                bridge = config.return_value(['interfaces', type, intf, 'bridge-group', 'bridge'])
+
+                # create new bridge member interface
+                config.set(base + [bridge, 'member', 'interface', intf])
+                # format as tag node to avoid loading problems
+                config.set_tag(base + [bridge, 'member', 'interface'])
+
+                # cost: migrate if configured
+                if config.exists(['interfaces', type, intf, 'bridge-group', 'cost']):
+                    cost = config.return_value(['interfaces', type, intf, 'bridge-group', 'cost'])
+                    # set new node
+                    config.set(base + [bridge, 'member', 'interface', intf, 'cost'], value=cost)
+
+                if config.exists(['interfaces', type, intf, 'bridge-group', 'priority']):
+                    priority = config.return_value(['interfaces', type, intf, 'bridge-group', 'priority'])
+                    # set new node
+                    config.set(base + [bridge, 'member', 'interface', intf, 'priority'], value=priority)
+
+                # Delete the old bridge-group assigned to an interface
+                config.delete(['interfaces', type, intf, 'bridge-group'])
+
+        try:
+            with open(file_name, 'w') as f:
+                f.write(config.to_string())
+        except OSError as e:
+            print("Failed to save the modified config: {}".format(e))
+            sys.exit(1)