Make attach-bridge iptables rule insertion idempotent

weave

@@ -403,6 +403,15 @@ add_iptables_rule() {
     fi
 }
 
+# Insert a rule in iptables, if it doesn't exist already
+insert_iptables_rule() {
+    IPTABLES_TABLE="$1"
+    shift 1
+    if ! run_iptables -t $IPTABLES_TABLE -C "$@" >/dev/null 2>&1 ; then
+        run_iptables -t $IPTABLES_TABLE -I "$@" >/dev/null
+    fi
+}
+
 # Delete a rule from iptables, if it exist
 delete_iptables_rule() {
     IPTABLES_TABLE="$1"
@@ -1832,7 +1841,7 @@ EOF
     attach-bridge)
         if detect_bridge_type ; then
             attach_bridge ${1:-$DOCKER_BRIDGE}
-            run_iptables -t nat -I POSTROUTING -o $BRIDGE -j ACCEPT
+            insert_iptables_rule nat POSTROUTING -o $BRIDGE -j ACCEPT
         else
             echo "Weave bridge not found. Please run 'weave launch' and try again" >&2
             exit 1