Merge #261: Fix netns restart

0972af55f1 netns: fix setup service restarts (Erik Arvstedt)
63a464431b netns: fail when netns already exists (Erik Arvstedt)

Pull request description:

ACKs for top commit:
  jonasnick:
    utACK 0972af55f1

Tree-SHA512: 0923d26c144eb19a11ec4c1f0cd512bb7a70b12c9b73c787f4af476c95c91ea012ff593d2b59e1ccea34193c6995ba60783049c5086ab286b323460905ca62e7

modules/netns-isolation.nix

@@ -152,7 +152,6 @@ in {
           requiredBy = bindsTo;
           before = bindsTo;
           script = ''
-            ${ip} netns delete ${netnsName} 2> /dev/null || true
             ${ip} netns add ${netnsName}
             ${ipNetns} link set lo up
             ${ip} link add ${veth} type veth peer name ${peer}
@@ -173,8 +172,13 @@ in {
             ${netnsIptables} -w -A INPUT -s ${allowedAddresses} -j ACCEPT
             ${netnsIptables} -w -A OUTPUT -d ${allowedAddresses} -j ACCEPT
           '';
+          # Link deletion is implicit in netns deletion, but it sometimes only happens
+          # after `netns delete` finishes. Add an extra `link del` to ensure that
+          # the link is deleted before the service stops, which is needed for service
+          # restart to succeed.
           preStop = ''
             ${ip} netns delete ${netnsName}
+            ${ip} link del ${peer} 2> /dev/null || true
           '';
           serviceConfig = {
             Type = "oneshot";