Consider this:

subscribe set(id=1,provider=1,receiver=2);
EXECUTE SCRIPT(set id=1, 
FILENAME='/tmp/some_add_column_script.sql',event node=1);

If the execute script  is submitted on node 1 before the COPY_SET on 
node 2 (copying the data from node 1) takes place the copy will pull the 
new column but that column won't yet exist on node 2 since the execute 
script won't get processed on node 2 until after the subscribe.

Any of the following should address this

1) When submitting an EXECUTE SCRIPT slonik will require all nodes to be 
'caught up'.

2) When submitting an execute script slonik will wait until all pending 
subscriptions be active (all sets, all nodes involved in any part of the 
execute script)

3) When submitting an execute script slonik will require all pending 
subscriptions to be active for the set id specified on the execute 
script command line.

I don't like 3 because often a single SQL script will need to involve 
tables from multiple sets, the 'set id' on execute script probably 
should be removed (in old versions of slony this controlled locking but 
it turned out to be not good enough, probably for the same reason)

I am inclined to go with 1, because it is easier to implement than 2. 
Thoughts?