Thanks Craig, the problem was that (if I remember correctly) there were absolutely no errors or warnings logged when I issued the ALTER statement. Everything seemed to operate normally except that the execution never completed. Even the fact that the node_status was set to 'i' didn't result in any log messages and the replication was working as it should.
-Selim
From: Craig Ringer [craig@2ndquadrant.com] Sent: Sunday, December 06, 2015 7:05 PM To: Selim Tuvi Cc: Sylvain MARECHAL; pgsql-general@postgresql.org Subject: Re: [GENERAL] BDR: ALTER statement hanging
If you're not sure what's going on on a node, look at its logs.
The background worker API and PostgreSQL's lack of autonomous transactions makes it quite challenging for BDR workers to capture logs and expose them to users at the SQL level. So always, if in doubt, examine the log files.