What causes git push to fail all of a sudden with “User was holding a relation lock for too long”? [closed]

As the message indicates, this is a problem on Bitbucket’s end. You can check the status of their services by visiting http://status.bitbucket.org/, which will inform you (for example):

Verifying – We are gradually restoring inbox content to mitigate any further replication issues. Any messages you’d expect to be there should re-appear slowly as we incrementally complete scheduled maintenance.
Jun 16, 03:22 UTC

Update – Maintenance on the primary database caused excessive lag on replicas used for authenticating Git and Mercurial operations over HTTPS. We’ve failed over to a spare replica and restored service. Please contact our support team if you’re still experiencing problems with your pushes or pulls.
Jun 16, 02:48 UTC

In progress – We’re seeing unanticipated complications on gitweb and hgweb traffic, which we are currently working to resolve. Please watch this page for updates.
Jun 16, 02:03 UTC

If no current issues are listed there, contact support.