Implement dummy body message to indicate high priority push This adds a dummy body sent alongside the push when the original message also contained a body to indicate a high priority push. The body can be configured and its contents are generally meaningless with most app servers because it will be stripped before pushed to the client.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.