What's the easiest/best way to debug remote processes?

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If log files are not sufficient for you, you could use a proxy script that sets uname and then executes your node. It feels like an ugly hack but should work :) Such a script could look like this (untested!):

#!/bin/bash
uname -c unlimited
EXECUTABLE=$1
shift
exec $EXECUTABLE $@

When you use the script as a launch-prefix, you should get core dumps.

If I remember properly, a remote shell (usually bash) does get invoked before roslaunch is invoked. On some modern linux systems, particularly Ubuntu, there's a check at the beginning of the default .bashrc that exits if you're running a non-interactive shell. You should look for that and put your ulimit before it.