holland can leave orphan grandchildren processes

[Andrew Garner]

When a holland plugin forks a command which in turn forks some subset of commands, but that command is terminated or fails in some way that does not kill its children, holland can sometimes leave children of its children processes running for some time, even if the plugin otherwise tracks the child exit status properly.

This is most common with xtrabackup, where the innobackupex perl script fails to clean up after itself in a variety of circumstances. This can leave the xtrabackup_* binary running in the background even after holland has stopped.

I think we can largely resolve this by always sending a SIGTERM to the holland process group to at least signal grandchild processes to exit which will cover most of the common cases where this occurs - and certainly fix a few nasty cases with innobackupex -> xtrabackup.

We can do something like:

# temporarily ignore the signal we will send our process group
with ignored_signals(SIGTERM):
    os.killpg(os.getpid(), SIGTERM)

And then exit as normal as needed.