Make sure only one get.py runs at once

[frabcus]

There are various failure modes:

e.g. user runs it at same time as cron e.g. deleting dataset while longing running existing cron job going

[frabcus]

Maybe use fcntl.flock

[frabcus]

I tried this as per http://stackoverflow.com/a/5255473

However, it never seems to time out (with the alarm). You get the alarm error, but only when the first process (holding the flock) exits.

I'm mildly worried one process might get stuck, then all the others do too, with no errors - just freezing.

signal.alarm(10)
fd = open("box.json", "r")
print "waiting for lock"
fcntl.flock(fd.fileno(), fcntl.LOCK_EX)
signal.alarm(0)

[frabcus]

Let's leave this for now, until we find the problems.