I think this must get the award for oldest (frequently excercised) bug
in the Emulab source. Thanks to the magic of the git pickaxe, I
discovered that code with this problem was first committed Monday, May
22, 2000, when it used to be in assign_hw/assign.cc

assign used to use a priority queue to 'randomly' select unassigned
nodes to try assigning. Turns out this is not very random! A node can
get unlucky, get a very low random priority assigned to it, and get
stuck in the queue for a very, very, very long time. As a result, I've
seen assign try to re-assign the same node several thousand times in a
row, when there are others waiting to be re-assigned. Of course, this is
going to result in very poor exploration of the state space.

I changed the unassigned node selection process to use a regular list,
and select an item from the list at random at the time of item removal.
This gets a much more even distribution of nodes, at the small cost of
linear iteration over the list of unassigned nodes.  This should be a
pretty minor cost, though, as iteration over this list should be cheap
and the list itself should generally be pretty small.

Also added some debugging statements.