Before this commit, in the watchtower client's DiskOverflowQueue, there
could be a situation _if no consumer was reading from the queue_ that
could lead to an in-memory build up of backup tasks instead of the
expected disk overflow. This commit fixes this by ensuring that if a
task is read from disk, then the mode is set to disk mode to ensure that
any new items are persisted to disk in this scenario.
The unit tests added in the previous commit is updated here in order to
show that the fix does in-fact fix the test.
In this commit, some temporary variables and logic is added to the
DiskOverflowQueue for easy stop/go control from unit tests. This is then
used to write a temporary unit tests that demonstrates a race condition
that can cause the queue to be in disk mode when it should be in memory
mode. This new code & test will be removed after the issue has been
fixed.
In this commit, a new generic DiskOverflowQueue implementation is added.
This allows a user to specify a maximum number of items that the queue
can hold in-memory. Any new items will then overflow to disk. The
producer and consumer of the queue items will interact with the queue
just like a normal in-memory queue.
