The driver manager is modeled after the device manager and is started by the client.
It's responsible for handling driver lifecycle and reattachment state, as well as
processing the incomming fingerprint and task events from each driver. The mananger
exposes a method for registering event handlers for task events that is used by the
task runner to update the server when a task has been updated with an event.
Since driver fingerprinting has been implemented by the driver manager, it is no
longer needed in the fingerprint mananger and has been removed.
The RestartCount is not really suitable for use as a source of
uniqueness within task invocations as it is not monotonic, and interacts
with the restart stanza in a users config, so conflates restarts due to
task failures, with restarts due to enviromental changes, such as consul
template or vault secrets changing.
Here we instead use a substring from a uuid, which is more random than
we strictly need, but is nicer than rolling our own random string
generator here.
This creates a new buffered channel and goroutine on the allocrunner for
serializing updates to allocations. This allows us to take updates off
the routine that is used from processing updates from the server,
without having complicated machinery for tracking update lifetimes, or
other external synchronization.
This results in a nice performance improvement and signficantly better
throughput on batch changes such as preempting a large number of jobs
for a larger placement.
This commit reduces the locking required to shutdown or destroy
allocrunners, and allows parallel shutdown and destroy of allocrunners during
shutdown.
Currently the docker driver does not remove tasks from its state map
when destroying the task, which leads to issues when restarting tasks in
place, and leaks expired handles over time.
