The allocrunner has a facility for passing data written by allocrunner hooks to
taskrunner hooks. Currently the only consumers of this facility are the
allocrunner CSI hook (which writes data) and the taskrunner volume hook (which
reads that same data).
The allocrunner hook for CSI volumes doesn't set the alloc hook resources
atomically. Instead, it gets the current resources and then writes a new version
back. Because the CSI hook is currently the only writer and all readers happen
long afterwards, this should be safe but #16623 shows there's some sequence of
events during restore where this breaks down.
Refactor hook resources so that hook data is accessed via setters and getters
that hold the mutex.
When an allocation runs for a task driver that can't support volume mounts,
the mounting will fail in a way that can be hard to understand. With host
volumes this usually means failing silently, whereas with CSI the operator
gets inscrutable internals exposed in the `nomad alloc status`.
This changeset adds a MountConfig field to the task driver Capabilities
response. We validate this when the `csi_hook` or `volume_hook` fires and
return a user-friendly error.
Note that we don't currently have a way to get driver capabilities up to the
server, except through attributes. Validating this when the user initially
submits the jobspec would be even better than what we're doing here (and could
be useful for all our other capabilities), but that's out of scope for this
changeset.
Also note that the MountConfig enum starts with "supports all" in order to
support community plugins in a backwards compatible way, rather than cutting
them off from volume mounting unexpectedly.
This commit implements support for creating driver mounts for CSI
Volumes.
It works by fetching the created mounts from the allocation resources
and then iterates through the volume requests, creating driver mount
configs as required.
It's a little bit messy primarily because there's _so_ much terminology
overlap and it's a bit difficult to follow.
