CSI: skip node unpublish on GC'd or down nodes

If the node has been GC'd or is down, we can't send it a node
unpublish. The CSI spec requires that we don't send the controller
unpublish before the node unpublish, but in the case where a node is
gone we can't know the final fate of the node unpublish step.

The `csi_hook` on the client will unpublish if the allocation has
stopped and if the host is terminated there's no mount for the volume
anyways. So we'll now assume that the node has unpublished at its
end. If it hasn't, any controller unpublish will potentially hang or
error and need to be retried.

nomad/csi_endpoint.go

@@ -686,6 +686,25 @@ RELEASE_CLAIM:
 
 func (v *CSIVolume) nodeUnpublishVolume(vol *structs.CSIVolume, claim *structs.CSIVolumeClaim) error {
 	v.logger.Trace("node unpublish", "vol", vol.ID)
+
+	store := v.srv.fsm.State()
+
+	// If the node has been GC'd or is down, we can't send it a node
+	// unpublish. We need to assume the node has unpublished at its
+	// end. If it hasn't, any controller unpublish will potentially
+	// hang or error and need to be retried.
+	if claim.NodeID != "" {
+		node, err := store.NodeByID(memdb.NewWatchSet(), claim.NodeID)
+		if err != nil {
+			return err
+		}
+		if node == nil || node.Status == structs.NodeStatusDown {
+			v.logger.Debug("skipping node unpublish for down or GC'd node")
+			claim.State = structs.CSIVolumeClaimStateNodeDetached
+			return v.checkpointClaim(vol, claim)
+		}
+	}
+
 	if claim.AllocationID != "" {
 		err := v.nodeUnpublishVolumeImpl(vol, claim)
 		if err != nil {
@@ -698,8 +717,7 @@ func (v *CSIVolume) nodeUnpublishVolume(vol *structs.CSIVolume, claim *structs.C
 	// The RPC sent from the 'nomad node detach' command or GC won't have an
 	// allocation ID set so we try to unpublish every terminal or invalid
 	// alloc on the node, all of which will be in PastClaims after denormalizing
-	state := v.srv.fsm.State()
-	vol, err := state.CSIVolumeDenormalize(memdb.NewWatchSet(), vol)
+	vol, err := store.CSIVolumeDenormalize(memdb.NewWatchSet(), vol)
 	if err != nil {
 		return err
 	}