csi: running allocations for the same job should block claims

When the scheduler checks feasibility for CSI volumes, the check is
fairly loose: earlier versions of the same job are not counted as
active claims. This allows the scheduler to place new allocations
for the new version of a job, under the assumption that we'll replace
the existing allocations and their volume claims.

But when the alloc runner claims the volume, we need to enforce the
active claims even if they're for allocations of an earlier version of
the job. Otherwise we'll try to mount a volume that's currently being
unmounted, and this will cause replacement allocations to frequently
fail.

This commit correctly enforces maximum volume claims for writers.

nomad/csi_endpoint_test.go

@@ -243,9 +243,11 @@ func TestCSIVolumeEndpoint_Claim(t *testing.T) {
 	// Create an initial volume claim request; we expect it to fail
 	// because there's no such volume yet.
 	claimReq := &structs.CSIVolumeClaimRequest{
-		VolumeID:     id0,
-		AllocationID: alloc.ID,
-		Claim:        structs.CSIVolumeClaimWrite,
+		VolumeID:       id0,
+		AllocationID:   alloc.ID,
+		Claim:          structs.CSIVolumeClaimWrite,
+		AccessMode:     structs.CSIVolumeAccessModeMultiNodeSingleWriter,
+		AttachmentMode: structs.CSIVolumeAttachmentModeFilesystem,
 		WriteRequest: structs.WriteRequest{
 			Region:    "global",
 			Namespace: structs.DefaultNamespace,

nomad/structs/csi.go

@@ -554,12 +554,7 @@ func (v *CSIVolume) claimWrite(claim *CSIVolumeClaim, alloc *Allocation) error {
 	}
 
 	if !v.HasFreeWriteClaims() {
-		// Check the blocking allocations to see if they belong to this job
-		for _, a := range v.WriteAllocs {
-			if a != nil && (a.Namespace != alloc.Namespace || a.JobID != alloc.JobID) {
-				return ErrCSIVolumeMaxClaims
-			}
-		}
+		return ErrCSIVolumeMaxClaims
 	}
 
 	// Allocations are copy on write, so we want to keep the id but don't need the