Backport of CSI: don't overwrite context with empty value from request into release/1.9.x

.changelog/24922.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+csi: Fixed a bug where volume context from the plugin would be erased on volume updates
+```

nomad/structs/csi.go

@@ -846,9 +846,13 @@ func (v *CSIVolume) Merge(other *CSIVolume) error {
 			"volume parameters cannot be updated"))
 	}
 
-	// Context is mutable and will be used during controller
-	// validation
-	v.Context = other.Context
+	// Context is mutable and will be used during controller validation, but we
+	// need to ensure we don't remove context that's been previously stored
+	// server-side if the user has submitted an update without adding it to the
+	// spec manually (which we should not require)
+	if len(other.Context) != 0 {
+		v.Context = other.Context
+	}
 	return errs.ErrorOrNil()
 }
 

nomad/structs/csi_test.go

@@ -711,6 +711,13 @@ func TestCSIVolume_Merge(t *testing.T) {
 						{Segments: map[string]string{"rack": "R2"}},
 					},
 				},
+				Context: map[string]string{
+					// a typical context for democratic-csi
+					"provisioner_driver": "nfs-client",
+					"node_attach_driver": "nfs",
+					"server":             "192.168.1.170",
+					"share":              "/srv/nfs_data/v/csi-volume-nfs",
+				},
 			},
 			update: &CSIVolume{
 				Topologies: []*CSITopology{
@@ -745,6 +752,14 @@ func TestCSIVolume_Merge(t *testing.T) {
 					test.Sprint("should add 2 requested capabilities"))
 				test.Eq(t, []string{"noatime", "another"}, v.MountOptions.MountFlags,
 					test.Sprint("should add mount flag"))
+				test.Eq(t, map[string]string{
+					"provisioner_driver": "nfs-client",
+					"node_attach_driver": "nfs",
+					"server":             "192.168.1.170",
+					"share":              "/srv/nfs_data/v/csi-volume-nfs",
+				}, v.Context,
+					test.Sprint("context should not be overwritten with empty update"))
+
 			},
 		},
 	}