Update the requirements for staging and target paths

The vague wording around the staging and target paths left open too
many possible implementations for raw block volumes, and implementors
ended up doing non-obvious things. This change clarifies the contract
between the CO and the SP when staging and publishing raw block volumes
on the node.

Specifically we now require that the CO creates the staging_target_path
and that it is always a directory. And we require that the SP create
the target_path and it should be a device file for blocks and a mount
point for filesystems.

Author: bswartz

csi.proto

@@ -980,11 +980,12 @@ message NodeStageVolumeRequest {
 
   // The path to which the volume MAY be staged. It MUST be an
   // absolute path in the root filesystem of the process serving this
-  // request. The CO SHALL ensure that there is only one
-  // `staging_target_path` per volume. The CO SHALL ensure that the
-  // process serving the request has `read` and `write` permission to
-  // the path, and is able to create files or directories at the path
-  // if it does not exist.
+  // request, and MUST be a directory. The CO SHALL ensure that there
+  // is only one `staging_target_path` per volume. The CO SHALL ensure
+  // that the path is directory and that the process serving the
+  // request has `read` and `write` permission to that directory. The
+  // CO SHALL be responsible for creating the directory if it does not
+  // exist.
   // This is a REQUIRED field.
   string staging_target_path = 3;
 
@@ -1043,9 +1044,14 @@ message NodePublishVolumeRequest {
   // The path to which the volume will be published. It MUST be an
   // absolute path in the root filesystem of the process serving this
   // request. The CO SHALL ensure uniqueness of target_path per volume.
-  // The CO SHALL ensure that the process serving the request has
-  // `read` and `write` permissions to the path, and is able to create
-  // files or directories at the path if it does not exist.
+  // The CO SHALL ensure that the parent directory of this path exists
+  // and that the process serving the request has `read` and `write`
+  // permissions to that parent directory.
+  // For volumes with an access type of block, the SP SHALL place the
+  // block device at target_path.
+  // For volumes with an access type of mount, the SP SHALL place the
+  // mounted directory at target_path.
+  // Creation of target_path is the responsibility of the SP.
   // This is a REQUIRED field.
   string target_path = 4;
 
@@ -1079,6 +1085,7 @@ message NodeUnpublishVolumeRequest {
 
   // The path at which the volume was published. It MUST be an absolute
   // path in the root filesystem of the process serving this request.
+  // The SP MUST delete the file or directory it created at this path.
   // This is a REQUIRED field.
   string target_path = 2;
 }