spec: Clarify target_path and staging_target_path

This patch clarifies the `target_path` and `staging_target_path` in the
spec. It is CO's responsibility to make sure `target_path` and
`staging_target_path` exist and is accessible. The SP should not use
symbolic link to publish/stage the volume. For block volumes, the CO
will create empty files at target paths. For mount volumes, the CO will
create empty directories at target paths.

Author: jieyu

csi.proto

@@ -944,10 +944,16 @@ message NodeStageVolumeRequest {
   // this capability. This is an OPTIONAL field.
   map<string, string> publish_info = 2;
 
-  // The path to which the volume will be published. It MUST be an
+  // The path to which the volume will 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.
+  // `staging_target_path` per volume. The SP MUST NOT use symbolic link
+  // to staged the volume. The CO SHALL ensure that the path exists, and
+  // that the process serving the request has `read` and `write`
+  // permission to the path. If the path does not exist, the CO SHALL
+  // create an empty file if the volume is being staged as a Block
+  // Volume, and an empty directory if the volume is being staged as a
+  // Mount Volume.
   // This is a REQUIRED field.
   string staging_target_path = 3;
 
@@ -960,7 +966,7 @@ message NodeStageVolumeRequest {
   // section on how to use this field.
   map<string, string> node_stage_secrets = 5;
 
-  // Attributes of the volume to publish. This field is OPTIONAL and
+  // Attributes of the volume to stage. This field is OPTIONAL and
   // MUST match the attributes of the `Volume` identified by
   // `volume_id`.
   map<string, string> volume_attributes = 6;
@@ -973,7 +979,7 @@ message NodeUnstageVolumeRequest {
   // The ID of the volume. This field is REQUIRED.
   string volume_id = 1;
 
-  // The path at which the volume was published. It MUST be an absolute
+  // The path at which the volume was staged. It MUST be an absolute
   // path in the root filesystem of the process serving this request.
   // This is a REQUIRED field.
   string staging_target_path = 2;
@@ -993,7 +999,7 @@ message NodePublishVolumeRequest {
   // this capability. This is an OPTIONAL field.
   map<string, string> publish_info = 2;
 
-  // The path to which the device was mounted by `NodeStageVolume`.
+  // The path to which the volume was staged by `NodeStageVolume`.
   // It MUST be an absolute path in the root filesystem of the process
   // serving this request.
   // It MUST be set if the Node Plugin implements the
@@ -1004,8 +1010,12 @@ 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 path exists, and that the process
-  // serving the request has `read` and `write` permissions to the path.
+  // The SP MUST NOT use symbolic link to publish the volume. The CO
+  // SHALL ensure that the path exists, and that the process serving the
+  // request has `read` and `write` permissions to the path. If the path
+  // does not exist, the CO SHALL create an empty file if the volume is
+  // being published as a Block Volume, and an empty directory if the
+  // volume is being published as a Mount Volume.
   // This is a REQUIRED field.
   string target_path = 4;