Make relative reference parameters in appprovider consistent with other services

[ishank011]

https://github.com/cs3org/reva/pull/2210#discussion_r765556535

[wkloucek]

The below diff would change the behavior with the restriction of not using relative CS3api references. I would opt for waiting until all storage provider support relative references, then this will be cleaner and easier, since we can just pass both parameters into the relative CS3api reference.

diff --git a/internal/http/services/appprovider/appprovider.go b/internal/http/services/appprovider/appprovider.go
index ed101a0f..57c2e387 100644
--- a/internal/http/services/appprovider/appprovider.go
+++ b/internal/http/services/appprovider/appprovider.go
@@ -154,11 +154,7 @@ func (s *svc) handleNew(w http.ResponseWriter, r *http.Request) {
                return
        }

-       dirPart, filePart := path.Split(filename)
-       if dirPart != "" || filePart != filename {
-               writeError(w, r, appErrorInvalidParameter, "the filename must not contain a path segment", nil)
-               return
-       }
+       relativePath, filename := path.Split(filename)

        statParentContainerReq := &provider.StatRequest{
                Ref: &provider.Reference{
@@ -176,13 +172,29 @@ func (s *svc) handleNew(w http.ResponseWriter, r *http.Request) {
                return
        }

-       if parentContainer.Info.Type != provider.ResourceType_RESOURCE_TYPE_CONTAINER {
-               writeError(w, r, appErrorInvalidParameter, "the parent container id does not point to a container", nil)
+       statRelativeParentContainerReq := &provider.StatRequest{
+               Ref: &provider.Reference{
+                       Path: path.Join(parentContainer.Info.Path, utils.MakeRelativePath(relativePath)),
+               },
+       }
+       relativeParentContainer, err := client.Stat(ctx, statRelativeParentContainerReq)
+       if err != nil {
+               writeError(w, r, appErrorServerError, "error sending a grpc stat request", err)
+               return
+       }
+
+       if relativeParentContainer.Status.Code != rpc.Code_CODE_OK {
+               writeError(w, r, appErrorNotFound, "the path relative to the parent container is not accessible or does not exist", err)
+               return
+       }
+
+       if relativeParentContainer.Info.Type != provider.ResourceType_RESOURCE_TYPE_CONTAINER {
+               writeError(w, r, appErrorInvalidParameter, "the path relative to the parent container id does not point to a container", nil)
                return
        }

        fileRef := &provider.Reference{
-               Path: path.Join(parentContainer.Info.Path, utils.MakeRelativePath(filename)),
+               Path: path.Join(relativeParentContainer.Info.Path, utils.MakeRelativePath(filename)),
        }

        statFileReq := &provider.StatRequest{