Fix get user share as grantee in json backend (#1650)

Signed-off-by: Jörn Friedrich Dreyer <jfd@butonic.de>

changelog/unreleased/fix-get-user-share-as-grantee.md

@@ -0,0 +1,5 @@
+Bugfix: Allow fetching shares as the grantee
+
+The json backend now allows a grantee to fetch a share by id.
+
+https://github.com/cs3org/reva/pull/1650

pkg/share/manager/json/json.go

@@ -278,6 +278,17 @@ func (m *mgr) get(ctx context.Context, ref *collaboration.ShareReference) (s *co
 		return s, nil
 	}
 
+	// or the grantee
+	if s.Grantee.Type == provider.GranteeType_GRANTEE_TYPE_USER && utils.UserEqual(user.Id, s.Grantee.GetUserId()) {
+		return s, nil
+	} else if s.Grantee.Type == provider.GranteeType_GRANTEE_TYPE_GROUP {
+		// check if all user groups match this share; TODO(labkode): filter shares created by us.
+		for _, g := range user.Groups {
+			if g == s.Grantee.GetGroupId().OpaqueId {
+				return s, nil
+			}
+		}
+	}
 	// we return not found to not disclose information
 	return nil, errtypes.NotFound(ref.String())
 }