Use explicit exception to detect invalid path (BackReferenceError), see comments on r773

fs/errors.py

@@ -188,6 +188,11 @@ class NoMMapError(ResourceError):
     default_message = "Can't get mmap for %(path)s"
 
 
+class BackReferenceError(FSError, ValueError):
+    """Exception raised when too many backrefs exist in a path (ex: '/..', '/docs/../..')."""
+    default_message = "Too many backrefs in '%(path)s'"
+
+
 def convert_fs_errors(func):
     """Function wrapper to convert FSError instances into OSError."""
     @wraps(func)

fs/expose/sftp.py

@@ -185,12 +185,10 @@ class SFTPServerInterface(paramiko.SFTPServerInterface):
     def canonicalize(self, path):
         try:
             return abspath(normpath(path)).encode(self.encoding)
-        except ValueError, e:
+        except BackReferenceError:
             # If the client tries to use backrefs to escape root, gently
             # nudge them back to /.
-            if 'too many backrefs' in e.args[0]:
             return '/'
-            raise
 
     @report_sftp_errors
     def chattr(self, path, attr):

fs/path.py

@@ -33,7 +33,7 @@ def normpath(path):
     >>> normpath("foo/../../bar")
     Traceback (most recent call last)
         ...
-    ValueError: too many backrefs in path 'foo/../../bar'
+    BackReferenceError: Too many backrefs in 'foo/../../bar'
 
     """
 
@@ -57,7 +57,10 @@ def normpath(path):
             else:
                 append(component)
     except IndexError:
-        raise ValueError("too many backrefs in path '%s'" % path)
+        # Imported here because errors imports this module (path),
+        # causing a circular import.
+        from fs.errors import BackReferenceError
+        BackReferenceError(details={ 'path': path })
     if path[0] == '/':
         return '/%s' % '/'.join(components)
     return '/'.join(components)
@@ -78,6 +81,7 @@ def iteratepath(path, numsplits=None):
     else:
         return path.split('/', numsplits)
 
+
 def recursepath(path, reverse=False):
     """Returns intermediate paths from the root to the given path