add a mkdir-p POST handler

this adds a t=mkdir-p call to directories (accessed by their uri as
/uri/<URI>?t=mkdir=p&path=/some/path) which returns the uri for a 
directory at a specified path before the given uri, regardless of
whether the directory exists or whether intermediate directories
need to be created to satisfy the request.

this is used by the migration code in MV to optimise the work of
path traversal which was other wise done on every file PUT

src/allmydata/webish.py

@@ -780,6 +780,30 @@ class POSTHandler(rend.Page):
         d.addCallback(lambda res: "directory created")
         return d
 
+    def _POST_mkdir_p(self, path):
+        path_ = tuple([seg.decode("utf-8") for seg in path.split('/') if seg ])
+        d = self._get_or_create_directories(self._node, path_)
+        d.addCallback(lambda node: node.get_uri())
+        return d
+
+    # this code stolen from PUTHandler: should be refactored to a more
+    # generally accesible place, perhaps...
+    def _get_or_create_directories(self, node, path):
+        if not IDirectoryNode.providedBy(node):
+            # unfortunately it is too late to provide the name of the
+            # blocking directory in the error message.
+            raise BlockingFileError("cannot create directory because there "
+                                    "is a file in the way")
+        if not path:
+            return defer.succeed(node)
+        d = node.get(path[0])
+        def _maybe_create(f):
+            f.trap(KeyError)
+            return node.create_empty_directory(path[0])
+        d.addErrback(_maybe_create)
+        d.addCallback(self._get_or_create_directories, path[1:])
+        return d
+
     def _POST_uri(self, name, newuri):
         d = self._check_replacement(name)
         d.addCallback(lambda res: self._node.set_uri(name, newuri))
@@ -914,6 +938,11 @@ class POSTHandler(rend.Page):
             if not name:
                 raise RuntimeError("mkdir requires a name")
             d = self._POST_mkdir(name)
+        elif t == "mkdir-p":
+            path = get_arg(req, "path")
+            if not path:
+                raise RuntimeError("mkdir-p requires a path")
+            d = self._POST_mkdir_p(path)
         elif t == "uri":
             if not name:
                 raise RuntimeError("set-uri requires a name")