IDirectoryNode: add has_child() method

src/allmydata/dirnode.py

@@ -234,6 +234,16 @@ class ImmutableDirectoryNode:
     def _hash_name(self, name):
         return hashutil.dir_name_hash(self._readkey, name)
 
+    def has_child(self, name):
+        d = self.get(name)
+        def _good(res):
+            return True
+        def _err(f):
+            f.trap(KeyError)
+            return False
+        d.addCallbacks(_good, _err)
+        return d
+
     def get(self, name):
         H_name = self._hash_name(name)
         d = self._rref.callRemote("get", self._index, H_name)

src/allmydata/interfaces.py

@@ -352,6 +352,10 @@ class IDirectoryNode(Interface):
         """I return a Deferred that fires with a dictionary mapping child
         name to an IFileNode or IDirectoryNode."""
 
+    def has_child(name):
+        """I return a Deferred that fires with a boolean, True if there
+        exists a child of the given name, False if not."""
+
     def get(name):
         """I return a Deferred that fires with a specific named child node,
         either an IFileNode or an IDirectoryNode."""

src/allmydata/test/test_dirnode.py

@@ -378,6 +378,12 @@ class Test(unittest.TestCase):
         # root/bar-ro/file4 = file2
         # root/bar-ro/baz/
 
+        # test has_child
+        d.addCallback(lambda res: rootnode.has_child("bar"))
+        d.addCallback(self.failUnlessEqual, True)
+        d.addCallback(lambda res: rootnode.has_child("missing"))
+        d.addCallback(self.failUnlessEqual, False)
+
         # test the manifest
         d.addCallback(lambda res: self.rootnode.build_manifest())
         def _check_manifest2(manifest):