Docserver: Add FileSystem.Exists method and start sanity checking path format

chrome/common/extensions/docs/server2/file_system.py

Index: chrome/common/extensions/docs/server2/file_system.py
diff --git a/chrome/common/extensions/docs/server2/file_system.py b/chrome/common/extensions/docs/server2/file_system.py
index 6686f4c99f2b10922daf84e89b05bf62ccf1cbe0..ddb50271742f49059c24dc2e2891b22fab934823 100644
--- a/chrome/common/extensions/docs/server2/file_system.py
+++ b/chrome/common/extensions/docs/server2/file_system.py
@@ -2,7 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import posixpath
+
 from future import Gettable, Future
+from path_util import IsDirectory, SplitParent, ToDirectory
 
 
 class _BaseFileSystemException(Exception):
@@ -77,6 +80,27 @@ class FileSystem(object):
     read_single = self.Read([path])
     return Future(delegate=Gettable(lambda: read_single.Get()[path]))
 
+  def Exists(self, path):
+    '''Returns a Future to the existence of |path|; True if |path| exists,
+    False if not. This method will not throw a FileNotFoundError unlike
+    the Read* methods, however it may still throw a FileSystemError.
+
+    There are several ways to implement this method via the interface but this
+    method exists to do so in a canonical and most efficient way for caching.
+    '''
+    if path in ('', '/'):
+      # There is always a root directory.
+      return Future(value=True)
+
+    parent, base = SplitParent(path)
+    list_future = self.ReadSingle(ToDirectory(parent))
+    def resolve():
+      try:
+        return base in list_future.Get()
+      except FileNotFoundError:
+        return False
+    return Future(delegate=Gettable(resolve))
+
   def Refresh(self):
     '''Asynchronously refreshes the content of the FileSystem, returning a
     future to its completion.
@@ -105,17 +129,22 @@ class FileSystem(object):
 
   def Walk(self, root):
     '''Recursively walk the directories in a file system, starting with root.
-    Emulates os.walk from the standard os module.
 
-    If the root cannot be found, raises a FileNotFoundError.
+    Behaviour is very similar to os.walk from the standard os module, yielding
+    (base, dirs, files) recursively, where |base| is the base path of |files|,
+    |dirs| relative to |root|, and |files| and |dirs| the list of files/dirs in
+    |base| respectively.
+
+    Note that directories will always end with a '/', files never will.
+
+    If |root| cannot be found, raises a FileNotFoundError.
     For any other failure, raises a FileSystemError.
     '''
-    basepath = root.rstrip('/') + '/'
+    root = ToDirectory(root)  # TODO(kalman): assert IsDirectory(root)
+    basepath = root
 
     def walk(root):
-      if not root.endswith('/'):
-        root += '/'
-
+      assert IsDirectory(root), root
       dirs, files = [], []
 
       for f in self.ReadSingle(root).Get():