Do not depend on UltiSnips internals

Use UltiSnips#SnippetsInCurrentScope to fetch snippets.
Add an entry in the FAQ about the :UltiSnipsAddFiletypes command.

README.md

@@ -2648,6 +2648,16 @@ g:UltiSnipsJumpForwardTrigger
 g:UltiSnipsJumpBackwardTrigger
 ```
 
+### Snippets added with `:UltiSnipsAddFiletypes` do not appear in the popup menu
+
+For efficiency, YCM only fetches UltiSnips snippets in specific scenarios like
+visiting a buffer or setting its filetype. You can force YCM to retrieve them by
+manually triggering the `FileType` autocommand:
+
+```viml
+:doautocmd FileType
+```
+
 ### Why isn't YCM just written in plain VimScript, FFS?
 
 Because of the identifier completion engine and subsequence-based filtering.

python/ycm/youcompleteme.py

@@ -51,12 +51,6 @@
                                             EventNotification )
 from ycm.client.shutdown_request import SendShutdownRequest
 
-try:
-  from UltiSnips import UltiSnips_Manager
-  USE_ULTISNIPS_DATA = True
-except ImportError:
-  USE_ULTISNIPS_DATA = False
-
 
 def PatchNoProxy():
   current_value = os.environ.get('no_proxy', '')
@@ -319,7 +313,7 @@ def OnBufferVisit( self ):
     if not self.IsServerAlive():
       return
     extra_data = {}
-    _AddUltiSnipsDataIfNeeded( extra_data )
+    self._AddUltiSnipsDataIfNeeded( extra_data )
     SendEventNotificationAsync( 'BufferVisit', extra_data )
 
 
@@ -690,25 +684,16 @@ def BuildExtraConfData( extra_conf_vim_data ):
         extra_conf_vim_data )
 
 
-def _AddUltiSnipsDataIfNeeded( extra_data ):
-  if not USE_ULTISNIPS_DATA:
-    return
-
-  try:
-    # Since UltiSnips may run in a different python interpreter (python 3) than
-    # YCM, UltiSnips_Manager singleton is not necessary the same as the one
-    # used by YCM. In particular, it means that we cannot rely on UltiSnips to
-    # set the current filetypes to the singleton. We need to do it ourself.
-    UltiSnips_Manager.reset_buffer_filetypes()
-    UltiSnips_Manager.add_buffer_filetypes(
-      vimsupport.GetVariableValue( '&filetype' ) )
-    rawsnips = UltiSnips_Manager._snips( '', True )
-  except:
-    return
-
-  # UltiSnips_Manager._snips() returns a class instance where:
-  # class.trigger - name of snippet trigger word ( e.g. defn or testcase )
-  # class.description - description of the snippet
-  extra_data[ 'ultisnips_snippets' ] = [
-    { 'trigger': x.trigger, 'description': x.description } for x in rawsnips
-  ]
+  def _AddUltiSnipsDataIfNeeded( self, extra_data ):
+    # See :h UltiSnips#SnippetsInCurrentScope.
+    try:
+      vim.eval( 'UltiSnips#SnippetsInCurrentScope( 1 )' )
+    except vim.error:
+      return
+
+    snippets = vimsupport.GetVariableValue( 'g:current_ulti_dict_info' )
+    extra_data[ 'ultisnips_snippets' ] = [
+      { 'trigger': trigger,
+        'description': snippet[ 'description' ] }
+      for trigger, snippet in iteritems( snippets )
+    ]