From e90fcd9edd66b78e520b407e6ffa3a66be2c9178 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Mon, 21 Nov 2016 15:18:18 +0000
Subject: [PATCH] Add filter_event_fields and filter_field to FilterCollection

---
 synapse/api/filtering.py | 69 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 3b3ef7075..27f8b99e3 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -18,6 +18,7 @@ from synapse.types import UserID, RoomID
 from twisted.internet import defer
 
 import ujson as json
+import re
 
 
 class Filtering(object):
@@ -71,6 +72,21 @@ class Filtering(object):
                 if key in user_filter_json["room"]:
                     self._check_definition(user_filter_json["room"][key])
 
+        if "event_fields" in user_filter_json:
+            if type(user_filter_json["event_fields"]) != list:
+                raise SynapseError(400, "event_fields must be a list of strings")
+            for field in user_filter_json["event_fields"]:
+                if not isinstance(field, basestring):
+                    raise SynapseError(400, "Event field must be a string")
+                # Don't allow '\\' in event field filters. This makes matching
+                # events a lot easier as we can then use a negative lookbehind
+                # assertion to split '\.' If we allowed \\ then it would
+                # incorrectly split '\\.'
+                if r'\\' in field:
+                    raise SynapseError(
+                        400, r'The escape character \ cannot itself be escaped'
+                    )
+
     def _check_definition_room_lists(self, definition):
         """Check that "rooms" and "not_rooms" are lists of room ids if they
         are present
@@ -152,6 +168,11 @@ class FilterCollection(object):
         self.include_leave = filter_json.get("room", {}).get(
             "include_leave", False
         )
+        self._event_fields = filter_json.get("event_fields", [])
+        # Negative lookbehind assertion for '\'
+        # (?<!stuff) matches if the current position in the string is not preceded
+        # by a match for 'stuff'.
+        self._split_field_regex = re.compile(r'(?<!\\)\.')
 
     def __repr__(self):
         return "<FilterCollection %s>" % (json.dumps(self._filter_json),)
@@ -186,6 +207,54 @@ class FilterCollection(object):
     def filter_room_account_data(self, events):
         return self._room_account_data.filter(self._room_filter.filter(events))
 
+    def filter_event_fields(self, event):
+        """Remove fields from an event in accordance with the 'event_fields' of a filter.
+
+        If there are no event fields specified then all fields are included.
+        The entries may include '.' charaters to indicate sub-fields.
+        So ['content.body'] will include the 'body' field of the 'content' object.
+        A literal '.' character in a field name may be escaped using a '\'.
+
+        Args:
+            event(dict): The raw event to filter
+        Returns:
+            dict: The same event with some fields missing, if required.
+        """
+        for field in self._event_fields:
+            self.filter_field(event, field)
+        return event
+
+    def filter_field(self, dictionary, field):
+        """Filter the given field from the given dictionary.
+
+        Args:
+            dictionary(dict): The dictionary to remove the field from.
+            field(str): The key to remove.
+        Returns:
+            dict: The same dictionary with the field removed.
+        """
+        # "content.body.thing\.with\.dots" => ["content", "body", "thing\.with\.dots"]
+        sub_fields = self._split_field_regex.split(field)
+        # remove escaping so we can use the right key names when deleting
+        sub_fields = [f.replace(r'\.', r'.') for f in sub_fields]
+
+        # common case e.g. 'origin_server_ts'
+        if len(sub_fields) == 1:
+            dictionary.pop(sub_fields[0], None)
+        # nested field e.g. 'content.body'
+        elif len(sub_fields) > 1:
+            # Pop the last field as that's the key to delete and we need the
+            # parent dict in order to remove the key. Drill down to the right dict.
+            key_to_delete = sub_fields.pop(-1)
+            sub_dict = dictionary
+            for sub_field in sub_fields:
+                if sub_field in sub_dict and type(sub_dict[sub_field]) == dict:
+                    sub_dict = sub_dict[sub_field]
+                else:
+                    return dictionary
+            sub_dict.pop(key_to_delete, None)
+        return dictionary
+
 
 class Filter(object):
     def __init__(self, filter_json):