Compare to the earliest known stream pos in the stream change cache. (#14435)

The internal methods of the StreamChangeCache were inconsistently
treating the earliest known stream position as valid. It is now treated as
invalid, meaning the cache cannot determine if an entity at the earliest
known stream position has changed or not.	

1 files changed, 116 insertions, 26 deletions

diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index 666f4b6895..042de8d7c8 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -27,13 +27,17 @@ EntityType = str
 
 
 class StreamChangeCache:
-    """Keeps track of the stream positions of the latest change in a set of entities.
+    """
+    Keeps track of the stream positions of the latest change in a set of entities.
+
+    The entity will is typically a room ID or user ID, but can be any string.
 
-    Typically the entity will be a room or user id.
+    Can be queried for whether a specific entity has changed after a stream position
+    or for a list of changed entities after a stream position. See the individual
+    methods for more information.
 
-    Given a list of entities and a stream position, it will give a subset of
-    entities that may have changed since that position. If position key is too
-    old then the cache will simply return all given entities.
+    Only tracks to a maximum cache size, any position earlier than the earliest
+    known stream position must be treated as unknown.
     """
 
     def __init__(
@@ -45,16 +49,20 @@ class StreamChangeCache:
     ) -> None:
         self._original_max_size: int = max_size
         self._max_size = math.floor(max_size)
-        self._entity_to_key: Dict[EntityType, int] = {}
 
-        # map from stream id to the a set of entities which changed at that stream id.
+        # map from stream id to the set of entities which changed at that stream id.
         self._cache: SortedDict[int, Set[EntityType]] = SortedDict()
+        # map from entity to the stream ID of the latest change for that entity.
+        #
+        # Must be kept in sync with _cache.
+        self._entity_to_key: Dict[EntityType, int] = {}
 
         # the earliest stream_pos for which we can reliably answer
         # get_all_entities_changed. In other words, one less than the earliest
         # stream_pos for which we know _cache is valid.
         #
         self._earliest_known_stream_pos = current_stream_pos
+
         self.name = name
         self.metrics = caches.register_cache(
             "cache", self.name, self._cache, resize_callback=self.set_cache_factor
@@ -82,22 +90,46 @@ class StreamChangeCache:
         return False
 
     def has_entity_changed(self, entity: EntityType, stream_pos: int) -> bool:
-        """Returns True if the entity may have been updated since stream_pos"""
+        """
+        Returns True if the entity may have been updated after stream_pos.
+
+        Args:
+            entity: The entity to check for changes.
+            stream_pos: The stream position to check for changes after.
+
+        Return:
+            True if the entity may have been updated, this happens if:
+                * The given stream position is at or earlier than the earliest
+                  known stream position.
+                * The given stream position is earlier than the latest change for
+                  the entity.
+
+            False otherwise:
+                * The entity is unknown.
+                * The given stream position is at or later than the latest change
+                  for the entity.
+        """
         assert isinstance(stream_pos, int)
 
-        if stream_pos < self._earliest_known_stream_pos:
+        # _cache is not valid at or before the earliest known stream position, so
+        # return that the entity has changed.
+        if stream_pos <= self._earliest_known_stream_pos:
             self.metrics.inc_misses()
             return True
 
+        # If the entity is unknown, it hasn't changed.
         latest_entity_change_pos = self._entity_to_key.get(entity, None)
         if latest_entity_change_pos is None:
             self.metrics.inc_hits()
             return False
 
+        # This is a known entity, return true if the stream position is earlier
+        # than the last change.
         if stream_pos < latest_entity_change_pos:
             self.metrics.inc_misses()
             return True
 
+        # Otherwise, the stream position is after the latest change: return false.
         self.metrics.inc_hits()
         return False
 
@@ -105,15 +137,27 @@ class StreamChangeCache:
         self, entities: Collection[EntityType], stream_pos: int
     ) -> Union[Set[EntityType], FrozenSet[EntityType]]:
         """
-        Returns subset of entities that have had new things since the given
-        position.  Entities unknown to the cache will be returned.  If the
-        position is too old it will just return the given list.
+        Returns the subset of the given entities that have had changes after the given position.
+
+        Entities unknown to the cache will be returned.
+
+        If the position is too old it will just return the given list.
+
+        Args:
+            entities: Entities to check for changes.
+            stream_pos: The stream position to check for changes after.
+
+        Return:
+            A subset of entities which have changed after the given stream position.
+
+            This will be all entities if the given stream position is at or earlier
+            than the earliest known stream position.
         """
         changed_entities = self.get_all_entities_changed(stream_pos)
         if changed_entities is not None:
             # We now do an intersection, trying to do so in the most efficient
             # way possible (some of these sets are *large*). First check in the
-            # given iterable is already set that we can reuse, otherwise we
+            # given iterable is already a set that we can reuse, otherwise we
             # create a set of the *smallest* of the two iterables and call
             # `intersection(..)` on it (this can be twice as fast as the reverse).
             if isinstance(entities, (set, frozenset)):
@@ -130,29 +174,57 @@ class StreamChangeCache:
         return result
 
     def has_any_entity_changed(self, stream_pos: int) -> bool:
-        """Returns if any entity has changed"""
-        assert type(stream_pos) is int
+        """
+        Returns true if any entity has changed after the given stream position.
+
+        Args:
+            stream_pos: The stream position to check for changes after.
+
+        Return:
+            True if any entity has changed after the given stream position or
+            if the given stream position is at or earlier than the earliest
+            known stream position.
+
+            False otherwise.
+        """
+        assert isinstance(stream_pos, int)
 
         if not self._cache:
             # If the cache is empty, nothing can have changed.
             return False
 
-        if stream_pos >= self._earliest_known_stream_pos:
-            self.metrics.inc_hits()
-            return self._cache.bisect_right(stream_pos) < len(self._cache)
-        else:
+        # _cache is not valid at or before the earliest known stream position, so
+        # return that an entity has changed.
+        if stream_pos <= self._earliest_known_stream_pos:
             self.metrics.inc_misses()
             return True
 
+        self.metrics.inc_hits()
+        return stream_pos < self._cache.peekitem()[0]
+
     def get_all_entities_changed(self, stream_pos: int) -> Optional[List[EntityType]]:
-        """Returns all entities that have had new things since the given
-        position. If the position is too old it will return None.
+        """
+        Returns all entities that have had changes after the given position.
+
+        If the stream change cache does not go far enough back, i.e. the position
+        is too old, it will return None.
 
         Returns the entities in the order that they were changed.
+
+        Args:
+            stream_pos: The stream position to check for changes after.
+
+        Return:
+            Entities which have changed after the given stream position.
+
+            None if the given stream position is at or earlier than the earliest
+            known stream position.
         """
-        assert type(stream_pos) is int
+        assert isinstance(stream_pos, int)
 
-        if stream_pos < self._earliest_known_stream_pos:
+        # _cache is not valid at or before the earliest known stream position, so
+        # return None to mark that it is unknown if an entity has changed.
+        if stream_pos <= self._earliest_known_stream_pos:
             return None
 
         changed_entities: List[EntityType] = []
@@ -162,11 +234,17 @@ class StreamChangeCache:
         return changed_entities
 
     def entity_has_changed(self, entity: EntityType, stream_pos: int) -> None:
-        """Informs the cache that the entity has been changed at the given
-        position.
         """
-        assert type(stream_pos) is int
+        Informs the cache that the entity has been changed at the given position.
+
+        Args:
+            entity: The entity to mark as changed.
+            stream_pos: The stream position to update the entity to.
+        """
+        assert isinstance(stream_pos, int)
 
+        # For a change before _cache is valid (e.g. at or before the earliest known
+        # stream position) there's nothing to do.
         if stream_pos <= self._earliest_known_stream_pos:
             return
 
@@ -189,6 +267,11 @@ class StreamChangeCache:
         self._evict()
 
     def _evict(self) -> None:
+        """
+        Ensure the cache has not exceeded the maximum size.
+
+        Evicts entries until it is at the maximum size.
+        """
         # if the cache is too big, remove entries
         while len(self._cache) > self._max_size:
             k, r = self._cache.popitem(0)
@@ -199,5 +282,12 @@ class StreamChangeCache:
     def get_max_pos_of_last_change(self, entity: EntityType) -> int:
         """Returns an upper bound of the stream id of the last change to an
         entity.
+
+        Args:
+            entity: The entity to check.
+
+        Return:
+            The stream position of the latest change for the given entity or
+            the earliest known stream position if the entitiy is unknown.
         """
         return self._entity_to_key.get(entity, self._earliest_known_stream_pos)