Extend StreamChangeCache to support multiple entities per stream ID (#…

…7303)

First some background: StreamChangeCache is used to keep track of what "entities" have 
changed since a given stream ID. So for example, we might use it to keep track of when the last
to-device message for a given user was received [1], and hence whether we need to pull any to-device messages from the database on a sync [2].

Now, it turns out that StreamChangeCache didn't support more than one thing being changed at
a given stream_id (this was part of the problem with #7206). However, it's entirely valid to send
to-device messages to more than one user at a time.

As it turns out, this did in fact work, because *some* methods of StreamChangeCache coped
ok with having multiple things changing on the same stream ID, and it seems we never actually
use the methods which don't work on the stream change caches where we allow multiple
changes at the same stream ID. But that feels horribly fragile, hence: let's update
StreamChangeCache to properly support this, and add some typing and some more tests while
we're at it.

[1]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L301
[2]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L47-L51

changelog.d/7303.misc

@@ -0,0 +1 @@
+Fix StreamChangeCache to work with multiple entities changing on the same stream id.

stubs/sortedcontainers/__init__.pyi

@@ -0,0 +1,13 @@
+from .sorteddict import (
+    SortedDict,
+    SortedKeysView,
+    SortedItemsView,
+    SortedValuesView,
+)
+
+__all__ = [
+    "SortedDict",
+    "SortedKeysView",
+    "SortedItemsView",
+    "SortedValuesView",
+]

stubs/sortedcontainers/sorteddict.pyi

@@ -0,0 +1,124 @@
+# stub for SortedDict. This is a lightly edited copy of
+# https://github.com/grantjenks/python-sortedcontainers/blob/eea42df1f7bad2792e8da77335ff888f04b9e5ae/sortedcontainers/sorteddict.pyi
+# (from https://github.com/grantjenks/python-sortedcontainers/pull/107)
+
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    Hashable,
+    Iterator,
+    Iterable,
+    ItemsView,
+    KeysView,
+    List,
+    Mapping,
+    Optional,
+    Sequence,
+    Type,
+    TypeVar,
+    Tuple,
+    Union,
+    ValuesView,
+    overload,
+)
+
+_T = TypeVar("_T")
+_S = TypeVar("_S")
+_T_h = TypeVar("_T_h", bound=Hashable)
+_KT = TypeVar("_KT", bound=Hashable)  # Key type.
+_VT = TypeVar("_VT")  # Value type.
+_KT_co = TypeVar("_KT_co", covariant=True, bound=Hashable)
+_VT_co = TypeVar("_VT_co", covariant=True)
+_SD = TypeVar("_SD", bound=SortedDict)
+_Key = Callable[[_T], Any]
+
+class SortedDict(Dict[_KT, _VT]):
+    @overload
+    def __init__(self, **kwargs: _VT) -> None: ...
+    @overload
+    def __init__(self, __map: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
+    @overload
+    def __init__(
+        self, __iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT
+    ) -> None: ...
+    @overload
+    def __init__(self, __key: _Key[_KT], **kwargs: _VT) -> None: ...
+    @overload
+    def __init__(
+        self, __key: _Key[_KT], __map: Mapping[_KT, _VT], **kwargs: _VT
+    ) -> None: ...
+    @overload
+    def __init__(
+        self, __key: _Key[_KT], __iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT
+    ) -> None: ...
+    @property
+    def key(self) -> Optional[_Key[_KT]]: ...
+    @property
+    def iloc(self) -> SortedKeysView[_KT]: ...
+    def clear(self) -> None: ...
+    def __delitem__(self, key: _KT) -> None: ...
+    def __iter__(self) -> Iterator[_KT]: ...
+    def __reversed__(self) -> Iterator[_KT]: ...
+    def __setitem__(self, key: _KT, value: _VT) -> None: ...
+    def _setitem(self, key: _KT, value: _VT) -> None: ...
+    def copy(self: _SD) -> _SD: ...
+    def __copy__(self: _SD) -> _SD: ...
+    @classmethod
+    @overload
+    def fromkeys(cls, seq: Iterable[_T_h]) -> SortedDict[_T_h, None]: ...
+    @classmethod
+    @overload
+    def fromkeys(cls, seq: Iterable[_T_h], value: _S) -> SortedDict[_T_h, _S]: ...
+    def keys(self) -> SortedKeysView[_KT]: ...
+    def items(self) -> SortedItemsView[_KT, _VT]: ...
+    def values(self) -> SortedValuesView[_VT]: ...
+    @overload
+    def pop(self, key: _KT) -> _VT: ...
+    @overload
+    def pop(self, key: _KT, default: _T = ...) -> Union[_VT, _T]: ...
+    def popitem(self, index: int = ...) -> Tuple[_KT, _VT]: ...
+    def peekitem(self, index: int = ...) -> Tuple[_KT, _VT]: ...
+    def setdefault(self, key: _KT, default: Optional[_VT] = ...) -> _VT: ...
+    @overload
+    def update(self, __map: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
+    @overload
+    def update(self, __iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
+    @overload
+    def update(self, **kwargs: _VT) -> None: ...
+    def __reduce__(
+        self,
+    ) -> Tuple[
+        Type[SortedDict[_KT, _VT]], Tuple[Callable[[_KT], Any], List[Tuple[_KT, _VT]]],
+    ]: ...
+    def __repr__(self) -> str: ...
+    def _check(self) -> None: ...
+    def islice(
+        self, start: Optional[int] = ..., stop: Optional[int] = ..., reverse=bool,
+    ) -> Iterator[_KT]: ...
+    def bisect_left(self, value: _KT) -> int: ...
+    def bisect_right(self, value: _KT) -> int: ...
+
+class SortedKeysView(KeysView[_KT_co], Sequence[_KT_co]):
+    @overload
+    def __getitem__(self, index: int) -> _KT_co: ...
+    @overload
+    def __getitem__(self, index: slice) -> List[_KT_co]: ...
+    def __delitem__(self, index: Union[int, slice]) -> None: ...
+
+class SortedItemsView(  # type: ignore
+    ItemsView[_KT_co, _VT_co], Sequence[Tuple[_KT_co, _VT_co]]
+):
+    def __iter__(self) -> Iterator[Tuple[_KT_co, _VT_co]]: ...
+    @overload
+    def __getitem__(self, index: int) -> Tuple[_KT_co, _VT_co]: ...
+    @overload
+    def __getitem__(self, index: slice) -> List[Tuple[_KT_co, _VT_co]]: ...
+    def __delitem__(self, index: Union[int, slice]) -> None: ...
+
+class SortedValuesView(ValuesView[_VT_co], Sequence[_VT_co]):
+    @overload
+    def __getitem__(self, index: int) -> _VT_co: ...
+    @overload
+    def __getitem__(self, index: slice) -> List[_VT_co]: ...
+    def __delitem__(self, index: Union[int, slice]) -> None: ...

synapse/util/caches/stream_change_cache.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import logging
+from typing import Dict, Iterable, List, Mapping, Optional, Set
 
 from six import integer_types
 
@@ -23,8 +24,11 @@
 
 logger = logging.getLogger(__name__)
 
+# for now, assume all entities in the cache are strings
+EntityType = str
 
-class StreamChangeCache(object):
+
+class StreamChangeCache:
     """Keeps track of the stream positions of the latest change in a set of entities.
 
     Typically the entity will be a room or user id.
@@ -34,10 +38,23 @@ class StreamChangeCache(object):
     old then the cache will simply return all given entities.
     """
 
-    def __init__(self, name, current_stream_pos, max_size=10000, prefilled_cache=None):
+    def __init__(
+        self,
+        name: str,
+        current_stream_pos: int,
+        max_size=10000,
+        prefilled_cache: Optional[Mapping[EntityType, int]] = None,
+    ):
         self._max_size = int(max_size * caches.CACHE_SIZE_FACTOR)
-        self._entity_to_key = {}
-        self._cache = SortedDict()
+        self._entity_to_key = {}  # type: Dict[EntityType, int]
+
+        # map from stream id to the a set of entities which changed at that stream id.
+        self._cache = SortedDict()  # type: SortedDict[int, Set[EntityType]]
+
+        # 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)
@@ -46,7 +63,7 @@ def __init__(self, name, current_stream_pos, max_size=10000, prefilled_cache=Non
             for entity, stream_pos in prefilled_cache.items():
                 self.entity_has_changed(entity, stream_pos)
 
-    def has_entity_changed(self, entity, stream_pos):
+    def has_entity_changed(self, entity: EntityType, stream_pos: int) -> bool:
         """Returns True if the entity may have been updated since stream_pos
         """
         assert type(stream_pos) in integer_types
@@ -67,36 +84,31 @@ def has_entity_changed(self, entity, stream_pos):
         self.metrics.inc_hits()
         return False
 
-    def get_entities_changed(self, entities, stream_pos):
+    def get_entities_changed(
+        self, entities: Iterable[EntityType], stream_pos: int
+    ) -> Set[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.
         """
-        assert type(stream_pos) is int
-
-        if stream_pos >= self._earliest_known_stream_pos:
-            changed_entities = {
-                self._cache[k]
-                for k in self._cache.islice(start=self._cache.bisect_right(stream_pos))
-            }
-
-            result = changed_entities.intersection(entities)
-
+        changed_entities = self.get_all_entities_changed(stream_pos)
+        if changed_entities is not None:
+            result = set(changed_entities).intersection(entities)
             self.metrics.inc_hits()
         else:
             result = set(entities)
             self.metrics.inc_misses()
 
         return result
 
-    def has_any_entity_changed(self, stream_pos):
+    def has_any_entity_changed(self, stream_pos: int) -> bool:
         """Returns if any entity has changed
         """
         assert type(stream_pos) is int
 
         if not self._cache:
-            # If we have no cache, nothing can have changed.
+            # If the cache is empty, nothing can have changed.
             return False
 
         if stream_pos >= self._earliest_known_stream_pos:
@@ -106,45 +118,58 @@ def has_any_entity_changed(self, stream_pos):
             self.metrics.inc_misses()
             return True
 
-    def get_all_entities_changed(self, stream_pos):
-        """Returns all entites that have had new things since the given
+    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 the entities in the order that they were changed.
         """
         assert type(stream_pos) is int
 
-        if stream_pos >= self._earliest_known_stream_pos:
-            return [
-                self._cache[k]
-                for k in self._cache.islice(start=self._cache.bisect_right(stream_pos))
-            ]
-        else:
+        if stream_pos < self._earliest_known_stream_pos:
             return None
 
-    def entity_has_changed(self, entity, stream_pos):
+        changed_entities = []  # type: List[EntityType]
+
+        for k in self._cache.islice(start=self._cache.bisect_right(stream_pos)):
+            changed_entities.extend(self._cache[k])
+        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
 
-        # FIXME: add a sanity check here that we are not overwriting existing
-        # data in self._cache
-
-        if stream_pos > self._earliest_known_stream_pos:
-            old_pos = self._entity_to_key.get(entity, None)
-            if old_pos is not None:
-                stream_pos = max(stream_pos, old_pos)
-                self._cache.pop(old_pos, None)
-            self._cache[stream_pos] = entity
-            self._entity_to_key[entity] = stream_pos
-
-            while len(self._cache) > self._max_size:
-                k, r = self._cache.popitem(0)
-                self._earliest_known_stream_pos = max(
-                    k, self._earliest_known_stream_pos
-                )
-                self._entity_to_key.pop(r, None)
-
-    def get_max_pos_of_last_change(self, entity):
+        if stream_pos <= self._earliest_known_stream_pos:
+            return
+
+        old_pos = self._entity_to_key.get(entity, None)
+        if old_pos is not None:
+            if old_pos >= stream_pos:
+                # nothing to do
+                return
+            e = self._cache[old_pos]
+            e.remove(entity)
+            if not e:
+                # cache at this point is now empty
+                del self._cache[old_pos]
+
+        e1 = self._cache.get(stream_pos)
+        if e1 is None:
+            e1 = self._cache[stream_pos] = set()
+        e1.add(entity)
+        self._entity_to_key[entity] = stream_pos
+
+        # if the cache is too big, remove entries
+        while len(self._cache) > self._max_size:
+            k, r = self._cache.popitem(0)
+            self._earliest_known_stream_pos = max(k, self._earliest_known_stream_pos)
+            for entity in r:
+                del self._entity_to_key[entity]
+
+    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.
         """