Sliding Sync: Make PerConnectionState immutable (#17600)

This is so that we can cache it.

We also move the sliding sync types to
`synapse/types/handlers/sliding_sync.py`. This is mainly in-prep for

The only change in behaviour is that
`RoomSyncConfig.combine_sync_config(..)` now returns a new room sync
config rather than mutating in-place.

Reviewable commit-by-commit.

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

changelog.d/17600.misc

@@ -0,0 +1 @@
+Make the sliding sync `PerConnectionState` class immutable.

scripts-dev/mypy_synapse_plugin.py

@@ -38,6 +38,7 @@ from mypy.types import (
     NoneType,
     TupleType,
     TypeAliasType,
+    TypeVarType,
     UninhabitedType,
     UnionType,
 )
@@ -233,6 +234,7 @@ IMMUTABLE_CUSTOM_TYPES = {
     "synapse.synapse_rust.push.FilteredPushRules",
     # This is technically not immutable, but close enough.
     "signedjson.types.VerifyKey",
+    "synapse.types.StrCollection",
 }
 
 # Immutable containers only if the values are also immutable.
@@ -298,7 +300,7 @@ def is_cacheable(
 
         elif rt.type.fullname in MUTABLE_CONTAINER_TYPES:
             # Mutable containers are mutable regardless of their underlying type.
-            return False, None
+            return False, f"container {rt.type.fullname} is mutable"
 
         elif "attrs" in rt.type.metadata:
             # attrs classes are only cachable iff it is frozen (immutable itself)
@@ -318,6 +320,9 @@ def is_cacheable(
             else:
                 return False, "non-frozen attrs class"
 
+        elif rt.type.is_enum:
+            # We assume Enum values are immutable
+            return True, None
         else:
             # Ensure we fail for unknown types, these generally means that the
             # above code is not complete.
@@ -326,6 +331,18 @@ def is_cacheable(
                 f"Don't know how to handle {rt.type.fullname} return type instance",
             )
 
+    elif isinstance(rt, TypeVarType):
+        # We consider TypeVars immutable if they are bound to a set of immutable
+        # types.
+        if rt.values:
+            for value in rt.values:
+                ok, note = is_cacheable(value, signature, verbose)
+                if not ok:
+                    return False, f"TypeVar bound not cacheable {value}"
+            return True, None
+
+        return False, "TypeVar is unbound"
+
     elif isinstance(rt, NoneType):
         # None is cachable.
         return True, None

synapse/handlers/sliding_sync/__init__.py

@@ -29,13 +29,6 @@ from synapse.handlers.sliding_sync.room_lists import (
     _RoomMembershipForUser,
 )
 from synapse.handlers.sliding_sync.store import SlidingSyncConnectionStore
-from synapse.handlers.sliding_sync.types import (
-    HaveSentRoomFlag,
-    MutablePerConnectionState,
-    PerConnectionState,
-    RoomSyncConfig,
-    StateValues,
-)
 from synapse.logging.opentracing import (
     SynapseTags,
     log_kv,
@@ -57,7 +50,15 @@ from synapse.types import (
     StreamKeyType,
     StreamToken,
 )
-from synapse.types.handlers import SlidingSyncConfig, SlidingSyncResult
+from synapse.types.handlers.sliding_sync import (
+    HaveSentRoomFlag,
+    MutablePerConnectionState,
+    PerConnectionState,
+    RoomSyncConfig,
+    SlidingSyncConfig,
+    SlidingSyncResult,
+    StateValues,
+)
 from synapse.types.state import StateFilter
 from synapse.util.async_helpers import concurrently_execute
 from synapse.visibility import filter_events_for_client

synapse/handlers/sliding_sync/extensions.py

@@ -20,11 +20,6 @@ from typing_extensions import assert_never
 
 from synapse.api.constants import AccountDataTypes, EduTypes
 from synapse.handlers.receipts import ReceiptEventSource
-from synapse.handlers.sliding_sync.types import (
-    HaveSentRoomFlag,
-    MutablePerConnectionState,
-    PerConnectionState,
-)
 from synapse.logging.opentracing import trace
 from synapse.storage.databases.main.receipts import ReceiptInRoom
 from synapse.types import (
@@ -35,7 +30,14 @@ from synapse.types import (
     StrCollection,
     StreamToken,
 )
-from synapse.types.handlers import OperationType, SlidingSyncConfig, SlidingSyncResult
+from synapse.types.handlers.sliding_sync import (
+    HaveSentRoomFlag,
+    MutablePerConnectionState,
+    OperationType,
+    PerConnectionState,
+    SlidingSyncConfig,
+    SlidingSyncResult,
+)
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer

synapse/handlers/sliding_sync/room_lists.py

@@ -40,11 +40,6 @@ from synapse.api.constants import (
 )
 from synapse.events import StrippedStateEvent
 from synapse.events.utils import parse_stripped_state_event
-from synapse.handlers.sliding_sync.types import (
-    HaveSentRoomFlag,
-    PerConnectionState,
-    RoomSyncConfig,
-)
 from synapse.logging.opentracing import start_active_span, trace
 from synapse.storage.databases.main.state import (
     ROOM_UNKNOWN_SENTINEL,
@@ -61,7 +56,14 @@ from synapse.types import (
     StreamToken,
     UserID,
 )
-from synapse.types.handlers import OperationType, SlidingSyncConfig, SlidingSyncResult
+from synapse.types.handlers.sliding_sync import (
+    HaveSentRoomFlag,
+    OperationType,
+    PerConnectionState,
+    RoomSyncConfig,
+    SlidingSyncConfig,
+    SlidingSyncResult,
+)
 from synapse.types.state import StateFilter
 
 if TYPE_CHECKING:
@@ -279,15 +281,11 @@ class SlidingSyncRoomLists:
                                     room_id
                                 )
                                 if existing_room_sync_config is not None:
-                                    existing_room_sync_config.combine_room_sync_config(
+                                    room_sync_config = existing_room_sync_config.combine_room_sync_config(
                                         room_sync_config
                                     )
-                                else:
+
-                                    # Make a copy so if we modify it later, it doesn't
+                                relevant_room_map[room_id] = room_sync_config
-                                    # affect all references.
-                                    relevant_room_map[room_id] = (
-                                        room_sync_config.deep_copy()
-                                    )
 
                                 room_ids_in_list.append(room_id)
 
@@ -351,11 +349,13 @@ class SlidingSyncRoomLists:
                     # and need to fetch more info about.
                     existing_room_sync_config = relevant_room_map.get(room_id)
                     if existing_room_sync_config is not None:
-                        existing_room_sync_config.combine_room_sync_config(
+                        room_sync_config = (
-                            room_sync_config
+                            existing_room_sync_config.combine_room_sync_config(
+                                room_sync_config
+                            )
                         )
-                    else:
+
-                        relevant_room_map[room_id] = room_sync_config
+                    relevant_room_map[room_id] = room_sync_config
 
         # Filtered subset of `relevant_room_map` for rooms that may have updates
         # (in the event stream)

synapse/handlers/sliding_sync/store.py

@@ -18,13 +18,13 @@ from typing import TYPE_CHECKING, Dict, Optional, Tuple
 import attr
 
 from synapse.api.errors import SlidingSyncUnknownPosition
-from synapse.handlers.sliding_sync.types import (
-    MutablePerConnectionState,
-    PerConnectionState,
-)
 from synapse.logging.opentracing import trace
 from synapse.types import SlidingSyncStreamToken
-from synapse.types.handlers import SlidingSyncConfig
+from synapse.types.handlers.sliding_sync import (
+    MutablePerConnectionState,
+    PerConnectionState,
+    SlidingSyncConfig,
+)
 
 if TYPE_CHECKING:
     pass

synapse/types/handlers/__init__.py

@@ -17,33 +17,9 @@
 # [This file includes modifications made by New Vector Limited]
 #
 #
-from enum import Enum
-from typing import TYPE_CHECKING, Dict, Final, List, Mapping, Optional, Sequence, Tuple
 
-import attr
-from typing_extensions import TypedDict
 
-from synapse._pydantic_compat import HAS_PYDANTIC_V2
+from typing import List, Optional, TypedDict
-
-if TYPE_CHECKING or HAS_PYDANTIC_V2:
-    from pydantic.v1 import Extra
-else:
-    from pydantic import Extra
-
-from synapse.events import EventBase
-from synapse.types import (
-    DeviceListUpdates,
-    JsonDict,
-    JsonMapping,
-    Requester,
-    SlidingSyncStreamToken,
-    StreamToken,
-    UserID,
-)
-from synapse.types.rest.client import SlidingSyncBody
-
-if TYPE_CHECKING:
-    from synapse.handlers.relations import BundledAggregations
 
 
 class ShutdownRoomParams(TypedDict):
@@ -101,335 +77,3 @@ class ShutdownRoomResponse(TypedDict):
     failed_to_kick_users: List[str]
     local_aliases: List[str]
     new_room_id: Optional[str]
-
-
-class SlidingSyncConfig(SlidingSyncBody):
-    """
-    Inherit from `SlidingSyncBody` since we need all of the same fields and add a few
-    extra fields that we need in the handler
-    """
-
-    user: UserID
-    requester: Requester
-
-    # Pydantic config
-    class Config:
-        # By default, ignore fields that we don't recognise.
-        extra = Extra.ignore
-        # By default, don't allow fields to be reassigned after parsing.
-        allow_mutation = False
-        # Allow custom types like `UserID` to be used in the model
-        arbitrary_types_allowed = True
-
-
-class OperationType(Enum):
-    """
-    Represents the operation types in a Sliding Sync window.
-
-    Attributes:
-        SYNC: Sets a range of entries. Clients SHOULD discard what they previous knew about
-            entries in this range.
-        INSERT: Sets a single entry. If the position is not empty then clients MUST move
-            entries to the left or the right depending on where the closest empty space is.
-        DELETE: Remove a single entry. Often comes before an INSERT to allow entries to move
-            places.
-        INVALIDATE: Remove a range of entries. Clients MAY persist the invalidated range for
-            offline support, but they should be treated as empty when additional operations
-            which concern indexes in the range arrive from the server.
-    """
-
-    SYNC: Final = "SYNC"
-    INSERT: Final = "INSERT"
-    DELETE: Final = "DELETE"
-    INVALIDATE: Final = "INVALIDATE"
-
-
-@attr.s(slots=True, frozen=True, auto_attribs=True)
-class SlidingSyncResult:
-    """
-    The Sliding Sync result to be serialized to JSON for a response.
-
-    Attributes:
-        next_pos: The next position token in the sliding window to request (next_batch).
-        lists: Sliding window API. A map of list key to list results.
-        rooms: Room subscription API. A map of room ID to room results.
-        extensions: Extensions API. A map of extension key to extension results.
-    """
-
-    @attr.s(slots=True, frozen=True, auto_attribs=True)
-    class RoomResult:
-        """
-        Attributes:
-            name: Room name or calculated room name.
-            avatar: Room avatar
-            heroes: List of stripped membership events (containing `user_id` and optionally
-                `avatar_url` and `displayname`) for the users used to calculate the room name.
-            is_dm: Flag to specify whether the room is a direct-message room (most likely
-                between two people).
-            initial: Flag which is set when this is the first time the server is sending this
-                data on this connection. Clients can use this flag to replace or update
-                their local state. When there is an update, servers MUST omit this flag
-                entirely and NOT send "initial":false as this is wasteful on bandwidth. The
-                absence of this flag means 'false'.
-            unstable_expanded_timeline: Flag which is set if we're returning more historic
-                events due to the timeline limit having increased. See "XXX: Odd behavior"
-                comment ing `synapse.handlers.sliding_sync`.
-            required_state: The current state of the room
-            timeline: Latest events in the room. The last event is the most recent.
-            bundled_aggregations: A mapping of event ID to the bundled aggregations for
-                the timeline events above. This allows clients to show accurate reaction
-                counts (or edits, threads), even if some of the reaction events were skipped
-                over in a gappy sync.
-            stripped_state: Stripped state events (for rooms where the usre is
-                invited/knocked). Same as `rooms.invite.$room_id.invite_state` in sync v2,
-                absent on joined/left rooms
-            prev_batch: A token that can be passed as a start parameter to the
-                `/rooms/<room_id>/messages` API to retrieve earlier messages.
-            limited: True if there are more events than `timeline_limit` looking
-                backwards from the `response.pos` to the `request.pos`.
-            num_live: The number of timeline events which have just occurred and are not historical.
-                The last N events are 'live' and should be treated as such. This is mostly
-                useful to determine whether a given @mention event should make a noise or not.
-                Clients cannot rely solely on the absence of `initial: true` to determine live
-                events because if a room not in the sliding window bumps into the window because
-                of an @mention it will have `initial: true` yet contain a single live event
-                (with potentially other old events in the timeline).
-            bump_stamp: The `stream_ordering` of the last event according to the
-                `bump_event_types`. This helps clients sort more readily without them
-                needing to pull in a bunch of the timeline to determine the last activity.
-                `bump_event_types` is a thing because for example, we don't want display
-                name changes to mark the room as unread and bump it to the top. For
-                encrypted rooms, we just have to consider any activity as a bump because we
-                can't see the content and the client has to figure it out for themselves.
-            joined_count: The number of users with membership of join, including the client's
-                own user ID. (same as sync `v2 m.joined_member_count`)
-            invited_count: The number of users with membership of invite. (same as sync v2
-                `m.invited_member_count`)
-            notification_count: The total number of unread notifications for this room. (same
-                as sync v2)
-            highlight_count: The number of unread notifications for this room with the highlight
-                flag set. (same as sync v2)
-        """
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class StrippedHero:
-            user_id: str
-            display_name: Optional[str]
-            avatar_url: Optional[str]
-
-        name: Optional[str]
-        avatar: Optional[str]
-        heroes: Optional[List[StrippedHero]]
-        is_dm: bool
-        initial: bool
-        unstable_expanded_timeline: bool
-        # Should be empty for invite/knock rooms with `stripped_state`
-        required_state: List[EventBase]
-        # Should be empty for invite/knock rooms with `stripped_state`
-        timeline_events: List[EventBase]
-        bundled_aggregations: Optional[Dict[str, "BundledAggregations"]]
-        # Optional because it's only relevant to invite/knock rooms
-        stripped_state: List[JsonDict]
-        # Only optional because it won't be included for invite/knock rooms with `stripped_state`
-        prev_batch: Optional[StreamToken]
-        # Only optional because it won't be included for invite/knock rooms with `stripped_state`
-        limited: Optional[bool]
-        # Only optional because it won't be included for invite/knock rooms with `stripped_state`
-        num_live: Optional[int]
-        bump_stamp: int
-        joined_count: int
-        invited_count: int
-        notification_count: int
-        highlight_count: int
-
-        def __bool__(self) -> bool:
-            return (
-                # If this is the first time the client is seeing the room, we should not filter it out
-                # under any circumstance.
-                self.initial
-                # We need to let the client know if there are any new events
-                or bool(self.required_state)
-                or bool(self.timeline_events)
-                or bool(self.stripped_state)
-            )
-
-    @attr.s(slots=True, frozen=True, auto_attribs=True)
-    class SlidingWindowList:
-        """
-        Attributes:
-            count: The total number of entries in the list. Always present if this list
-                is.
-            ops: The sliding list operations to perform.
-        """
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class Operation:
-            """
-            Attributes:
-                op: The operation type to perform.
-                range: Which index positions are affected by this operation. These are
-                    both inclusive.
-                room_ids: Which room IDs are affected by this operation. These IDs match
-                    up to the positions in the `range`, so the last room ID in this list
-                    matches the 9th index. The room data is held in a separate object.
-            """
-
-            op: OperationType
-            range: Tuple[int, int]
-            room_ids: List[str]
-
-        count: int
-        ops: List[Operation]
-
-    @attr.s(slots=True, frozen=True, auto_attribs=True)
-    class Extensions:
-        """Responses for extensions
-
-        Attributes:
-            to_device: The to-device extension (MSC3885)
-            e2ee: The E2EE device extension (MSC3884)
-        """
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class ToDeviceExtension:
-            """The to-device extension (MSC3885)
-
-            Attributes:
-                next_batch: The to-device stream token the client should use
-                    to get more results
-                events: A list of to-device messages for the client
-            """
-
-            next_batch: str
-            events: Sequence[JsonMapping]
-
-            def __bool__(self) -> bool:
-                return bool(self.events)
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class E2eeExtension:
-            """The E2EE device extension (MSC3884)
-
-            Attributes:
-                device_list_updates: List of user_ids whose devices have changed or left (only
-                    present on incremental syncs).
-                device_one_time_keys_count: Map from key algorithm to the number of
-                    unclaimed one-time keys currently held on the server for this device. If
-                    an algorithm is unlisted, the count for that algorithm is assumed to be
-                    zero. If this entire parameter is missing, the count for all algorithms
-                    is assumed to be zero.
-                device_unused_fallback_key_types: List of unused fallback key algorithms
-                    for this device.
-            """
-
-            # Only present on incremental syncs
-            device_list_updates: Optional[DeviceListUpdates]
-            device_one_time_keys_count: Mapping[str, int]
-            device_unused_fallback_key_types: Sequence[str]
-
-            def __bool__(self) -> bool:
-                # Note that "signed_curve25519" is always returned in key count responses
-                # regardless of whether we uploaded any keys for it. This is necessary until
-                # https://github.com/matrix-org/matrix-doc/issues/3298 is fixed.
-                #
-                # Also related:
-                # https://github.com/element-hq/element-android/issues/3725 and
-                # https://github.com/matrix-org/synapse/issues/10456
-                default_otk = self.device_one_time_keys_count.get("signed_curve25519")
-                more_than_default_otk = len(self.device_one_time_keys_count) > 1 or (
-                    default_otk is not None and default_otk > 0
-                )
-
-                return bool(
-                    more_than_default_otk
-                    or self.device_list_updates
-                    or self.device_unused_fallback_key_types
-                )
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class AccountDataExtension:
-            """The Account Data extension (MSC3959)
-
-            Attributes:
-                global_account_data_map: Mapping from `type` to `content` of global account
-                    data events.
-                account_data_by_room_map: Mapping from room_id to mapping of `type` to
-                    `content` of room account data events.
-            """
-
-            global_account_data_map: Mapping[str, JsonMapping]
-            account_data_by_room_map: Mapping[str, Mapping[str, JsonMapping]]
-
-            def __bool__(self) -> bool:
-                return bool(
-                    self.global_account_data_map or self.account_data_by_room_map
-                )
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class ReceiptsExtension:
-            """The Receipts extension (MSC3960)
-
-            Attributes:
-                room_id_to_receipt_map: Mapping from room_id to `m.receipt` ephemeral
-                    event (type, content)
-            """
-
-            room_id_to_receipt_map: Mapping[str, JsonMapping]
-
-            def __bool__(self) -> bool:
-                return bool(self.room_id_to_receipt_map)
-
-        @attr.s(slots=True, frozen=True, auto_attribs=True)
-        class TypingExtension:
-            """The Typing Notification extension (MSC3961)
-
-            Attributes:
-                room_id_to_typing_map: Mapping from room_id to `m.typing` ephemeral
-                    event (type, content)
-            """
-
-            room_id_to_typing_map: Mapping[str, JsonMapping]
-
-            def __bool__(self) -> bool:
-                return bool(self.room_id_to_typing_map)
-
-        to_device: Optional[ToDeviceExtension] = None
-        e2ee: Optional[E2eeExtension] = None
-        account_data: Optional[AccountDataExtension] = None
-        receipts: Optional[ReceiptsExtension] = None
-        typing: Optional[TypingExtension] = None
-
-        def __bool__(self) -> bool:
-            return bool(
-                self.to_device
-                or self.e2ee
-                or self.account_data
-                or self.receipts
-                or self.typing
-            )
-
-    next_pos: SlidingSyncStreamToken
-    lists: Mapping[str, SlidingWindowList]
-    rooms: Dict[str, RoomResult]
-    extensions: Extensions
-
-    def __bool__(self) -> bool:
-        """Make the result appear empty if there are no updates. This is used
-        to tell if the notifier needs to wait for more events when polling for
-        events.
-        """
-        # We don't include `self.lists` here, as a) `lists` is always non-empty even if
-        # there are no changes, and b) since we're sorting rooms by `stream_ordering` of
-        # the latest activity, anything that would cause the order to change would end
-        # up in `self.rooms` and cause us to send down the change.
-        return bool(self.rooms or self.extensions)
-
-    @staticmethod
-    def empty(next_pos: SlidingSyncStreamToken) -> "SlidingSyncResult":
-        "Return a new empty result"
-        return SlidingSyncResult(
-            next_pos=next_pos,
-            lists={},
-            rooms={},
-            extensions=SlidingSyncResult.Extensions(),
-        )

synapse/types/handlers/sliding_sync.py

@@ -18,30 +18,382 @@ from collections import ChainMap
 from enum import Enum
 from typing import (
     TYPE_CHECKING,
+    AbstractSet,
     Callable,
     Dict,
     Final,
     Generic,
+    List,
     Mapping,
     MutableMapping,
     Optional,
+    Sequence,
     Set,
+    Tuple,
     TypeVar,
     cast,
 )
 
 import attr
 
+from synapse._pydantic_compat import HAS_PYDANTIC_V2
 from synapse.api.constants import EventTypes
 from synapse.types import MultiWriterStreamToken, RoomStreamToken, StrCollection, UserID
-from synapse.types.handlers import SlidingSyncConfig
+
+if TYPE_CHECKING or HAS_PYDANTIC_V2:
+    from pydantic.v1 import Extra
+else:
+    from pydantic import Extra
+
+from synapse.events import EventBase
+from synapse.types import (
+    DeviceListUpdates,
+    JsonDict,
+    JsonMapping,
+    Requester,
+    SlidingSyncStreamToken,
+    StreamToken,
+)
+from synapse.types.rest.client import SlidingSyncBody
 
 if TYPE_CHECKING:
-    pass
+    from synapse.handlers.relations import BundledAggregations
 
 logger = logging.getLogger(__name__)
 
 
+class SlidingSyncConfig(SlidingSyncBody):
+    """
+    Inherit from `SlidingSyncBody` since we need all of the same fields and add a few
+    extra fields that we need in the handler
+    """
+
+    user: UserID
+    requester: Requester
+
+    # Pydantic config
+    class Config:
+        # By default, ignore fields that we don't recognise.
+        extra = Extra.ignore
+        # By default, don't allow fields to be reassigned after parsing.
+        allow_mutation = False
+        # Allow custom types like `UserID` to be used in the model
+        arbitrary_types_allowed = True
+
+
+class OperationType(Enum):
+    """
+    Represents the operation types in a Sliding Sync window.
+
+    Attributes:
+        SYNC: Sets a range of entries. Clients SHOULD discard what they previous knew about
+            entries in this range.
+        INSERT: Sets a single entry. If the position is not empty then clients MUST move
+            entries to the left or the right depending on where the closest empty space is.
+        DELETE: Remove a single entry. Often comes before an INSERT to allow entries to move
+            places.
+        INVALIDATE: Remove a range of entries. Clients MAY persist the invalidated range for
+            offline support, but they should be treated as empty when additional operations
+            which concern indexes in the range arrive from the server.
+    """
+
+    SYNC: Final = "SYNC"
+    INSERT: Final = "INSERT"
+    DELETE: Final = "DELETE"
+    INVALIDATE: Final = "INVALIDATE"
+
+
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class SlidingSyncResult:
+    """
+    The Sliding Sync result to be serialized to JSON for a response.
+
+    Attributes:
+        next_pos: The next position token in the sliding window to request (next_batch).
+        lists: Sliding window API. A map of list key to list results.
+        rooms: Room subscription API. A map of room ID to room results.
+        extensions: Extensions API. A map of extension key to extension results.
+    """
+
+    @attr.s(slots=True, frozen=True, auto_attribs=True)
+    class RoomResult:
+        """
+        Attributes:
+            name: Room name or calculated room name.
+            avatar: Room avatar
+            heroes: List of stripped membership events (containing `user_id` and optionally
+                `avatar_url` and `displayname`) for the users used to calculate the room name.
+            is_dm: Flag to specify whether the room is a direct-message room (most likely
+                between two people).
+            initial: Flag which is set when this is the first time the server is sending this
+                data on this connection. Clients can use this flag to replace or update
+                their local state. When there is an update, servers MUST omit this flag
+                entirely and NOT send "initial":false as this is wasteful on bandwidth. The
+                absence of this flag means 'false'.
+            unstable_expanded_timeline: Flag which is set if we're returning more historic
+                events due to the timeline limit having increased. See "XXX: Odd behavior"
+                comment ing `synapse.handlers.sliding_sync`.
+            required_state: The current state of the room
+            timeline: Latest events in the room. The last event is the most recent.
+            bundled_aggregations: A mapping of event ID to the bundled aggregations for
+                the timeline events above. This allows clients to show accurate reaction
+                counts (or edits, threads), even if some of the reaction events were skipped
+                over in a gappy sync.
+            stripped_state: Stripped state events (for rooms where the usre is
+                invited/knocked). Same as `rooms.invite.$room_id.invite_state` in sync v2,
+                absent on joined/left rooms
+            prev_batch: A token that can be passed as a start parameter to the
+                `/rooms/<room_id>/messages` API to retrieve earlier messages.
+            limited: True if there are more events than `timeline_limit` looking
+                backwards from the `response.pos` to the `request.pos`.
+            num_live: The number of timeline events which have just occurred and are not historical.
+                The last N events are 'live' and should be treated as such. This is mostly
+                useful to determine whether a given @mention event should make a noise or not.
+                Clients cannot rely solely on the absence of `initial: true` to determine live
+                events because if a room not in the sliding window bumps into the window because
+                of an @mention it will have `initial: true` yet contain a single live event
+                (with potentially other old events in the timeline).
+            bump_stamp: The `stream_ordering` of the last event according to the
+                `bump_event_types`. This helps clients sort more readily without them
+                needing to pull in a bunch of the timeline to determine the last activity.
+                `bump_event_types` is a thing because for example, we don't want display
+                name changes to mark the room as unread and bump it to the top. For
+                encrypted rooms, we just have to consider any activity as a bump because we
+                can't see the content and the client has to figure it out for themselves.
+            joined_count: The number of users with membership of join, including the client's
+                own user ID. (same as sync `v2 m.joined_member_count`)
+            invited_count: The number of users with membership of invite. (same as sync v2
+                `m.invited_member_count`)
+            notification_count: The total number of unread notifications for this room. (same
+                as sync v2)
+            highlight_count: The number of unread notifications for this room with the highlight
+                flag set. (same as sync v2)
+        """
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class StrippedHero:
+            user_id: str
+            display_name: Optional[str]
+            avatar_url: Optional[str]
+
+        name: Optional[str]
+        avatar: Optional[str]
+        heroes: Optional[List[StrippedHero]]
+        is_dm: bool
+        initial: bool
+        unstable_expanded_timeline: bool
+        # Should be empty for invite/knock rooms with `stripped_state`
+        required_state: List[EventBase]
+        # Should be empty for invite/knock rooms with `stripped_state`
+        timeline_events: List[EventBase]
+        bundled_aggregations: Optional[Dict[str, "BundledAggregations"]]
+        # Optional because it's only relevant to invite/knock rooms
+        stripped_state: List[JsonDict]
+        # Only optional because it won't be included for invite/knock rooms with `stripped_state`
+        prev_batch: Optional[StreamToken]
+        # Only optional because it won't be included for invite/knock rooms with `stripped_state`
+        limited: Optional[bool]
+        # Only optional because it won't be included for invite/knock rooms with `stripped_state`
+        num_live: Optional[int]
+        bump_stamp: int
+        joined_count: int
+        invited_count: int
+        notification_count: int
+        highlight_count: int
+
+        def __bool__(self) -> bool:
+            return (
+                # If this is the first time the client is seeing the room, we should not filter it out
+                # under any circumstance.
+                self.initial
+                # We need to let the client know if there are any new events
+                or bool(self.required_state)
+                or bool(self.timeline_events)
+                or bool(self.stripped_state)
+            )
+
+    @attr.s(slots=True, frozen=True, auto_attribs=True)
+    class SlidingWindowList:
+        """
+        Attributes:
+            count: The total number of entries in the list. Always present if this list
+                is.
+            ops: The sliding list operations to perform.
+        """
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class Operation:
+            """
+            Attributes:
+                op: The operation type to perform.
+                range: Which index positions are affected by this operation. These are
+                    both inclusive.
+                room_ids: Which room IDs are affected by this operation. These IDs match
+                    up to the positions in the `range`, so the last room ID in this list
+                    matches the 9th index. The room data is held in a separate object.
+            """
+
+            op: OperationType
+            range: Tuple[int, int]
+            room_ids: List[str]
+
+        count: int
+        ops: List[Operation]
+
+    @attr.s(slots=True, frozen=True, auto_attribs=True)
+    class Extensions:
+        """Responses for extensions
+
+        Attributes:
+            to_device: The to-device extension (MSC3885)
+            e2ee: The E2EE device extension (MSC3884)
+        """
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class ToDeviceExtension:
+            """The to-device extension (MSC3885)
+
+            Attributes:
+                next_batch: The to-device stream token the client should use
+                    to get more results
+                events: A list of to-device messages for the client
+            """
+
+            next_batch: str
+            events: Sequence[JsonMapping]
+
+            def __bool__(self) -> bool:
+                return bool(self.events)
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class E2eeExtension:
+            """The E2EE device extension (MSC3884)
+
+            Attributes:
+                device_list_updates: List of user_ids whose devices have changed or left (only
+                    present on incremental syncs).
+                device_one_time_keys_count: Map from key algorithm to the number of
+                    unclaimed one-time keys currently held on the server for this device. If
+                    an algorithm is unlisted, the count for that algorithm is assumed to be
+                    zero. If this entire parameter is missing, the count for all algorithms
+                    is assumed to be zero.
+                device_unused_fallback_key_types: List of unused fallback key algorithms
+                    for this device.
+            """
+
+            # Only present on incremental syncs
+            device_list_updates: Optional[DeviceListUpdates]
+            device_one_time_keys_count: Mapping[str, int]
+            device_unused_fallback_key_types: Sequence[str]
+
+            def __bool__(self) -> bool:
+                # Note that "signed_curve25519" is always returned in key count responses
+                # regardless of whether we uploaded any keys for it. This is necessary until
+                # https://github.com/matrix-org/matrix-doc/issues/3298 is fixed.
+                #
+                # Also related:
+                # https://github.com/element-hq/element-android/issues/3725 and
+                # https://github.com/matrix-org/synapse/issues/10456
+                default_otk = self.device_one_time_keys_count.get("signed_curve25519")
+                more_than_default_otk = len(self.device_one_time_keys_count) > 1 or (
+                    default_otk is not None and default_otk > 0
+                )
+
+                return bool(
+                    more_than_default_otk
+                    or self.device_list_updates
+                    or self.device_unused_fallback_key_types
+                )
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class AccountDataExtension:
+            """The Account Data extension (MSC3959)
+
+            Attributes:
+                global_account_data_map: Mapping from `type` to `content` of global account
+                    data events.
+                account_data_by_room_map: Mapping from room_id to mapping of `type` to
+                    `content` of room account data events.
+            """
+
+            global_account_data_map: Mapping[str, JsonMapping]
+            account_data_by_room_map: Mapping[str, Mapping[str, JsonMapping]]
+
+            def __bool__(self) -> bool:
+                return bool(
+                    self.global_account_data_map or self.account_data_by_room_map
+                )
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class ReceiptsExtension:
+            """The Receipts extension (MSC3960)
+
+            Attributes:
+                room_id_to_receipt_map: Mapping from room_id to `m.receipt` ephemeral
+                    event (type, content)
+            """
+
+            room_id_to_receipt_map: Mapping[str, JsonMapping]
+
+            def __bool__(self) -> bool:
+                return bool(self.room_id_to_receipt_map)
+
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class TypingExtension:
+            """The Typing Notification extension (MSC3961)
+
+            Attributes:
+                room_id_to_typing_map: Mapping from room_id to `m.typing` ephemeral
+                    event (type, content)
+            """
+
+            room_id_to_typing_map: Mapping[str, JsonMapping]
+
+            def __bool__(self) -> bool:
+                return bool(self.room_id_to_typing_map)
+
+        to_device: Optional[ToDeviceExtension] = None
+        e2ee: Optional[E2eeExtension] = None
+        account_data: Optional[AccountDataExtension] = None
+        receipts: Optional[ReceiptsExtension] = None
+        typing: Optional[TypingExtension] = None
+
+        def __bool__(self) -> bool:
+            return bool(
+                self.to_device
+                or self.e2ee
+                or self.account_data
+                or self.receipts
+                or self.typing
+            )
+
+    next_pos: SlidingSyncStreamToken
+    lists: Mapping[str, SlidingWindowList]
+    rooms: Dict[str, RoomResult]
+    extensions: Extensions
+
+    def __bool__(self) -> bool:
+        """Make the result appear empty if there are no updates. This is used
+        to tell if the notifier needs to wait for more events when polling for
+        events.
+        """
+        # We don't include `self.lists` here, as a) `lists` is always non-empty even if
+        # there are no changes, and b) since we're sorting rooms by `stream_ordering` of
+        # the latest activity, anything that would cause the order to change would end
+        # up in `self.rooms` and cause us to send down the change.
+        return bool(self.rooms or self.extensions)
+
+    @staticmethod
+    def empty(next_pos: SlidingSyncStreamToken) -> "SlidingSyncResult":
+        "Return a new empty result"
+        return SlidingSyncResult(
+            next_pos=next_pos,
+            lists={},
+            rooms={},
+            extensions=SlidingSyncResult.Extensions(),
+        )
+
+
 class StateValues:
     """
     Understood values of the (type, state_key) tuple in `required_state`.
@@ -60,7 +412,7 @@ class StateValues:
 
 # We can't freeze this class because we want to update it in place with the
 # de-duplicated data.
-@attr.s(slots=True, auto_attribs=True)
+@attr.s(slots=True, auto_attribs=True, frozen=True)
 class RoomSyncConfig:
     """
     Holds the config for what data we should fetch for a room in the sync response.
@@ -74,7 +426,7 @@ class RoomSyncConfig:
     """
 
     timeline_limit: int
-    required_state_map: Dict[str, Set[str]]
+    required_state_map: Mapping[str, AbstractSet[str]]
 
     @classmethod
     def from_room_config(
@@ -146,27 +498,22 @@ class RoomSyncConfig:
             required_state_map=required_state_map,
         )
 
-    def deep_copy(self) -> "RoomSyncConfig":
-        required_state_map: Dict[str, Set[str]] = {
-            state_type: state_key_set.copy()
-            for state_type, state_key_set in self.required_state_map.items()
-        }
-
-        return RoomSyncConfig(
-            timeline_limit=self.timeline_limit,
-            required_state_map=required_state_map,
-        )
-
     def combine_room_sync_config(
         self, other_room_sync_config: "RoomSyncConfig"
-    ) -> None:
+    ) -> "RoomSyncConfig":
         """
-        Combine this `RoomSyncConfig` with another `RoomSyncConfig` and take the
+        Combine this `RoomSyncConfig` with another `RoomSyncConfig` and return the
         superset union of the two.
         """
+        timeline_limit = self.timeline_limit
+        required_state_map = {
+            event_type: set(state_keys)
+            for event_type, state_keys in self.required_state_map.items()
+        }
+
         # Take the highest timeline limit
-        if self.timeline_limit < other_room_sync_config.timeline_limit:
+        if timeline_limit < other_room_sync_config.timeline_limit:
-            self.timeline_limit = other_room_sync_config.timeline_limit
+            timeline_limit = other_room_sync_config.timeline_limit
 
         # Union the required state
         for (
@@ -175,14 +522,14 @@ class RoomSyncConfig:
         ) in other_room_sync_config.required_state_map.items():
             # If we already have a wildcard for everything, we don't need to add
             # anything else
-            if StateValues.WILDCARD in self.required_state_map.get(
+            if StateValues.WILDCARD in required_state_map.get(
                 StateValues.WILDCARD, set()
             ):
                 break
 
             # If we already have a wildcard `state_key` for this `state_type`, we don't need
             # to add anything else
-            if StateValues.WILDCARD in self.required_state_map.get(state_type, set()):
+            if StateValues.WILDCARD in required_state_map.get(state_type, set()):
                 continue
 
             # If we're getting wildcards for the `state_type` and `state_key`, that's
@@ -191,16 +538,14 @@ class RoomSyncConfig:
                 state_type == StateValues.WILDCARD
                 and StateValues.WILDCARD in state_key_set
             ):
-                self.required_state_map = {state_type: {StateValues.WILDCARD}}
+                required_state_map = {state_type: {StateValues.WILDCARD}}
                 # We can break, since we don't need to add anything else
                 break
 
             for state_key in state_key_set:
                 # If we already have a wildcard for this specific `state_key`, we don't need
                 # to add it since the wildcard already covers it.
-                if state_key in self.required_state_map.get(
+                if state_key in required_state_map.get(StateValues.WILDCARD, set()):
-                    StateValues.WILDCARD, set()
-                ):
                     continue
 
                 # If we're getting a wildcard for the `state_type`, get rid of any other
@@ -211,7 +556,7 @@ class RoomSyncConfig:
                     # Make a copy so we don't run into an error: `dictionary changed size
                     # during iteration`, when we remove items
                     for existing_state_type, existing_state_key_set in list(
-                        self.required_state_map.items()
+                        required_state_map.items()
                     ):
                         # Make a copy so we don't run into an error: `Set changed size during
                         # iteration`, when we filter out and remove items
@@ -221,19 +566,21 @@ class RoomSyncConfig:
 
                         # If we've the left the `set()` empty, remove it from the map
                         if existing_state_key_set == set():
-                            self.required_state_map.pop(existing_state_type, None)
+                            required_state_map.pop(existing_state_type, None)
 
                 # If we're getting a wildcard `state_key`, get rid of any other state_keys
                 # for this `state_type` since the wildcard will cover it already.
                 if state_key == StateValues.WILDCARD:
-                    self.required_state_map[state_type] = {state_key}
+                    required_state_map[state_type] = {state_key}
                     break
                 # Otherwise, just add it to the set
                 else:
-                    if self.required_state_map.get(state_type) is None:
+                    if required_state_map.get(state_type) is None:
-                        self.required_state_map[state_type] = {state_key}
+                        required_state_map[state_type] = {state_key}
                     else:
-                        self.required_state_map[state_type].add(state_key)
+                        required_state_map[state_type].add(state_key)
+
+        return RoomSyncConfig(timeline_limit, required_state_map)
 
     def must_await_full_state(
         self,
@@ -324,7 +671,7 @@ class HaveSentRoomFlag(Enum):
     LIVE = "live"
 
 
-T = TypeVar("T")
+T = TypeVar("T", str, RoomStreamToken, MultiWriterStreamToken)
 
 
 @attr.s(auto_attribs=True, slots=True, frozen=True)
@@ -439,7 +786,7 @@ class MutableRoomStatusMap(RoomStatusMap[T]):
             self._statuses[room_id] = HaveSentRoom.previously(from_token)
 
 
-@attr.s(auto_attribs=True)
+@attr.s(auto_attribs=True, frozen=True)
 class PerConnectionState:
     """The per-connection state. A snapshot of what we've sent down the
     connection before.

tests/handlers/test_sliding_sync.py

@@ -18,7 +18,6 @@
 #
 #
 import logging
-from copy import deepcopy
 from typing import Dict, List, Optional
 from unittest.mock import patch
 
@@ -47,7 +46,7 @@ from synapse.rest.client import knock, login, room
 from synapse.server import HomeServer
 from synapse.storage.util.id_generators import MultiWriterIdGenerator
 from synapse.types import JsonDict, StreamToken, UserID
-from synapse.types.handlers import SlidingSyncConfig
+from synapse.types.handlers.sliding_sync import SlidingSyncConfig
 from synapse.util import Clock
 
 from tests.replication._base import BaseMultiWorkerStreamTestCase
@@ -566,23 +565,11 @@ class RoomSyncConfigTestCase(TestCase):
         """
         Combine A into B and B into A to make sure we get the same result.
         """
-        # Since we're mutating these in place, make a copy for each of our trials
+        combined_config = a.combine_room_sync_config(b)
-        room_sync_config_a = deepcopy(a)
+        self._assert_room_config_equal(combined_config, expected, "B into A")
-        room_sync_config_b = deepcopy(b)
 
-        # Combine B into A
+        combined_config = a.combine_room_sync_config(b)
-        room_sync_config_a.combine_room_sync_config(room_sync_config_b)
+        self._assert_room_config_equal(combined_config, expected, "A into B")
-
-        self._assert_room_config_equal(room_sync_config_a, expected, "B into A")
-
-        # Since we're mutating these in place, make a copy for each of our trials
-        room_sync_config_a = deepcopy(a)
-        room_sync_config_b = deepcopy(b)
-
-        # Combine A into B
-        room_sync_config_b.combine_room_sync_config(room_sync_config_a)
-
-        self._assert_room_config_equal(room_sync_config_b, expected, "A into B")
 
 
 class GetRoomMembershipForUserAtToTokenTestCase(HomeserverTestCase):