Track currently syncing users by device for presence (#16172)

Refactoring to use both the user ID & the device ID when tracking
the currently syncing users in the presence handler.

This is done both locally and over replication. Note that the device
ID is discarded but will be used in a future change.

4 files changed, 129 insertions, 63 deletions

diff --git a/changelog.d/16172.misc b/changelog.d/16172.misc
new file mode 100644
index 0000000000..4d709cb56e
--- /dev/null
+++ b/changelog.d/16172.misc
@@ -0,0 +1 @@
+Track per-device information in the presence code.
diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 50c68c86ce..2f841863ae 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -23,6 +23,7 @@ The methods that define policy are:
 """
 import abc
 import contextlib
+import itertools
 import logging
 from bisect import bisect
 from contextlib import contextmanager
@@ -188,15 +189,17 @@ class BasePresenceHandler(abc.ABC):
         """
 
     @abc.abstractmethod
-    def get_currently_syncing_users_for_replication(self) -> Iterable[str]:
-        """Get an iterable of syncing users on this worker, to send to the presence handler
+    def get_currently_syncing_users_for_replication(
+        self,
+    ) -> Iterable[Tuple[str, Optional[str]]]:
+        """Get an iterable of syncing users and devices on this worker, to send to the presence handler
 
         This is called when a replication connection is established. It should return
-        a list of user ids, which are then sent as USER_SYNC commands to inform the
-        process handling presence about those users.
+        a list of tuples of user ID & device ID, which are then sent as USER_SYNC commands
+        to inform the process handling presence about those users/devices.
 
         Returns:
-            An iterable of user_id strings.
+            An iterable of tuples of user ID and device ID.
         """
 
     async def get_state(self, target_user: UserID) -> UserPresenceState:
@@ -284,7 +287,12 @@ class BasePresenceHandler(abc.ABC):
         """
 
     async def update_external_syncs_row(  # noqa: B027 (no-op by design)
-        self, process_id: str, user_id: str, is_syncing: bool, sync_time_msec: int
+        self,
+        process_id: str,
+        user_id: str,
+        device_id: Optional[str],
+        is_syncing: bool,
+        sync_time_msec: int,
     ) -> None:
         """Update the syncing users for an external process as a delta.
 
@@ -295,6 +303,7 @@ class BasePresenceHandler(abc.ABC):
                 syncing against. This allows synapse to process updates
                 as user start and stop syncing against a given process.
             user_id: The user who has started or stopped syncing
+            device_id: The user's device that has started or stopped syncing
             is_syncing: Whether or not the user is now syncing
             sync_time_msec: Time in ms when the user was last syncing
         """
@@ -425,16 +434,18 @@ class WorkerPresenceHandler(BasePresenceHandler):
             hs.config.worker.writers.presence,
         )
 
-        # The number of ongoing syncs on this process, by user id.
+        # The number of ongoing syncs on this process, by (user ID, device ID).
         # Empty if _presence_enabled is false.
-        self._user_to_num_current_syncs: Dict[str, int] = {}
+        self._user_device_to_num_current_syncs: Dict[
+            Tuple[str, Optional[str]], int
+        ] = {}
 
         self.notifier = hs.get_notifier()
         self.instance_id = hs.get_instance_id()
 
-        # user_id -> last_sync_ms. Lists the users that have stopped syncing but
-        # we haven't notified the presence writer of that yet
-        self.users_going_offline: Dict[str, int] = {}
+        # (user_id, device_id) -> last_sync_ms. Lists the devices that have stopped
+        # syncing but we haven't notified the presence writer of that yet
+        self._user_devices_going_offline: Dict[Tuple[str, Optional[str]], int] = {}
 
         self._bump_active_client = ReplicationBumpPresenceActiveTime.make_client(hs)
         self._set_state_client = ReplicationPresenceSetState.make_client(hs)
@@ -457,39 +468,47 @@ class WorkerPresenceHandler(BasePresenceHandler):
                 ClearUserSyncsCommand(self.instance_id)
             )
 
-    def send_user_sync(self, user_id: str, is_syncing: bool, last_sync_ms: int) -> None:
+    def send_user_sync(
+        self,
+        user_id: str,
+        device_id: Optional[str],
+        is_syncing: bool,
+        last_sync_ms: int,
+    ) -> None:
         if self._presence_enabled:
             self.hs.get_replication_command_handler().send_user_sync(
-                self.instance_id, user_id, is_syncing, last_sync_ms
+                self.instance_id, user_id, device_id, is_syncing, last_sync_ms
             )
 
-    def mark_as_coming_online(self, user_id: str) -> None:
+    def mark_as_coming_online(self, user_id: str, device_id: Optional[str]) -> None:
         """A user has started syncing. Send a UserSync to the presence writer,
         unless they had recently stopped syncing.
         """
-        going_offline = self.users_going_offline.pop(user_id, None)
+        going_offline = self._user_devices_going_offline.pop((user_id, device_id), None)
         if not going_offline:
             # Safe to skip because we haven't yet told the presence writer they
             # were offline
-            self.send_user_sync(user_id, True, self.clock.time_msec())
+            self.send_user_sync(user_id, device_id, True, self.clock.time_msec())
 
-    def mark_as_going_offline(self, user_id: str) -> None:
+    def mark_as_going_offline(self, user_id: str, device_id: Optional[str]) -> None:
         """A user has stopped syncing. We wait before notifying the presence
         writer as its likely they'll come back soon. This allows us to avoid
         sending a stopped syncing immediately followed by a started syncing
         notification to the presence writer
         """
-        self.users_going_offline[user_id] = self.clock.time_msec()
+        self._user_devices_going_offline[(user_id, device_id)] = self.clock.time_msec()
 
     def send_stop_syncing(self) -> None:
         """Check if there are any users who have stopped syncing a while ago and
         haven't come back yet. If there are poke the presence writer about them.
         """
         now = self.clock.time_msec()
-        for user_id, last_sync_ms in list(self.users_going_offline.items()):
+        for (user_id, device_id), last_sync_ms in list(
+            self._user_devices_going_offline.items()
+        ):
             if now - last_sync_ms > UPDATE_SYNCING_USERS_MS:
-                self.users_going_offline.pop(user_id, None)
-                self.send_user_sync(user_id, False, last_sync_ms)
+                self._user_devices_going_offline.pop((user_id, device_id), None)
+                self.send_user_sync(user_id, device_id, False, last_sync_ms)
 
     async def user_syncing(
         self,
@@ -515,23 +534,23 @@ class WorkerPresenceHandler(BasePresenceHandler):
             is_sync=True,
         )
 
-        curr_sync = self._user_to_num_current_syncs.get(user_id, 0)
-        self._user_to_num_current_syncs[user_id] = curr_sync + 1
+        curr_sync = self._user_device_to_num_current_syncs.get((user_id, device_id), 0)
+        self._user_device_to_num_current_syncs[(user_id, device_id)] = curr_sync + 1
 
         # If this is the first in-flight sync, notify replication
-        if self._user_to_num_current_syncs[user_id] == 1:
-            self.mark_as_coming_online(user_id)
+        if self._user_device_to_num_current_syncs[(user_id, device_id)] == 1:
+            self.mark_as_coming_online(user_id, device_id)
 
         def _end() -> None:
             # We check that the user_id is in user_to_num_current_syncs because
             # user_to_num_current_syncs may have been cleared if we are
             # shutting down.
-            if user_id in self._user_to_num_current_syncs:
-                self._user_to_num_current_syncs[user_id] -= 1
+            if (user_id, device_id) in self._user_device_to_num_current_syncs:
+                self._user_device_to_num_current_syncs[(user_id, device_id)] -= 1
 
                 # If there are no more in-flight syncs, notify replication
-                if self._user_to_num_current_syncs[user_id] == 0:
-                    self.mark_as_going_offline(user_id)
+                if self._user_device_to_num_current_syncs[(user_id, device_id)] == 0:
+                    self.mark_as_going_offline(user_id, device_id)
 
         @contextlib.contextmanager
         def _user_syncing() -> Generator[None, None, None]:
@@ -598,10 +617,12 @@ class WorkerPresenceHandler(BasePresenceHandler):
         # If this is a federation sender, notify about presence updates.
         await self.maybe_send_presence_to_interested_destinations(state_to_notify)
 
-    def get_currently_syncing_users_for_replication(self) -> Iterable[str]:
+    def get_currently_syncing_users_for_replication(
+        self,
+    ) -> Iterable[Tuple[str, Optional[str]]]:
         return [
-            user_id
-            for user_id, count in self._user_to_num_current_syncs.items()
+            user_id_device_id
+            for user_id_device_id, count in self._user_device_to_num_current_syncs.items()
             if count > 0
         ]
 
@@ -723,17 +744,23 @@ class PresenceHandler(BasePresenceHandler):
 
         # Keeps track of the number of *ongoing* syncs on this process. While
         # this is non zero a user will never go offline.
-        self.user_to_num_current_syncs: Dict[str, int] = {}
+        self._user_device_to_num_current_syncs: Dict[
+            Tuple[str, Optional[str]], int
+        ] = {}
 
         # Keeps track of the number of *ongoing* syncs on other processes.
+        #
         # While any sync is ongoing on another process the user will never
         # go offline.
+        #
         # Each process has a unique identifier and an update frequency. If
         # no update is received from that process within the update period then
         # we assume that all the sync requests on that process have stopped.
-        # Stored as a dict from process_id to set of user_id, and a dict of
-        # process_id to millisecond timestamp last updated.
-        self.external_process_to_current_syncs: Dict[str, Set[str]] = {}
+        # Stored as a dict from process_id to set of (user_id, device_id), and
+        # a dict of process_id to millisecond timestamp last updated.
+        self.external_process_to_current_syncs: Dict[
+            str, Set[Tuple[str, Optional[str]]]
+        ] = {}
         self.external_process_last_updated_ms: Dict[str, int] = {}
 
         self.external_sync_linearizer = Linearizer(name="external_sync_linearizer")
@@ -938,7 +965,10 @@ class PresenceHandler(BasePresenceHandler):
             # that were syncing on that process to see if they need to be timed
             # out.
             users_to_check.update(
-                self.external_process_to_current_syncs.pop(process_id, ())
+                user_id
+                for user_id, device_id in self.external_process_to_current_syncs.pop(
+                    process_id, ()
+                )
             )
             self.external_process_last_updated_ms.pop(process_id)
 
@@ -951,11 +981,15 @@ class PresenceHandler(BasePresenceHandler):
 
         syncing_user_ids = {
             user_id
-            for user_id, count in self.user_to_num_current_syncs.items()
+            for (user_id, _), count in self._user_device_to_num_current_syncs.items()
             if count
         }
-        for user_ids in self.external_process_to_current_syncs.values():
-            syncing_user_ids.update(user_ids)
+        syncing_user_ids.update(
+            user_id
+            for user_id, _ in itertools.chain(
+                *self.external_process_to_current_syncs.values()
+            )
+        )
 
         changes = handle_timeouts(
             states,
@@ -1013,8 +1047,8 @@ class PresenceHandler(BasePresenceHandler):
         if not affect_presence or not self._presence_enabled:
             return _NullContextManager()
 
-        curr_sync = self.user_to_num_current_syncs.get(user_id, 0)
-        self.user_to_num_current_syncs[user_id] = curr_sync + 1
+        curr_sync = self._user_device_to_num_current_syncs.get((user_id, device_id), 0)
+        self._user_device_to_num_current_syncs[(user_id, device_id)] = curr_sync + 1
 
         # Note that this causes last_active_ts to be incremented which is not
         # what the spec wants.
@@ -1027,7 +1061,7 @@ class PresenceHandler(BasePresenceHandler):
 
         async def _end() -> None:
             try:
-                self.user_to_num_current_syncs[user_id] -= 1
+                self._user_device_to_num_current_syncs[(user_id, device_id)] -= 1
 
                 prev_state = await self.current_state_for_user(user_id)
                 await self._update_states(
@@ -1049,12 +1083,19 @@ class PresenceHandler(BasePresenceHandler):
 
         return _user_syncing()
 
-    def get_currently_syncing_users_for_replication(self) -> Iterable[str]:
+    def get_currently_syncing_users_for_replication(
+        self,
+    ) -> Iterable[Tuple[str, Optional[str]]]:
         # since we are the process handling presence, there is nothing to do here.
         return []
 
     async def update_external_syncs_row(
-        self, process_id: str, user_id: str, is_syncing: bool, sync_time_msec: int
+        self,
+        process_id: str,
+        user_id: str,
+        device_id: Optional[str],
+        is_syncing: bool,
+        sync_time_msec: int,
     ) -> None:
         """Update the syncing users for an external process as a delta.
 
@@ -1063,6 +1104,7 @@ class PresenceHandler(BasePresenceHandler):
                 syncing against. This allows synapse to process updates
                 as user start and stop syncing against a given process.
             user_id: The user who has started or stopped syncing
+            device_id: The user's device that has started or stopped syncing
             is_syncing: Whether or not the user is now syncing
             sync_time_msec: Time in ms when the user was last syncing
         """
@@ -1073,26 +1115,27 @@ class PresenceHandler(BasePresenceHandler):
                 process_id, set()
             )
 
-            # USER_SYNC is sent when a user starts or stops syncing on a remote
-            # process. (But only for the initial and last device.)
+            # USER_SYNC is sent when a user's device starts or stops syncing on
+            # a remote # process. (But only for the initial and last sync for that
+            # device.)
             #
-            # When a user *starts* syncing it also calls set_state(...) which
+            # When a device *starts* syncing it also calls set_state(...) which
             # will update the state, last_active_ts, and last_user_sync_ts.
-            # Simply ensure the user is tracked as syncing in this case.
+            # Simply ensure the user & device is tracked as syncing in this case.
             #
-            # When a user *stops* syncing, update the last_user_sync_ts and mark
+            # When a device *stops* syncing, update the last_user_sync_ts and mark
             # them as no longer syncing. Note this doesn't quite match the
             # monolith behaviour, which updates last_user_sync_ts at the end of
             # every sync, not just the last in-flight sync.
-            if is_syncing and user_id not in process_presence:
-                process_presence.add(user_id)
-            elif not is_syncing and user_id in process_presence:
+            if is_syncing and (user_id, device_id) not in process_presence:
+                process_presence.add((user_id, device_id))
+            elif not is_syncing and (user_id, device_id) in process_presence:
                 new_state = prev_state.copy_and_replace(
                     last_user_sync_ts=sync_time_msec
                 )
                 await self._update_states([new_state])
 
-                process_presence.discard(user_id)
+                process_presence.discard((user_id, device_id))
 
             self.external_process_last_updated_ms[process_id] = self.clock.time_msec()
 
@@ -1106,7 +1149,9 @@ class PresenceHandler(BasePresenceHandler):
             process_presence = self.external_process_to_current_syncs.pop(
                 process_id, set()
             )
-            prev_states = await self.current_state_for_users(process_presence)
+            prev_states = await self.current_state_for_users(
+                {user_id for user_id, device_id in process_presence}
+            )
             time_now_ms = self.clock.time_msec()
 
             await self._update_states(
diff --git a/synapse/replication/tcp/commands.py b/synapse/replication/tcp/commands.py
index 58a871c6d9..e616b5e1c8 100644
--- a/synapse/replication/tcp/commands.py
+++ b/synapse/replication/tcp/commands.py
@@ -267,27 +267,38 @@ class UserSyncCommand(Command):
     NAME = "USER_SYNC"
 
     def __init__(
-        self, instance_id: str, user_id: str, is_syncing: bool, last_sync_ms: int
+        self,
+        instance_id: str,
+        user_id: str,
+        device_id: Optional[str],
+        is_syncing: bool,
+        last_sync_ms: int,
     ):
         self.instance_id = instance_id
         self.user_id = user_id
+        self.device_id = device_id
         self.is_syncing = is_syncing
         self.last_sync_ms = last_sync_ms
 
     @classmethod
     def from_line(cls: Type["UserSyncCommand"], line: str) -> "UserSyncCommand":
-        instance_id, user_id, state, last_sync_ms = line.split(" ", 3)
+        device_id: Optional[str]
+        instance_id, user_id, device_id, state, last_sync_ms = line.split(" ", 4)
+
+        if device_id == "None":
+            device_id = None
 
         if state not in ("start", "end"):
             raise Exception("Invalid USER_SYNC state %r" % (state,))
 
-        return cls(instance_id, user_id, state == "start", int(last_sync_ms))
+        return cls(instance_id, user_id, device_id, state == "start", int(last_sync_ms))
 
     def to_line(self) -> str:
         return " ".join(
             (
                 self.instance_id,
                 self.user_id,
+                str(self.device_id),
                 "start" if self.is_syncing else "end",
                 str(self.last_sync_ms),
             )
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 92c5a55acc..d9045d7b73 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -428,7 +428,11 @@ class ReplicationCommandHandler:
 
         if self._is_presence_writer:
             return self._presence_handler.update_external_syncs_row(
-                cmd.instance_id, cmd.user_id, cmd.is_syncing, cmd.last_sync_ms
+                cmd.instance_id,
+                cmd.user_id,
+                cmd.device_id,
+                cmd.is_syncing,
+                cmd.last_sync_ms,
             )
         else:
             return None
@@ -699,9 +703,9 @@ class ReplicationCommandHandler:
         )
 
         now = self._clock.time_msec()
-        for user_id in currently_syncing:
+        for user_id, device_id in currently_syncing:
             connection.send_command(
-                UserSyncCommand(self._instance_id, user_id, True, now)
+                UserSyncCommand(self._instance_id, user_id, device_id, True, now)
             )
 
     def lost_connection(self, connection: IReplicationConnection) -> None:
@@ -753,11 +757,16 @@ class ReplicationCommandHandler:
         self.send_command(FederationAckCommand(self._instance_name, token))
 
     def send_user_sync(
-        self, instance_id: str, user_id: str, is_syncing: bool, last_sync_ms: int
+        self,
+        instance_id: str,
+        user_id: str,
+        device_id: Optional[str],
+        is_syncing: bool,
+        last_sync_ms: int,
     ) -> None:
         """Poke the master that a user has started/stopped syncing."""
         self.send_command(
-            UserSyncCommand(instance_id, user_id, is_syncing, last_sync_ms)
+            UserSyncCommand(instance_id, user_id, device_id, is_syncing, last_sync_ms)
         )
 
     def send_user_ip(