Add support for MSC3202: sending one-time key counts and fallback key usage states to Application Services. (#11617)

Co-authored-by: Erik Johnston <erik@matrix.org>

11 files changed, 528 insertions, 38 deletions

diff --git a/changelog.d/11617.feature b/changelog.d/11617.feature
new file mode 100644
index 0000000000..cf03f00e7c
--- /dev/null
+++ b/changelog.d/11617.feature
@@ -0,0 +1 @@
+Add support for MSC3202: sending one-time key counts and fallback key usage states to Application Services.
\ No newline at end of file
diff --git a/synapse/appservice/__init__.py b/synapse/appservice/__init__.py
index a340a8c9c7..4d3f8e4923 100644
--- a/synapse/appservice/__init__.py
+++ b/synapse/appservice/__init__.py
@@ -31,6 +31,14 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+# Type for the `device_one_time_key_counts` field in an appservice transaction
+#   user ID -> {device ID -> {algorithm -> count}}
+TransactionOneTimeKeyCounts = Dict[str, Dict[str, Dict[str, int]]]
+
+# Type for the `device_unused_fallback_keys` field in an appservice transaction
+#   user ID -> {device ID -> [algorithm]}
+TransactionUnusedFallbackKeys = Dict[str, Dict[str, List[str]]]
+
 
 class ApplicationServiceState(Enum):
     DOWN = "down"
@@ -72,6 +80,7 @@ class ApplicationService:
         rate_limited: bool = True,
         ip_range_whitelist: Optional[IPSet] = None,
         supports_ephemeral: bool = False,
+        msc3202_transaction_extensions: bool = False,
     ):
         self.token = token
         self.url = (
@@ -84,6 +93,7 @@ class ApplicationService:
         self.id = id
         self.ip_range_whitelist = ip_range_whitelist
         self.supports_ephemeral = supports_ephemeral
+        self.msc3202_transaction_extensions = msc3202_transaction_extensions
 
         if "|" in self.id:
             raise Exception("application service ID cannot contain '|' character")
@@ -339,12 +349,16 @@ class AppServiceTransaction:
         events: List[EventBase],
         ephemeral: List[JsonDict],
         to_device_messages: List[JsonDict],
+        one_time_key_counts: TransactionOneTimeKeyCounts,
+        unused_fallback_keys: TransactionUnusedFallbackKeys,
     ):
         self.service = service
         self.id = id
         self.events = events
         self.ephemeral = ephemeral
         self.to_device_messages = to_device_messages
+        self.one_time_key_counts = one_time_key_counts
+        self.unused_fallback_keys = unused_fallback_keys
 
     async def send(self, as_api: "ApplicationServiceApi") -> bool:
         """Sends this transaction using the provided AS API interface.
@@ -359,6 +373,8 @@ class AppServiceTransaction:
             events=self.events,
             ephemeral=self.ephemeral,
             to_device_messages=self.to_device_messages,
+            one_time_key_counts=self.one_time_key_counts,
+            unused_fallback_keys=self.unused_fallback_keys,
             txn_id=self.id,
         )
 
diff --git a/synapse/appservice/api.py b/synapse/appservice/api.py
index 73be7ff3d4..a0ea958af6 100644
--- a/synapse/appservice/api.py
+++ b/synapse/appservice/api.py
@@ -19,6 +19,11 @@ from prometheus_client import Counter
 
 from synapse.api.constants import EventTypes, Membership, ThirdPartyEntityKind
 from synapse.api.errors import CodeMessageException
+from synapse.appservice import (
+    ApplicationService,
+    TransactionOneTimeKeyCounts,
+    TransactionUnusedFallbackKeys,
+)
 from synapse.events import EventBase
 from synapse.events.utils import serialize_event
 from synapse.http.client import SimpleHttpClient
@@ -26,7 +31,6 @@ from synapse.types import JsonDict, ThirdPartyInstanceID
 from synapse.util.caches.response_cache import ResponseCache
 
 if TYPE_CHECKING:
-    from synapse.appservice import ApplicationService
     from synapse.server import HomeServer
 
 logger = logging.getLogger(__name__)
@@ -219,6 +223,8 @@ class ApplicationServiceApi(SimpleHttpClient):
         events: List[EventBase],
         ephemeral: List[JsonDict],
         to_device_messages: List[JsonDict],
+        one_time_key_counts: TransactionOneTimeKeyCounts,
+        unused_fallback_keys: TransactionUnusedFallbackKeys,
         txn_id: Optional[int] = None,
     ) -> bool:
         """
@@ -252,7 +258,7 @@ class ApplicationServiceApi(SimpleHttpClient):
         uri = service.url + ("/transactions/%s" % urllib.parse.quote(str(txn_id)))
 
         # Never send ephemeral events to appservices that do not support it
-        body: Dict[str, List[JsonDict]] = {"events": serialized_events}
+        body: JsonDict = {"events": serialized_events}
         if service.supports_ephemeral:
             body.update(
                 {
@@ -262,6 +268,16 @@ class ApplicationServiceApi(SimpleHttpClient):
                 }
             )
 
+        if service.msc3202_transaction_extensions:
+            if one_time_key_counts:
+                body[
+                    "org.matrix.msc3202.device_one_time_key_counts"
+                ] = one_time_key_counts
+            if unused_fallback_keys:
+                body[
+                    "org.matrix.msc3202.device_unused_fallback_keys"
+                ] = unused_fallback_keys
+
         try:
             await self.put_json(
                 uri=uri,
diff --git a/synapse/appservice/scheduler.py b/synapse/appservice/scheduler.py
index b4e602e880..72417151ba 100644
--- a/synapse/appservice/scheduler.py
+++ b/synapse/appservice/scheduler.py
@@ -54,12 +54,19 @@ from typing import (
     Callable,
     Collection,
     Dict,
+    Iterable,
     List,
     Optional,
     Set,
+    Tuple,
 )
 
-from synapse.appservice import ApplicationService, ApplicationServiceState
+from synapse.appservice import (
+    ApplicationService,
+    ApplicationServiceState,
+    TransactionOneTimeKeyCounts,
+    TransactionUnusedFallbackKeys,
+)
 from synapse.appservice.api import ApplicationServiceApi
 from synapse.events import EventBase
 from synapse.logging.context import run_in_background
@@ -96,7 +103,7 @@ class ApplicationServiceScheduler:
         self.as_api = hs.get_application_service_api()
 
         self.txn_ctrl = _TransactionController(self.clock, self.store, self.as_api)
-        self.queuer = _ServiceQueuer(self.txn_ctrl, self.clock)
+        self.queuer = _ServiceQueuer(self.txn_ctrl, self.clock, hs)
 
     async def start(self) -> None:
         logger.info("Starting appservice scheduler")
@@ -153,7 +160,9 @@ class _ServiceQueuer:
     appservice at a given time.
     """
 
-    def __init__(self, txn_ctrl: "_TransactionController", clock: Clock):
+    def __init__(
+        self, txn_ctrl: "_TransactionController", clock: Clock, hs: "HomeServer"
+    ):
         # dict of {service_id: [events]}
         self.queued_events: Dict[str, List[EventBase]] = {}
         # dict of {service_id: [events]}
@@ -165,6 +174,10 @@ class _ServiceQueuer:
         self.requests_in_flight: Set[str] = set()
         self.txn_ctrl = txn_ctrl
         self.clock = clock
+        self._msc3202_transaction_extensions_enabled: bool = (
+            hs.config.experimental.msc3202_transaction_extensions
+        )
+        self._store = hs.get_datastores().main
 
     def start_background_request(self, service: ApplicationService) -> None:
         # start a sender for this appservice if we don't already have one
@@ -202,15 +215,84 @@ class _ServiceQueuer:
                 if not events and not ephemeral and not to_device_messages_to_send:
                     return
 
+                one_time_key_counts: Optional[TransactionOneTimeKeyCounts] = None
+                unused_fallback_keys: Optional[TransactionUnusedFallbackKeys] = None
+
+                if (
+                    self._msc3202_transaction_extensions_enabled
+                    and service.msc3202_transaction_extensions
+                ):
+                    # Compute the one-time key counts and fallback key usage states
+                    # for the users which are mentioned in this transaction,
+                    # as well as the appservice's sender.
+                    (
+                        one_time_key_counts,
+                        unused_fallback_keys,
+                    ) = await self._compute_msc3202_otk_counts_and_fallback_keys(
+                        service, events, ephemeral, to_device_messages_to_send
+                    )
+
                 try:
                     await self.txn_ctrl.send(
-                        service, events, ephemeral, to_device_messages_to_send
+                        service,
+                        events,
+                        ephemeral,
+                        to_device_messages_to_send,
+                        one_time_key_counts,
+                        unused_fallback_keys,
                     )
                 except Exception:
                     logger.exception("AS request failed")
         finally:
             self.requests_in_flight.discard(service.id)
 
+    async def _compute_msc3202_otk_counts_and_fallback_keys(
+        self,
+        service: ApplicationService,
+        events: Iterable[EventBase],
+        ephemerals: Iterable[JsonDict],
+        to_device_messages: Iterable[JsonDict],
+    ) -> Tuple[TransactionOneTimeKeyCounts, TransactionUnusedFallbackKeys]:
+        """
+        Given a list of the events, ephemeral messages and to-device messages,
+        - first computes a list of application services users that may have
+          interesting updates to the one-time key counts or fallback key usage.
+        - then computes one-time key counts and fallback key usages for those users.
+        Given a list of application service users that are interesting,
+        compute one-time key counts and fallback key usages for the users.
+        """
+
+        # Set of 'interesting' users who may have updates
+        users: Set[str] = set()
+
+        # The sender is always included
+        users.add(service.sender)
+
+        # All AS users that would receive the PDUs or EDUs sent to these rooms
+        # are classed as 'interesting'.
+        rooms_of_interesting_users: Set[str] = set()
+        # PDUs
+        rooms_of_interesting_users.update(event.room_id for event in events)
+        # EDUs
+        rooms_of_interesting_users.update(
+            ephemeral["room_id"] for ephemeral in ephemerals
+        )
+
+        # Look up the AS users in those rooms
+        for room_id in rooms_of_interesting_users:
+            users.update(
+                await self._store.get_app_service_users_in_room(room_id, service)
+            )
+
+        # Add recipients of to-device messages.
+        # device_message["user_id"] is the ID of the recipient.
+        users.update(device_message["user_id"] for device_message in to_device_messages)
+
+        # Compute and return the counts / fallback key usage states
+        otk_counts = await self._store.count_bulk_e2e_one_time_keys_for_as(users)
+        unused_fbks = await self._store.get_e2e_bulk_unused_fallback_key_types(users)
+        return otk_counts, unused_fbks
+
 
 class _TransactionController:
     """Transaction manager.
@@ -238,6 +320,8 @@ class _TransactionController:
         events: List[EventBase],
         ephemeral: Optional[List[JsonDict]] = None,
         to_device_messages: Optional[List[JsonDict]] = None,
+        one_time_key_counts: Optional[TransactionOneTimeKeyCounts] = None,
+        unused_fallback_keys: Optional[TransactionUnusedFallbackKeys] = None,
     ) -> None:
         """
         Create a transaction with the given data and send to the provided
@@ -248,6 +332,10 @@ class _TransactionController:
             events: The persistent events to include in the transaction.
             ephemeral: The ephemeral events to include in the transaction.
             to_device_messages: The to-device messages to include in the transaction.
+            one_time_key_counts: Counts of remaining one-time keys for relevant
+                appservice devices in the transaction.
+            unused_fallback_keys: Lists of unused fallback keys for relevant
+                appservice devices in the transaction.
         """
         try:
             txn = await self.store.create_appservice_txn(
@@ -255,6 +343,8 @@ class _TransactionController:
                 events=events,
                 ephemeral=ephemeral or [],
                 to_device_messages=to_device_messages or [],
+                one_time_key_counts=one_time_key_counts or {},
+                unused_fallback_keys=unused_fallback_keys or {},
             )
             service_is_up = await self._is_service_up(service)
             if service_is_up:
diff --git a/synapse/config/appservice.py b/synapse/config/appservice.py
index 7fad2e0422..439bfe1526 100644
--- a/synapse/config/appservice.py
+++ b/synapse/config/appservice.py
@@ -166,6 +166,16 @@ def _load_appservice(
 
     supports_ephemeral = as_info.get("de.sorunome.msc2409.push_ephemeral", False)
 
+    # Opt-in flag for the MSC3202-specific transactional behaviour.
+    # When enabled, appservice transactions contain the following information:
+    #  - device One-Time Key counts
+    #  - device unused fallback key usage states
+    msc3202_transaction_extensions = as_info.get("org.matrix.msc3202", False)
+    if not isinstance(msc3202_transaction_extensions, bool):
+        raise ValueError(
+            "The `org.matrix.msc3202` option should be true or false if specified."
+        )
+
     return ApplicationService(
         token=as_info["as_token"],
         hostname=hostname,
@@ -174,8 +184,9 @@ def _load_appservice(
         hs_token=as_info["hs_token"],
         sender=user_id,
         id=as_info["id"],
-        supports_ephemeral=supports_ephemeral,
         protocols=protocols,
         rate_limited=rate_limited,
         ip_range_whitelist=ip_range_whitelist,
+        supports_ephemeral=supports_ephemeral,
+        msc3202_transaction_extensions=msc3202_transaction_extensions,
     )
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 772eb35013..41338b39df 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -47,11 +47,6 @@ class ExperimentalConfig(Config):
         # MSC3030 (Jump to date API endpoint)
         self.msc3030_enabled: bool = experimental.get("msc3030_enabled", False)
 
-        # The portion of MSC3202 which is related to device masquerading.
-        self.msc3202_device_masquerading_enabled: bool = experimental.get(
-            "msc3202_device_masquerading", False
-        )
-
         # MSC2409 (this setting only relates to optionally sending to-device messages).
         # Presence, typing and read receipt EDUs are already sent to application services that
         # have opted in to receive them. If enabled, this adds to-device messages to that list.
@@ -59,6 +54,17 @@ class ExperimentalConfig(Config):
             "msc2409_to_device_messages_enabled", False
         )
 
+        # The portion of MSC3202 which is related to device masquerading.
+        self.msc3202_device_masquerading_enabled: bool = experimental.get(
+            "msc3202_device_masquerading", False
+        )
+
+        # Portion of MSC3202 related to transaction extensions:
+        # sending one-time key counts and fallback key usage to application services.
+        self.msc3202_transaction_extensions: bool = experimental.get(
+            "msc3202_transaction_extensions", False
+        )
+
         # MSC3706 (server-side support for partial state in /send_join responses)
         self.msc3706_enabled: bool = experimental.get("msc3706_enabled", False)
 
diff --git a/synapse/storage/databases/main/appservice.py b/synapse/storage/databases/main/appservice.py
index 304814af5d..0694446558 100644
--- a/synapse/storage/databases/main/appservice.py
+++ b/synapse/storage/databases/main/appservice.py
@@ -20,14 +20,18 @@ from synapse.appservice import (
     ApplicationService,
     ApplicationServiceState,
     AppServiceTransaction,
+    TransactionOneTimeKeyCounts,
+    TransactionUnusedFallbackKeys,
 )
 from synapse.config.appservice import load_appservices
 from synapse.events import EventBase
-from synapse.storage._base import SQLBaseStore, db_to_json
+from synapse.storage._base import db_to_json
 from synapse.storage.database import DatabasePool, LoggingDatabaseConnection
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
+from synapse.storage.databases.main.roommember import RoomMemberWorkerStore
 from synapse.types import JsonDict
 from synapse.util import json_encoder
+from synapse.util.caches.descriptors import _CacheContext, cached
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -56,7 +60,7 @@ def _make_exclusive_regex(
     return exclusive_user_pattern
 
 
-class ApplicationServiceWorkerStore(SQLBaseStore):
+class ApplicationServiceWorkerStore(RoomMemberWorkerStore):
     def __init__(
         self,
         database: DatabasePool,
@@ -124,6 +128,18 @@ class ApplicationServiceWorkerStore(SQLBaseStore):
                 return service
         return None
 
+    @cached(iterable=True, cache_context=True)
+    async def get_app_service_users_in_room(
+        self,
+        room_id: str,
+        app_service: "ApplicationService",
+        cache_context: _CacheContext,
+    ) -> List[str]:
+        users_in_room = await self.get_users_in_room(
+            room_id, on_invalidate=cache_context.invalidate
+        )
+        return list(filter(app_service.is_interested_in_user, users_in_room))
+
 
 class ApplicationServiceStore(ApplicationServiceWorkerStore):
     # This is currently empty due to there not being any AS storage functions
@@ -199,6 +215,8 @@ class ApplicationServiceTransactionWorkerStore(
         events: List[EventBase],
         ephemeral: List[JsonDict],
         to_device_messages: List[JsonDict],
+        one_time_key_counts: TransactionOneTimeKeyCounts,
+        unused_fallback_keys: TransactionUnusedFallbackKeys,
     ) -> AppServiceTransaction:
         """Atomically creates a new transaction for this application service
         with the given list of events. Ephemeral events are NOT persisted to the
@@ -209,6 +227,10 @@ class ApplicationServiceTransactionWorkerStore(
             events: A list of persistent events to put in the transaction.
             ephemeral: A list of ephemeral events to put in the transaction.
             to_device_messages: A list of to-device messages to put in the transaction.
+            one_time_key_counts: Counts of remaining one-time keys for relevant
+                appservice devices in the transaction.
+            unused_fallback_keys: Lists of unused fallback keys for relevant
+                appservice devices in the transaction.
 
         Returns:
             A new transaction.
@@ -244,6 +266,8 @@ class ApplicationServiceTransactionWorkerStore(
                 events=events,
                 ephemeral=ephemeral,
                 to_device_messages=to_device_messages,
+                one_time_key_counts=one_time_key_counts,
+                unused_fallback_keys=unused_fallback_keys,
             )
 
         return await self.db_pool.runInteraction(
@@ -335,12 +359,17 @@ class ApplicationServiceTransactionWorkerStore(
 
         events = await self.get_events_as_list(event_ids)
 
+        # TODO: to-device messages, one-time key counts and unused fallback keys
+        #       are not yet populated for catch-up transactions.
+        #       We likely want to populate those for reliability.
         return AppServiceTransaction(
             service=service,
             id=entry["txn_id"],
             events=events,
             ephemeral=[],
             to_device_messages=[],
+            one_time_key_counts={},
+            unused_fallback_keys={},
         )
 
     def _get_last_txn(self, txn, service_id: Optional[str]) -> int:
diff --git a/synapse/storage/databases/main/end_to_end_keys.py b/synapse/storage/databases/main/end_to_end_keys.py
index 1f8447b507..9b293475c8 100644
--- a/synapse/storage/databases/main/end_to_end_keys.py
+++ b/synapse/storage/databases/main/end_to_end_keys.py
@@ -29,6 +29,10 @@ import attr
 from canonicaljson import encode_canonical_json
 
 from synapse.api.constants import DeviceKeyAlgorithms
+from synapse.appservice import (
+    TransactionOneTimeKeyCounts,
+    TransactionUnusedFallbackKeys,
+)
 from synapse.logging.opentracing import log_kv, set_tag, trace
 from synapse.storage._base import SQLBaseStore, db_to_json
 from synapse.storage.database import (
@@ -439,6 +443,114 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore, CacheInvalidationWorker
             "count_e2e_one_time_keys", _count_e2e_one_time_keys
         )
 
+    async def count_bulk_e2e_one_time_keys_for_as(
+        self, user_ids: Collection[str]
+    ) -> TransactionOneTimeKeyCounts:
+        """
+        Counts, in bulk, the one-time keys for all the users specified.
+        Intended to be used by application services for populating OTK counts in
+        transactions.
+
+        Return structure is of the shape:
+          user_id -> device_id -> algorithm -> count
+          Empty algorithm -> count dicts are created if needed to represent a
+          lack of unused one-time keys.
+        """
+
+        def _count_bulk_e2e_one_time_keys_txn(
+            txn: LoggingTransaction,
+        ) -> TransactionOneTimeKeyCounts:
+            user_in_where_clause, user_parameters = make_in_list_sql_clause(
+                self.database_engine, "user_id", user_ids
+            )
+            sql = f"""
+                SELECT user_id, device_id, algorithm, COUNT(key_id)
+                FROM devices
+                LEFT JOIN e2e_one_time_keys_json USING (user_id, device_id)
+                WHERE {user_in_where_clause}
+                GROUP BY user_id, device_id, algorithm
+            """
+            txn.execute(sql, user_parameters)
+
+            result: TransactionOneTimeKeyCounts = {}
+
+            for user_id, device_id, algorithm, count in txn:
+                # We deliberately construct empty dictionaries for
+                # users and devices without any unused one-time keys.
+                # We *could* omit these empty dicts if there have been no
+                # changes since the last transaction, but we currently don't
+                # do any change tracking!
+                device_count_by_algo = result.setdefault(user_id, {}).setdefault(
+                    device_id, {}
+                )
+                if algorithm is not None:
+                    # algorithm will be None if this device has no keys.
+                    device_count_by_algo[algorithm] = count
+
+            return result
+
+        return await self.db_pool.runInteraction(
+            "count_bulk_e2e_one_time_keys", _count_bulk_e2e_one_time_keys_txn
+        )
+
+    async def get_e2e_bulk_unused_fallback_key_types(
+        self, user_ids: Collection[str]
+    ) -> TransactionUnusedFallbackKeys:
+        """
+        Finds, in bulk, the types of unused fallback keys for all the users specified.
+        Intended to be used by application services for populating unused fallback
+        keys in transactions.
+
+        Return structure is of the shape:
+          user_id -> device_id -> algorithms
+          Empty lists are created for devices if there are no unused fallback
+          keys. This matches the response structure of MSC3202.
+        """
+        if len(user_ids) == 0:
+            return {}
+
+        def _get_bulk_e2e_unused_fallback_keys_txn(
+            txn: LoggingTransaction,
+        ) -> TransactionUnusedFallbackKeys:
+            user_in_where_clause, user_parameters = make_in_list_sql_clause(
+                self.database_engine, "devices.user_id", user_ids
+            )
+            # We can't use USING here because we require the `.used` condition
+            # to be part of the JOIN condition so that we generate empty lists
+            # when all keys are used (as opposed to just when there are no keys at all).
+            sql = f"""
+                SELECT devices.user_id, devices.device_id, algorithm
+                FROM devices
+                LEFT JOIN e2e_fallback_keys_json AS fallback_keys
+                    ON devices.user_id = fallback_keys.user_id
+                    AND devices.device_id = fallback_keys.device_id
+                    AND NOT fallback_keys.used
+                WHERE
+                    {user_in_where_clause}
+            """
+            txn.execute(sql, user_parameters)
+
+            result: TransactionUnusedFallbackKeys = {}
+
+            for user_id, device_id, algorithm in txn:
+                # We deliberately construct empty dictionaries and lists for
+                # users and devices without any unused fallback keys.
+                # We *could* omit these empty dicts if there have been no
+                # changes since the last transaction, but we currently don't
+                # do any change tracking!
+                device_unused_keys = result.setdefault(user_id, {}).setdefault(
+                    device_id, []
+                )
+                if algorithm is not None:
+                    # algorithm will be None if this device has no keys.
+                    device_unused_keys.append(algorithm)
+
+            return result
+
+        return await self.db_pool.runInteraction(
+            "_get_bulk_e2e_unused_fallback_keys", _get_bulk_e2e_unused_fallback_keys_txn
+        )
+
     async def set_e2e_fallback_keys(
         self, user_id: str, device_id: str, fallback_keys: JsonDict
     ) -> None:
diff --git a/tests/appservice/test_scheduler.py b/tests/appservice/test_scheduler.py
index 8fb6687f89..b9dc4dfe1b 100644
--- a/tests/appservice/test_scheduler.py
+++ b/tests/appservice/test_scheduler.py
@@ -68,6 +68,8 @@ class ApplicationServiceSchedulerTransactionCtrlTestCase(unittest.TestCase):
             events=events,
             ephemeral=[],
             to_device_messages=[],  # txn made and saved
+            one_time_key_counts={},
+            unused_fallback_keys={},
         )
         self.assertEquals(0, len(self.txnctrl.recoverers))  # no recoverer made
         txn.complete.assert_called_once_with(self.store)  # txn completed
@@ -92,6 +94,8 @@ class ApplicationServiceSchedulerTransactionCtrlTestCase(unittest.TestCase):
             events=events,
             ephemeral=[],
             to_device_messages=[],  # txn made and saved
+            one_time_key_counts={},
+            unused_fallback_keys={},
         )
         self.assertEquals(0, txn.send.call_count)  # txn not sent though
         self.assertEquals(0, txn.complete.call_count)  # or completed
@@ -114,7 +118,12 @@ class ApplicationServiceSchedulerTransactionCtrlTestCase(unittest.TestCase):
         self.successResultOf(defer.ensureDeferred(self.txnctrl.send(service, events)))
 
         self.store.create_appservice_txn.assert_called_once_with(
-            service=service, events=events, ephemeral=[], to_device_messages=[]
+            service=service,
+            events=events,
+            ephemeral=[],
+            to_device_messages=[],
+            one_time_key_counts={},
+            unused_fallback_keys={},
         )
         self.assertEquals(1, self.recoverer_fn.call_count)  # recoverer made
         self.assertEquals(1, self.recoverer.recover.call_count)  # and invoked
@@ -216,7 +225,7 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
         service = Mock(id=4)
         event = Mock()
         self.scheduler.enqueue_for_appservice(service, events=[event])
-        self.txn_ctrl.send.assert_called_once_with(service, [event], [], [])
+        self.txn_ctrl.send.assert_called_once_with(service, [event], [], [], None, None)
 
     def test_send_single_event_with_queue(self):
         d = defer.Deferred()
@@ -231,11 +240,13 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
         # (call enqueue_for_appservice multiple times deliberately)
         self.scheduler.enqueue_for_appservice(service, events=[event2])
         self.scheduler.enqueue_for_appservice(service, events=[event3])
-        self.txn_ctrl.send.assert_called_with(service, [event], [], [])
+        self.txn_ctrl.send.assert_called_with(service, [event], [], [], None, None)
         self.assertEquals(1, self.txn_ctrl.send.call_count)
         # Resolve the send event: expect the queued events to be sent
         d.callback(service)
-        self.txn_ctrl.send.assert_called_with(service, [event2, event3], [], [])
+        self.txn_ctrl.send.assert_called_with(
+            service, [event2, event3], [], [], None, None
+        )
         self.assertEquals(2, self.txn_ctrl.send.call_count)
 
     def test_multiple_service_queues(self):
@@ -261,15 +272,15 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
         # send events for different ASes and make sure they are sent
         self.scheduler.enqueue_for_appservice(srv1, events=[srv_1_event])
         self.scheduler.enqueue_for_appservice(srv1, events=[srv_1_event2])
-        self.txn_ctrl.send.assert_called_with(srv1, [srv_1_event], [], [])
+        self.txn_ctrl.send.assert_called_with(srv1, [srv_1_event], [], [], None, None)
         self.scheduler.enqueue_for_appservice(srv2, events=[srv_2_event])
         self.scheduler.enqueue_for_appservice(srv2, events=[srv_2_event2])
-        self.txn_ctrl.send.assert_called_with(srv2, [srv_2_event], [], [])
+        self.txn_ctrl.send.assert_called_with(srv2, [srv_2_event], [], [], None, None)
 
         # make sure callbacks for a service only send queued events for THAT
         # service
         srv_2_defer.callback(srv2)
-        self.txn_ctrl.send.assert_called_with(srv2, [srv_2_event2], [], [])
+        self.txn_ctrl.send.assert_called_with(srv2, [srv_2_event2], [], [], None, None)
         self.assertEquals(3, self.txn_ctrl.send.call_count)
 
     def test_send_large_txns(self):
@@ -288,13 +299,19 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
             self.scheduler.enqueue_for_appservice(service, [event], [])
 
         # Expect the first event to be sent immediately.
-        self.txn_ctrl.send.assert_called_with(service, [event_list[0]], [], [])
+        self.txn_ctrl.send.assert_called_with(
+            service, [event_list[0]], [], [], None, None
+        )
         srv_1_defer.callback(service)
         # Then send the next 100 events
-        self.txn_ctrl.send.assert_called_with(service, event_list[1:101], [], [])
+        self.txn_ctrl.send.assert_called_with(
+            service, event_list[1:101], [], [], None, None
+        )
         srv_2_defer.callback(service)
         # Then the final 99 events
-        self.txn_ctrl.send.assert_called_with(service, event_list[101:], [], [])
+        self.txn_ctrl.send.assert_called_with(
+            service, event_list[101:], [], [], None, None
+        )
         self.assertEquals(3, self.txn_ctrl.send.call_count)
 
     def test_send_single_ephemeral_no_queue(self):
@@ -302,14 +319,18 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
         service = Mock(id=4, name="service")
         event_list = [Mock(name="event")]
         self.scheduler.enqueue_for_appservice(service, ephemeral=event_list)
-        self.txn_ctrl.send.assert_called_once_with(service, [], event_list, [])
+        self.txn_ctrl.send.assert_called_once_with(
+            service, [], event_list, [], None, None
+        )
 
     def test_send_multiple_ephemeral_no_queue(self):
         # Expect the event to be sent immediately.
         service = Mock(id=4, name="service")
         event_list = [Mock(name="event1"), Mock(name="event2"), Mock(name="event3")]
         self.scheduler.enqueue_for_appservice(service, ephemeral=event_list)
-        self.txn_ctrl.send.assert_called_once_with(service, [], event_list, [])
+        self.txn_ctrl.send.assert_called_once_with(
+            service, [], event_list, [], None, None
+        )
 
     def test_send_single_ephemeral_with_queue(self):
         d = defer.Deferred()
@@ -324,13 +345,13 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
         # Send more events: expect send() to NOT be called multiple times.
         self.scheduler.enqueue_for_appservice(service, ephemeral=event_list_2)
         self.scheduler.enqueue_for_appservice(service, ephemeral=event_list_3)
-        self.txn_ctrl.send.assert_called_with(service, [], event_list_1, [])
+        self.txn_ctrl.send.assert_called_with(service, [], event_list_1, [], None, None)
         self.assertEquals(1, self.txn_ctrl.send.call_count)
         # Resolve txn_ctrl.send
         d.callback(service)
         # Expect the queued events to be sent
         self.txn_ctrl.send.assert_called_with(
-            service, [], event_list_2 + event_list_3, []
+            service, [], event_list_2 + event_list_3, [], None, None
         )
         self.assertEquals(2, self.txn_ctrl.send.call_count)
 
@@ -343,7 +364,9 @@ class ApplicationServiceSchedulerQueuerTestCase(unittest.HomeserverTestCase):
         second_chunk = [Mock(name="event%i" % (i + 101)) for i in range(50)]
         event_list = first_chunk + second_chunk
         self.scheduler.enqueue_for_appservice(service, ephemeral=event_list)
-        self.txn_ctrl.send.assert_called_once_with(service, [], first_chunk, [])
+        self.txn_ctrl.send.assert_called_once_with(
+            service, [], first_chunk, [], None, None
+        )
         d.callback(service)
-        self.txn_ctrl.send.assert_called_with(service, [], second_chunk, [])
+        self.txn_ctrl.send.assert_called_with(service, [], second_chunk, [], None, None)
         self.assertEquals(2, self.txn_ctrl.send.call_count)
diff --git a/tests/handlers/test_appservice.py b/tests/handlers/test_appservice.py
index 9918ff6807..6e0ec37963 100644
--- a/tests/handlers/test_appservice.py
+++ b/tests/handlers/test_appservice.py
@@ -16,17 +16,25 @@ from typing import Dict, Iterable, List, Optional
 from unittest.mock import Mock
 
 from twisted.internet import defer
+from twisted.test.proto_helpers import MemoryReactor
 
 import synapse.rest.admin
 import synapse.storage
-from synapse.appservice import ApplicationService
+from synapse.appservice import (
+    ApplicationService,
+    TransactionOneTimeKeyCounts,
+    TransactionUnusedFallbackKeys,
+)
 from synapse.handlers.appservice import ApplicationServicesHandler
-from synapse.rest.client import login, receipts, room, sendtodevice
+from synapse.rest.client import login, receipts, register, room, sendtodevice
+from synapse.server import HomeServer
 from synapse.types import RoomStreamToken
+from synapse.util import Clock
 from synapse.util.stringutils import random_string
 
 from tests import unittest
 from tests.test_utils import make_awaitable, simple_async_mock
+from tests.unittest import override_config
 from tests.utils import MockClock
 
 
@@ -428,7 +436,14 @@ class ApplicationServicesHandlerSendEventsTestCase(unittest.HomeserverTestCase):
         #
         # The uninterested application service should not have been notified at all.
         self.send_mock.assert_called_once()
-        service, _events, _ephemeral, to_device_messages = self.send_mock.call_args[0]
+        (
+            service,
+            _events,
+            _ephemeral,
+            to_device_messages,
+            _otks,
+            _fbks,
+        ) = self.send_mock.call_args[0]
 
         # Assert that this was the same to-device message that local_user sent
         self.assertEqual(service, interested_appservice)
@@ -540,7 +555,7 @@ class ApplicationServicesHandlerSendEventsTestCase(unittest.HomeserverTestCase):
         service_id_to_message_count: Dict[str, int] = {}
 
         for call in self.send_mock.call_args_list:
-            service, _events, _ephemeral, to_device_messages = call[0]
+            service, _events, _ephemeral, to_device_messages, _otks, _fbks = call[0]
 
             # Check that this was made to an interested service
             self.assertIn(service, interested_appservices)
@@ -582,3 +597,174 @@ class ApplicationServicesHandlerSendEventsTestCase(unittest.HomeserverTestCase):
         self._services.append(appservice)
 
         return appservice
+
+
+class ApplicationServicesHandlerOtkCountsTestCase(unittest.HomeserverTestCase):
+    # Argument indices for pulling out arguments from a `send_mock`.
+    ARG_OTK_COUNTS = 4
+    ARG_FALLBACK_KEYS = 5
+
+    servlets = [
+        synapse.rest.admin.register_servlets_for_client_rest_resource,
+        login.register_servlets,
+        register.register_servlets,
+        room.register_servlets,
+        sendtodevice.register_servlets,
+        receipts.register_servlets,
+    ]
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        # Mock the ApplicationServiceScheduler's _TransactionController's send method so that
+        # we can track what's going out
+        self.send_mock = simple_async_mock()
+        hs.get_application_service_handler().scheduler.txn_ctrl.send = self.send_mock  # type: ignore[assignment]  # We assign to a method.
+
+        # Define an application service for the tests
+        self._service_token = "VERYSECRET"
+        self._service = ApplicationService(
+            self._service_token,
+            "as1.invalid",
+            "as1",
+            "@as.sender:test",
+            namespaces={
+                "users": [
+                    {"regex": "@_as_.*:test", "exclusive": True},
+                    {"regex": "@as.sender:test", "exclusive": True},
+                ]
+            },
+            msc3202_transaction_extensions=True,
+        )
+        self.hs.get_datastores().main.services_cache = [self._service]
+
+        # Register some appservice users
+        self._sender_user, self._sender_device = self.register_appservice_user(
+            "as.sender", self._service_token
+        )
+        self._namespaced_user, self._namespaced_device = self.register_appservice_user(
+            "_as_user1", self._service_token
+        )
+
+        # Register a real user as well.
+        self._real_user = self.register_user("real.user", "meow")
+        self._real_user_token = self.login("real.user", "meow")
+
+    async def _add_otks_for_device(
+        self, user_id: str, device_id: str, otk_count: int
+    ) -> None:
+        """
+        Add some dummy keys. It doesn't matter if they're not a real algorithm;
+        that should be opaque to the server anyway.
+        """
+        await self.hs.get_datastores().main.add_e2e_one_time_keys(
+            user_id,
+            device_id,
+            self.clock.time_msec(),
+            [("algo", f"k{i}", "{}") for i in range(otk_count)],
+        )
+
+    async def _add_fallback_key_for_device(
+        self, user_id: str, device_id: str, used: bool
+    ) -> None:
+        """
+        Adds a fake fallback key to a device, optionally marking it as used
+        right away.
+        """
+        store = self.hs.get_datastores().main
+        await store.set_e2e_fallback_keys(user_id, device_id, {"algo:fk": "fall back!"})
+        if used is True:
+            # Mark the key as used
+            await store.db_pool.simple_update_one(
+                table="e2e_fallback_keys_json",
+                keyvalues={
+                    "user_id": user_id,
+                    "device_id": device_id,
+                    "algorithm": "algo",
+                    "key_id": "fk",
+                },
+                updatevalues={"used": True},
+                desc="_get_fallback_key_set_used",
+            )
+
+    def _set_up_devices_and_a_room(self) -> str:
+        """
+        Helper to set up devices for all the users
+        and a room for the users to talk in.
+        """
+
+        async def preparation():
+            await self._add_otks_for_device(self._sender_user, self._sender_device, 42)
+            await self._add_fallback_key_for_device(
+                self._sender_user, self._sender_device, used=True
+            )
+            await self._add_otks_for_device(
+                self._namespaced_user, self._namespaced_device, 36
+            )
+            await self._add_fallback_key_for_device(
+                self._namespaced_user, self._namespaced_device, used=False
+            )
+
+            # Register a device for the real user, too, so that we can later ensure
+            # that we don't leak information to the AS about the non-AS user.
+            await self.hs.get_datastores().main.store_device(
+                self._real_user, "REALDEV", "UltraMatrix 3000"
+            )
+            await self._add_otks_for_device(self._real_user, "REALDEV", 50)
+
+        self.get_success(preparation())
+
+        room_id = self.helper.create_room_as(
+            self._real_user, is_public=True, tok=self._real_user_token
+        )
+        self.helper.join(
+            room_id,
+            self._namespaced_user,
+            tok=self._service_token,
+            appservice_user_id=self._namespaced_user,
+        )
+
+        # Check it was called for sanity. (This was to send the join event to the AS.)
+        self.send_mock.assert_called()
+        self.send_mock.reset_mock()
+
+        return room_id
+
+    @override_config(
+        {"experimental_features": {"msc3202_transaction_extensions": True}}
+    )
+    def test_application_services_receive_otk_counts_and_fallback_key_usages_with_pdus(
+        self,
+    ) -> None:
+        """
+        Tests that:
+        - the AS receives one-time key counts and unused fallback keys for:
+            - the specified sender; and
+            - any user who is in receipt of the PDUs
+        """
+
+        room_id = self._set_up_devices_and_a_room()
+
+        # Send a message into the AS's room
+        self.helper.send(room_id, "woof woof", tok=self._real_user_token)
+
+        # Capture what was sent as an AS transaction.
+        self.send_mock.assert_called()
+        last_args, _last_kwargs = self.send_mock.call_args
+        otks: Optional[TransactionOneTimeKeyCounts] = last_args[self.ARG_OTK_COUNTS]
+        unused_fallbacks: Optional[TransactionUnusedFallbackKeys] = last_args[
+            self.ARG_FALLBACK_KEYS
+        ]
+
+        self.assertEqual(
+            otks,
+            {
+                "@as.sender:test": {self._sender_device: {"algo": 42}},
+                "@_as_user1:test": {self._namespaced_device: {"algo": 36}},
+            },
+        )
+        self.assertEqual(
+            unused_fallbacks,
+            {
+                "@as.sender:test": {self._sender_device: []},
+                "@_as_user1:test": {self._namespaced_device: ["algo"]},
+            },
+        )
diff --git a/tests/storage/test_appservice.py b/tests/storage/test_appservice.py
index 50703ccaee..d2f654214e 100644
--- a/tests/storage/test_appservice.py
+++ b/tests/storage/test_appservice.py
@@ -267,7 +267,7 @@ class ApplicationServiceTransactionStoreTestCase(unittest.HomeserverTestCase):
         events = cast(List[EventBase], [Mock(event_id="e1"), Mock(event_id="e2")])
         txn = self.get_success(
             defer.ensureDeferred(
-                self.store.create_appservice_txn(service, events, [], [])
+                self.store.create_appservice_txn(service, events, [], [], {}, {})
             )
         )
         self.assertEquals(txn.id, 1)
@@ -283,7 +283,7 @@ class ApplicationServiceTransactionStoreTestCase(unittest.HomeserverTestCase):
         self.get_success(self._insert_txn(service.id, 9644, events))
         self.get_success(self._insert_txn(service.id, 9645, events))
         txn = self.get_success(
-            self.store.create_appservice_txn(service, events, [], [])
+            self.store.create_appservice_txn(service, events, [], [], {}, {})
         )
         self.assertEquals(txn.id, 9646)
         self.assertEquals(txn.events, events)
@@ -296,7 +296,7 @@ class ApplicationServiceTransactionStoreTestCase(unittest.HomeserverTestCase):
         events = cast(List[EventBase], [Mock(event_id="e1"), Mock(event_id="e2")])
         self.get_success(self._set_last_txn(service.id, 9643))
         txn = self.get_success(
-            self.store.create_appservice_txn(service, events, [], [])
+            self.store.create_appservice_txn(service, events, [], [], {}, {})
         )
         self.assertEquals(txn.id, 9644)
         self.assertEquals(txn.events, events)
@@ -320,7 +320,7 @@ class ApplicationServiceTransactionStoreTestCase(unittest.HomeserverTestCase):
         self.get_success(self._insert_txn(self.as_list[3]["id"], 9643, events))
 
         txn = self.get_success(
-            self.store.create_appservice_txn(service, events, [], [])
+            self.store.create_appservice_txn(service, events, [], [], {}, {})
         )
         self.assertEquals(txn.id, 9644)
         self.assertEquals(txn.events, events)