Support MSC3814: Dehydrated Devices

changelog.d/15929.feature

@@ -0,0 +1 @@
+Implement [MSC3814](https://github.com/matrix-org/matrix-spec-proposals/pull/3814), dehydrated devices v2/shrivelled sessions and move [MSC2697](https://github.com/matrix-org/matrix-spec-proposals/pull/2697) behind a config flag. Contributed by Nico from Famedly and H-Shay.

synapse/config/experimental.py

@@ -247,6 +247,27 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         # MSC3026 (busy presence state)
         self.msc3026_enabled: bool = experimental.get("msc3026_enabled", False)
 
+        # MSC2697 (device dehydration)
+        # Enabled by default since this option was added after adding the feature.
+        # It is not recommended that both MSC2697 and MSC3814 both be enabled at
+        # once.
+        self.msc2697_enabled: bool = experimental.get("msc2697_enabled", True)
+
+        # MSC3814 (dehydrated devices with SSSS)
+        # This is an alternative method to achieve the same goals as MSC2697.
+        # It is not recommended that both MSC2697 and MSC3814 both be enabled at
+        # once.
+        self.msc3814_enabled: bool = experimental.get("msc3814_enabled", False)
+
+        if self.msc2697_enabled and self.msc3814_enabled:
+            raise ConfigError(
+                "MSC2697 and MSC3814 should not both be enabled.",
+                (
+                    "experimental_features",
+                    "msc3814_enabled",
+                ),
+            )
+
         # MSC3244 (room version capabilities)
         self.msc3244_enabled: bool = experimental.get("msc3244_enabled", True)
 

synapse/handlers/device.py

@@ -653,6 +653,7 @@ async def notify_user_signature_update(
     async def store_dehydrated_device(
         self,
         user_id: str,
+        device_id: Optional[str],
         device_data: JsonDict,
         initial_device_display_name: Optional[str] = None,
     ) -> str:
@@ -661,14 +662,15 @@ async def store_dehydrated_device(
 
         Args:
             user_id: the user that we are storing the device for
+            device_id: device id supplied by client
             device_data: the dehydrated device information
             initial_device_display_name: The display name to use for the device
         Returns:
             device id of the dehydrated device
         """
         device_id = await self.check_device_registered(
             user_id,
-            None,
+            device_id,
             initial_device_display_name,
         )
         old_device_id = await self.store.store_dehydrated_device(

synapse/handlers/devicemessage.py

@@ -13,10 +13,11 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Any, Dict
+from http import HTTPStatus
+from typing import TYPE_CHECKING, Any, Dict, Optional
 
 from synapse.api.constants import EduTypes, EventContentFields, ToDeviceEventTypes
-from synapse.api.errors import SynapseError
+from synapse.api.errors import Codes, SynapseError
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.logging.context import run_in_background
 from synapse.logging.opentracing import (
@@ -48,6 +49,9 @@ def __init__(self, hs: "HomeServer"):
         self.store = hs.get_datastores().main
         self.notifier = hs.get_notifier()
         self.is_mine = hs.is_mine
+        if hs.config.experimental.msc3814_enabled:
+            self.event_sources = hs.get_event_sources()
+            self.device_handler = hs.get_device_handler()
 
         # We only need to poke the federation sender explicitly if its on the
         # same instance. Other federation sender instances will get notified by
@@ -303,3 +307,103 @@ async def send_device_message(
                 # Enqueue a new federation transaction to send the new
                 # device messages to each remote destination.
                 self.federation_sender.send_device_messages(destination)
+
+    async def get_events_for_dehydrated_device(
+        self,
+        requester: Requester,
+        device_id: str,
+        since_token: Optional[str],
+        limit: int,
+    ) -> JsonDict:
+        """Fetches up to `limit` events sent to `device_id` starting from `since_token`
+        and returns the new since token. If there are no more messages, returns an empty
+        array.
+
+        Args:
+            requester: the user requesting the messages
+            device_id: ID of the dehydrated device
+            since_token: stream id to start from when fetching messages
+            limit: the number of messages to fetch
+        Returns:
+            A dict containing the to-device messages, as well as a token that the client
+            can provide in the next call to fetch the next batch of messages
+        """
+
+        user_id = requester.user.to_string()
+
+        # only allow fetching messages for the dehydrated device id currently associated
+        # with the user
+        dehydrated_device = await self.device_handler.get_dehydrated_device(user_id)
+        if dehydrated_device is None:
+            raise SynapseError(
+                HTTPStatus.FORBIDDEN,
+                "No dehydrated device exists",
+                Codes.FORBIDDEN,
+            )
+
+        dehydrated_device_id, _ = dehydrated_device
+        if device_id != dehydrated_device_id:
+            raise SynapseError(
+                HTTPStatus.FORBIDDEN,
+                "You may only fetch messages for your dehydrated device",
+                Codes.FORBIDDEN,
+            )
+
+        since_stream_id = 0
+        if since_token:
+            if not since_token.startswith("d"):
+                raise SynapseError(
+                    HTTPStatus.BAD_REQUEST,
+                    "from parameter %r has an invalid format" % (since_token,),
+                    errcode=Codes.INVALID_PARAM,
+                )
+
+            try:
+                since_stream_id = int(since_token[1:])
+            except Exception:
+                raise SynapseError(
+                    HTTPStatus.BAD_REQUEST,
+                    "from parameter %r has an invalid format" % (since_token,),
+                    errcode=Codes.INVALID_PARAM,
+                )
+
+            # if we have a since token, delete any to-device messages before that token
+            # (since we now know that the device has received them)
+            deleted = await self.store.delete_messages_for_device(
+                user_id, device_id, since_stream_id
+            )
+            logger.debug(
+                "Deleted %d to-device messages up to %d for user_id %s device_id %s",
+                deleted,
+                since_stream_id,
+                user_id,
+                device_id,
+            )
+
+        to_token = self.event_sources.get_current_token().to_device_key
+
+        messages, stream_id = await self.store.get_messages_for_device(
+            user_id, device_id, since_stream_id, to_token, limit
+        )
+
+        for message in messages:
+            # Remove the message id before sending to client
+            message_id = message.pop("message_id", None)
+            if message_id:
+                set_tag(SynapseTags.TO_DEVICE_EDU_ID, message_id)
+
+        logger.debug(
+            "Returning %d to-device messages between %d and %d (current token: %d) for "
+            "dehydrated device %s, user_id %s",
+            len(messages),
+            since_stream_id,
+            stream_id,
+            to_token,
+            device_id,
+            user_id,
+        )
+
+        return {
+            "events": messages,
+            "next_batch": f"d{stream_id}",
+        }