Implement MSC3852: Expose `last_seen_user_agent` to users for their own devices; also expose to Admin API (#13549)

3 files changed, 38 insertions, 1 deletions

diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 7d17c958bb..c1ff417539 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -90,3 +90,6 @@ class ExperimentalConfig(Config):
 
         # MSC3848: Introduce errcodes for specific event sending failures
         self.msc3848_enabled: bool = experimental.get("msc3848_enabled", False)
+
+        # MSC3852: Expose last seen user agent field on /_matrix/client/v3/devices.
+        self.msc3852_enabled: bool = experimental.get("msc3852_enabled", False)
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 1a8379854c..f5c586f657 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -74,6 +74,7 @@ class DeviceWorkerHandler:
         self._state_storage = hs.get_storage_controllers().state
         self._auth_handler = hs.get_auth_handler()
         self.server_name = hs.hostname
+        self._msc3852_enabled = hs.config.experimental.msc3852_enabled
 
     @trace
     async def get_devices_by_user(self, user_id: str) -> List[JsonDict]:
@@ -747,7 +748,13 @@ def _update_device_from_client_ips(
     device: JsonDict, client_ips: Mapping[Tuple[str, str], Mapping[str, Any]]
 ) -> None:
     ip = client_ips.get((device["user_id"], device["device_id"]), {})
-    device.update({"last_seen_ts": ip.get("last_seen"), "last_seen_ip": ip.get("ip")})
+    device.update(
+        {
+            "last_seen_user_agent": ip.get("user_agent"),
+            "last_seen_ts": ip.get("last_seen"),
+            "last_seen_ip": ip.get("ip"),
+        }
+    )
 
 
 class DeviceListUpdater:
diff --git a/synapse/rest/client/devices.py b/synapse/rest/client/devices.py
index 6fab102437..ed6ce78d47 100644
--- a/synapse/rest/client/devices.py
+++ b/synapse/rest/client/devices.py
@@ -42,12 +42,26 @@ class DevicesRestServlet(RestServlet):
         self.hs = hs
         self.auth = hs.get_auth()
         self.device_handler = hs.get_device_handler()
+        self._msc3852_enabled = hs.config.experimental.msc3852_enabled
 
     async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
         devices = await self.device_handler.get_devices_by_user(
             requester.user.to_string()
         )
+
+        # If MSC3852 is disabled, then the "last_seen_user_agent" field will be
+        # removed from each device. If it is enabled, then the field name will
+        # be replaced by the unstable identifier.
+        #
+        # When MSC3852 is accepted, this block of code can just be removed to
+        # expose "last_seen_user_agent" to clients.
+        for device in devices:
+            last_seen_user_agent = device["last_seen_user_agent"]
+            del device["last_seen_user_agent"]
+            if self._msc3852_enabled:
+                device["org.matrix.msc3852.last_seen_user_agent"] = last_seen_user_agent
+
         return 200, {"devices": devices}
 
 
@@ -108,6 +122,7 @@ class DeviceRestServlet(RestServlet):
         self.auth = hs.get_auth()
         self.device_handler = hs.get_device_handler()
         self.auth_handler = hs.get_auth_handler()
+        self._msc3852_enabled = hs.config.experimental.msc3852_enabled
 
     async def on_GET(
         self, request: SynapseRequest, device_id: str
@@ -118,6 +133,18 @@ class DeviceRestServlet(RestServlet):
         )
         if device is None:
             raise NotFoundError("No device found")
+
+        # If MSC3852 is disabled, then the "last_seen_user_agent" field will be
+        # removed from each device. If it is enabled, then the field name will
+        # be replaced by the unstable identifier.
+        #
+        # When MSC3852 is accepted, this block of code can just be removed to
+        # expose "last_seen_user_agent" to clients.
+        last_seen_user_agent = device["last_seen_user_agent"]
+        del device["last_seen_user_agent"]
+        if self._msc3852_enabled:
+            device["org.matrix.msc3852.last_seen_user_agent"] = last_seen_user_agent
+
         return 200, device
 
     @interactive_auth_handler