Only emit key changes which are different from what we had before (#1279)

We did this already for local `/keys/upload` but didn't for
remote `/users/devices`. This meant any resyncs would spam produce
events, hammering disk i/o and spamming the logs.

keyserver/internal/device_list_update.go

@@ -91,6 +91,9 @@ type DeviceListUpdaterDatabase interface {
 
 	// PrevIDsExists returns true if all prev IDs exist for this user.
 	PrevIDsExists(ctx context.Context, userID string, prevIDs []int) (bool, error)
+
+	// DeviceKeysJSON populates the KeyJSON for the given keys. If any proided `keys` have a `KeyJSON` or `StreamID` already then it will be replaced.
+	DeviceKeysJSON(ctx context.Context, keys []api.DeviceMessage) error
 }
 
 // KeyChangeProducer is the interface for producers.KeyChange useful for testing.
@@ -354,6 +357,7 @@ func (u *DeviceListUpdater) processServer(serverName gomatrixserverlib.ServerNam
 func (u *DeviceListUpdater) updateDeviceList(res *gomatrixserverlib.RespUserDevices) error {
 	ctx := context.Background() // we've got the keys, don't time out when persisting them to the database.
 	keys := make([]api.DeviceMessage, len(res.Devices))
+	existingKeys := make([]api.DeviceMessage, len(res.Devices))
 	for i, device := range res.Devices {
 		keyJSON, err := json.Marshal(device.Keys)
 		if err != nil {
@@ -369,7 +373,21 @@ func (u *DeviceListUpdater) updateDeviceList(res *gomatrixserverlib.RespUserDevi
 				KeyJSON:     keyJSON,
 			},
 		}
+		existingKeys[i] = api.DeviceMessage{
+			DeviceKeys: api.DeviceKeys{
+				UserID:   res.UserID,
+				DeviceID: device.DeviceID,
+			},
+		}
 	}
+	// fetch what keys we had already and only emit changes
+	if err := u.db.DeviceKeysJSON(ctx, existingKeys); err != nil {
+		// non-fatal, log and continue
+		util.GetLogger(ctx).WithError(err).WithField("user_id", res.UserID).Errorf(
+			"failed to query device keys json for calculating diffs",
+		)
+	}
+
 	err := u.db.StoreRemoteDeviceKeys(ctx, keys, []string{res.UserID})
 	if err != nil {
 		return fmt.Errorf("failed to store remote device keys: %w", err)
@@ -378,7 +396,7 @@ func (u *DeviceListUpdater) updateDeviceList(res *gomatrixserverlib.RespUserDevi
 	if err != nil {
 		return fmt.Errorf("failed to mark device list as fresh: %w", err)
 	}
-	err = u.producer.ProduceKeyChanges(keys)
+	err = emitDeviceKeyChanges(u.producer, existingKeys, keys)
 	if err != nil {
 		return fmt.Errorf("failed to emit key changes for fresh device list: %w", err)
 	}

keyserver/internal/device_list_update_test.go

@@ -91,6 +91,10 @@ func (d *mockDeviceListUpdaterDatabase) PrevIDsExists(ctx context.Context, userI
 	return d.prevIDsExist(userID, prevIDs), nil
 }
 
+func (d *mockDeviceListUpdaterDatabase) DeviceKeysJSON(ctx context.Context, keys []api.DeviceMessage) error {
+	return nil
+}
+
 type roundTripper struct {
 	fn func(*http.Request) (*http.Response, error)
 }

keyserver/internal/internal.go

@@ -505,7 +505,7 @@ func (a *KeyInternalAPI) uploadLocalDeviceKeys(ctx context.Context, req *api.Per
 		}
 		return
 	}
-	err = a.emitDeviceKeyChanges(existingKeys, keysToStore)
+	err = emitDeviceKeyChanges(a.Producer, existingKeys, keysToStore)
 	if err != nil {
 		util.GetLogger(ctx).Errorf("Failed to emitDeviceKeyChanges: %s", err)
 	}
@@ -550,7 +550,7 @@ func (a *KeyInternalAPI) uploadOneTimeKeys(ctx context.Context, req *api.Perform
 
 }
 
-func (a *KeyInternalAPI) emitDeviceKeyChanges(existing, new []api.DeviceMessage) error {
+func emitDeviceKeyChanges(producer KeyChangeProducer, existing, new []api.DeviceMessage) error {
 	// find keys in new that are not in existing
 	var keysAdded []api.DeviceMessage
 	for _, newKey := range new {
@@ -567,7 +567,7 @@ func (a *KeyInternalAPI) emitDeviceKeyChanges(existing, new []api.DeviceMessage)
 			keysAdded = append(keysAdded, newKey)
 		}
 	}
-	return a.Producer.ProduceKeyChanges(keysAdded)
+	return producer.ProduceKeyChanges(keysAdded)
 }
 
 func appendDisplayNames(existing, new []api.DeviceMessage) []api.DeviceMessage {