bugfix: ensure we send back device list updates at the correct time

Otherwise we can cause lost device list updates if the client resets
the connection (as the device list update won't be in any in-memory cache).

state/device_data_table.go

@@ -70,6 +70,9 @@ func (t *DeviceDataTable) Select(userID, deviceID string, swap bool) (result *in
 		if !swap {
 			return nil // don't swap
 		}
+		// the caller will only look at sent, so make sure what is new is now in sent
+		result.DeviceLists.Sent = result.DeviceLists.New
+
 		// swap over the fields
 		writeBack := *result
 		writeBack.DeviceLists.Sent = result.DeviceLists.New

tests-integration/extensions_test.go

@@ -193,6 +193,48 @@ func TestExtensionE2EE(t *testing.T) {
 	if time.Since(start) >= (500 * time.Millisecond) {
 		t.Fatalf("sync request did not return immediately with OTK counts")
 	}
+
+	// check that if we lose a device list update and restart from nothing, we see the same update
+	v2.queueResponse(alice, sync2.SyncResponse{
+		DeviceLists: struct {
+			Changed []string `json:"changed,omitempty"`
+			Left    []string `json:"left,omitempty"`
+		}{
+			Changed: wantChanged,
+			Left:    wantLeft,
+		},
+	})
+	v2.waitUntilEmpty(t, alice)
+	res = v3.mustDoV3RequestWithPos(t, aliceToken, res.Pos, sync3.Request{
+		Lists: map[string]sync3.RequestList{"a": {
+			Ranges: sync3.SliceRanges{
+				[2]int64{0, 10}, // doesn't matter
+			},
+		}},
+		// enable the E2EE extension
+		Extensions: extensions.Request{
+			E2EE: &extensions.E2EERequest{
+				Core: extensions.Core{Enabled: &boolTrue},
+			},
+		},
+	})
+	m.MatchResponse(t, res, m.MatchDeviceLists(wantChanged, wantLeft))
+	// we actually lost this update: start again and we should see it.
+	res = v3.mustDoV3Request(t, aliceToken, sync3.Request{
+		Lists: map[string]sync3.RequestList{"a": {
+			Ranges: sync3.SliceRanges{
+				[2]int64{0, 10}, // doesn't matter
+			},
+		}},
+		// enable the E2EE extension
+		Extensions: extensions.Request{
+			E2EE: &extensions.E2EERequest{
+				Core: extensions.Core{Enabled: &boolTrue},
+			},
+		},
+	})
+	m.MatchResponse(t, res, m.MatchDeviceLists(wantChanged, wantLeft))
+
 }
 
 // Checks that to-device messages are passed from v2 to v3