feat(GODT-2891): Allow message create & delete during sync

Incoming messages which arrive into labels we know during sync are now
presented to the IMAP clients.

We also allow messages to be deleted while syncing if deleted on other
clients.

Other operations such as moving, marking messages as read and label
operations need to be considered in a follow up patch as they are far
more complex.

internal/bridge/sync_test.go

@@ -504,6 +504,81 @@ func TestBridge_EventReplayAfterSyncHasFinished(t *testing.T) {
 	}, server.WithTLS(false))
 }
 
+func TestBridge_MessageCreateDuringSync(t *testing.T) {
+	numMsg := 1 << 8
+
+	withEnv(t, func(ctx context.Context, s *server.Server, netCtl *proton.NetCtl, locator bridge.Locator, storeKey []byte) {
+		userID, addrID, err := s.CreateUser("imap", password)
+		require.NoError(t, err)
+
+		labelID, err := s.CreateLabel(userID, "folder", "", proton.LabelTypeFolder)
+		require.NoError(t, err)
+
+		withClient(ctx, t, s, "imap", password, func(ctx context.Context, c *proton.Client) {
+			createNumMessages(ctx, t, c, addrID, labelID, numMsg)
+		})
+
+		var allowSyncToProgress atomic.Bool
+		allowSyncToProgress.Store(false)
+
+		// Simulate 429 to prevent sync from progressing.
+		s.AddStatusHook(func(request *http.Request) (int, bool) {
+			if request.Method == "GET" && strings.Contains(request.URL.Path, "/mail/v4/messages/") {
+				if !allowSyncToProgress.Load() {
+					return http.StatusTooManyRequests, true
+				}
+			}
+
+			return 0, false
+		})
+
+		// The initial user should be fully synced.
+		withBridge(ctx, t, s.GetHostURL(), netCtl, locator, storeKey, func(bridge *bridge.Bridge, _ *bridge.Mocks) {
+			syncStartedCh, syncStartedDone := chToType[events.Event, events.SyncStarted](bridge.GetEvents(events.SyncStarted{}))
+			defer syncStartedDone()
+
+			addressCreatedCh, addressCreatedDone := chToType[events.Event, events.UserAddressCreated](bridge.GetEvents(events.UserAddressCreated{}))
+			defer addressCreatedDone()
+
+			userID, err := bridge.LoginFull(ctx, "imap", password, nil, nil)
+			require.NoError(t, err)
+
+			require.Equal(t, userID, (<-syncStartedCh).UserID)
+
+			// create 20 more messages and move them to inbox
+			withClient(ctx, t, s, "imap", password, func(ctx context.Context, c *proton.Client) {
+				createNumMessages(ctx, t, c, addrID, proton.InboxLabel, 20)
+			})
+
+			// User AddrID2 event as a check point to see when the new address was created.
+			addrID, err := s.CreateAddress(userID, "bar@proton.ch", password)
+			require.NoError(t, err)
+
+			// At most two events can be published, one for the first address, then for the second.
+			// if the second event is not `addrID` then something went wrong.
+			event := <-addressCreatedCh
+			require.Equal(t, addrID, event.AddressID)
+			allowSyncToProgress.Store(true)
+
+			info, err := bridge.GetUserInfo(userID)
+			require.NoError(t, err)
+
+			client, err := eventuallyDial(fmt.Sprintf("%v:%v", constants.Host, bridge.GetIMAPPort()))
+			require.NoError(t, err)
+			require.NoError(t, client.Login(info.Addresses[0], string(info.BridgePass)))
+			defer func() { _ = client.Logout() }()
+
+			require.Eventually(t, func() bool {
+				// Finally check if the 20 messages are in INBOX.
+				status, err := client.Status("INBOX", []imap.StatusItem{imap.StatusMessages})
+				require.NoError(t, err)
+
+				return uint32(20) == status.Messages
+			}, 10*time.Second, time.Second)
+		})
+	}, server.WithTLS(false))
+}
+
 func withClient(ctx context.Context, t *testing.T, s *server.Server, username string, password []byte, fn func(context.Context, *proton.Client)) { //nolint:unparam
 	m := proton.New(
 		proton.WithHostURL(s.GetHostURL()),