Merge pull request #7692 from element-hq/bma/fix7689

Fix blank screen at launch when the homeserver expires the sliding sync session

appnav/src/main/kotlin/io/element/android/appnav/LoggedInFlowNode.kt

@@ -96,6 +96,7 @@ import io.element.android.libraries.matrix.api.verification.SessionVerificationS
 import io.element.android.libraries.matrix.api.verification.VerificationRequest
 import io.element.android.libraries.preferences.api.store.AppPreferencesStore
 import io.element.android.libraries.push.api.notifications.conversations.NotificationConversationService
+import io.element.android.libraries.ui.common.nodes.EmptyState
 import io.element.android.libraries.ui.common.nodes.emptyNode
 import io.element.android.services.analytics.api.AnalyticsLongRunningTransaction
 import io.element.android.services.analytics.api.AnalyticsService
@@ -113,6 +114,7 @@ import java.time.Duration
 import java.time.Instant
 import java.util.Optional
 import java.util.UUID
+import kotlin.time.Duration.Companion.milliseconds
 import kotlin.time.Duration.Companion.minutes
 import kotlin.time.Duration.Companion.seconds
 import kotlin.time.toKotlinDuration
@@ -324,7 +326,9 @@ class LoggedInFlowNode(
 
     override fun resolve(navTarget: NavTarget, buildContext: BuildContext): Node {
         return when (navTarget) {
-            NavTarget.Placeholder -> emptyNode(buildContext)
+            // Rendered while the FtueState is Unknown. Render a loading state rather than nothing at all,
+            // since there is no guarantee that this state will be left.
+            NavTarget.Placeholder -> emptyNode(buildContext, state = EmptyState(delayBeforeShowingContent = 500.milliseconds))
             NavTarget.LoggedInPermanent -> {
                 val callback = object : LoggedInNode.Callback {
                     override fun navigateToNotificationTroubleshoot() {

appnav/src/main/kotlin/io/element/android/appnav/session/SyncOrchestrator.kt

@@ -28,10 +28,14 @@ import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.debounce
 import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onCompletion
+import kotlinx.coroutines.flow.onEach
 import kotlinx.coroutines.launch
 import timber.log.Timber
 import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicInteger
+import kotlin.time.Duration
 import kotlin.time.Duration.Companion.milliseconds
 import kotlin.time.Duration.Companion.seconds
 
@@ -60,6 +64,12 @@ class SyncOrchestrator(
 
     private val started = AtomicBoolean(false)
 
+    /**
+     * Number of consecutive attempts at restarting a sync service which stopped on its own.
+     * Reset once the sync service has been running for [RESTART_BACKOFF_RESET_DELAY].
+     */
+    private val restartAttempt = AtomicInteger(0)
+
     /**
      * Starting observing the app state and network state to start/stop the sync service.
      *
@@ -91,6 +101,17 @@ class SyncOrchestrator(
     @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
     internal fun observeStates() = coroutineScope.launch {
         Timber.tag(tag).d("start observing the app and network state")
+        // Reset the restart backoff once the sync service has been steadily running for a while. Note that
+        // SyncService.startSync() moves the state to Running right away, so resetting on any Running value
+        // would defeat the backoff when the sync service dies again immediately after each restart.
+        syncService.syncState
+            .debounce(RESTART_BACKOFF_RESET_DELAY)
+            .onEach { syncState ->
+                if (syncState == SyncState.Running) {
+                    restartAttempt.set(0)
+                }
+            }
+            .launchIn(this)
         val isAppActiveFlows = listOf(
             appForegroundStateService.isInForeground,
             appForegroundStateService.isInCall,
@@ -117,14 +138,26 @@ class SyncOrchestrator(
                 SyncStateAction.StopSync
             } else if (syncState == SyncState.Idle && isAppActive && isNetworkAvailable) {
                 SyncStateAction.StartSync
+            } else if (syncState == SyncState.Error && isAppActive && isNetworkAvailable) {
+                // The sync service has given up, for instance because the homeserver has expired our sliding sync
+                // session and answered 400 M_UNKNOWN_POS. Nothing else will ever restart it, so do it from here,
+                // else the application is stuck until its data is cleared.
+                SyncStateAction.RestartSync
             } else {
                 SyncStateAction.NoOp
             }
         }
             .distinctUntilChanged()
             .debounce { action ->
+                when (action) {
                     // Don't stop the sync immediately, wait a bit to avoid starting/stopping the sync too often
-                if (action == SyncStateAction.StopSync) 3.seconds else 0.seconds
+                    SyncStateAction.StopSync -> 3.seconds
+                    // Back off before restarting a dead sync service, to avoid hammering a failing homeserver.
+                    // A state change in the meantime cancels the pending restart.
+                    SyncStateAction.RestartSync -> restartDelay(restartAttempt.get())
+                    SyncStateAction.StartSync,
+                    SyncStateAction.NoOp -> 0.seconds
+                }
             }
             .onCompletion {
                 Timber.tag(tag).d("has been stopped")
@@ -134,6 +167,11 @@ class SyncOrchestrator(
                     SyncStateAction.StartSync -> {
                         syncService.startSync()
                     }
+                    SyncStateAction.RestartSync -> {
+                        val attempt = restartAttempt.incrementAndGet()
+                        Timber.tag(tag).w("sync service is dead, restarting it (attempt $attempt)")
+                        syncService.startSync()
+                    }
                     SyncStateAction.StopSync -> {
                         syncService.stopSync()
                     }
@@ -143,8 +181,35 @@ class SyncOrchestrator(
     }
 }
 
+@VisibleForTesting
+internal val FIRST_RESTART_DELAY = 1.seconds
+
+/**
+ * Exponential backoff, from [FIRST_RESTART_DELAY], doubling on every attempt, up to [FIRST_RESTART_DELAY * 32].
+ */
+@VisibleForTesting
+internal val RESTART_BACKOFF_DELAYS = listOf(
+    FIRST_RESTART_DELAY,
+    FIRST_RESTART_DELAY * 2,
+    FIRST_RESTART_DELAY * 4,
+    FIRST_RESTART_DELAY * 8,
+    FIRST_RESTART_DELAY * 16,
+    FIRST_RESTART_DELAY * 32,
+)
+
+/**
+ * Duration during which the sync service has to stay running before the restart backoff is reset.
+ */
+@VisibleForTesting
+internal val RESTART_BACKOFF_RESET_DELAY = 30.seconds
+
+private fun restartDelay(attempt: Int): Duration {
+    return RESTART_BACKOFF_DELAYS[attempt.coerceIn(0, RESTART_BACKOFF_DELAYS.lastIndex)]
+}
+
 private enum class SyncStateAction {
     StartSync,
+    RestartSync,
     StopSync,
     NoOp,
 }

appnav/src/test/kotlin/io/element/android/appnav/SyncOrchestratorTest.kt

@@ -8,6 +8,9 @@
 
 package io.element.android.appnav
 
+import io.element.android.appnav.session.FIRST_RESTART_DELAY
+import io.element.android.appnav.session.RESTART_BACKOFF_DELAYS
+import io.element.android.appnav.session.RESTART_BACKOFF_RESET_DELAY
 import io.element.android.appnav.session.SyncOrchestrator
 import io.element.android.features.networkmonitor.api.NetworkStatus
 import io.element.android.features.networkmonitor.test.FakeNetworkMonitor
@@ -414,6 +417,131 @@ class SyncOrchestratorTest {
         startSyncRecorder.assertions().isNeverCalled()
     }
 
+    @Test
+    fun `when the sync service is in error, it will be restarted after a delay`() = runTest {
+        val startSyncRecorder = lambdaRecorder<Result<Unit>> { Result.success(Unit) }
+        val syncService = FakeSyncService(initialSyncState = SyncState.Error).apply {
+            startSyncLambda = startSyncRecorder
+        }
+        val syncOrchestrator = createSyncOrchestrator(
+            matrixClient = FakeMatrixClient(syncService = syncService),
+            networkMonitor = FakeNetworkMonitor(initialStatus = NetworkStatus.Connected),
+            appForegroundStateService = FakeAppForegroundStateService(initialForegroundValue = true),
+        )
+
+        // We start observing
+        syncOrchestrator.observeStates()
+
+        // The restart does not happen right away, we back off first
+        advanceTimeBy(FIRST_RESTART_DELAY / 2)
+        startSyncRecorder.assertions().isNeverCalled()
+
+        // But it does happen once the backoff has elapsed
+        advanceTimeBy(FIRST_RESTART_DELAY)
+        startSyncRecorder.assertions().isCalledOnce()
+    }
+
+    @Test
+    fun `when the sync service keeps dying, the delay between the restarts increases`() = runTest {
+        val startSyncRecorder = lambdaRecorder<Result<Unit>> { Result.success(Unit) }
+        val syncService = FakeSyncService(initialSyncState = SyncState.Error).apply {
+            startSyncLambda = startSyncRecorder
+        }
+        val syncOrchestrator = createSyncOrchestrator(
+            matrixClient = FakeMatrixClient(syncService = syncService),
+            networkMonitor = FakeNetworkMonitor(initialStatus = NetworkStatus.Connected),
+            appForegroundStateService = FakeAppForegroundStateService(initialForegroundValue = true),
+        )
+
+        // We start observing
+        syncOrchestrator.observeStates()
+
+        advanceTimeBy(FIRST_RESTART_DELAY * 2)
+        startSyncRecorder.assertions().isCalledOnce()
+
+        // The sync service starts, but dies again straight away
+        syncService.emitSyncState(SyncState.Running)
+        advanceTimeBy(200.milliseconds)
+        syncService.emitSyncState(SyncState.Error)
+
+        // The second restart is not attempted after the first delay anymore
+        advanceTimeBy(FIRST_RESTART_DELAY + 200.milliseconds)
+        startSyncRecorder.assertions().isCalledOnce()
+
+        // But after twice that delay
+        advanceTimeBy(FIRST_RESTART_DELAY)
+        startSyncRecorder.assertions().isCalledExactly(2)
+    }
+
+    @Test
+    fun `when the sync service recovers, the delay between the restarts is reset`() = runTest {
+        val startSyncRecorder = lambdaRecorder<Result<Unit>> { Result.success(Unit) }
+        val syncService = FakeSyncService(initialSyncState = SyncState.Error).apply {
+            startSyncLambda = startSyncRecorder
+        }
+        val syncOrchestrator = createSyncOrchestrator(
+            matrixClient = FakeMatrixClient(syncService = syncService),
+            networkMonitor = FakeNetworkMonitor(initialStatus = NetworkStatus.Connected),
+            appForegroundStateService = FakeAppForegroundStateService(initialForegroundValue = true),
+        )
+
+        // We start observing
+        syncOrchestrator.observeStates()
+
+        advanceTimeBy(FIRST_RESTART_DELAY * 2)
+        startSyncRecorder.assertions().isCalledOnce()
+
+        // The sync service is now running, and keeps running for a while
+        syncService.emitSyncState(SyncState.Running)
+        advanceTimeBy(RESTART_BACKOFF_RESET_DELAY + 1.seconds)
+        startSyncRecorder.assertions().isCalledOnce()
+
+        // It dies again, and is restarted after the first delay, not a longer one
+        syncService.emitSyncState(SyncState.Error)
+        advanceTimeBy(FIRST_RESTART_DELAY / 2)
+        startSyncRecorder.assertions().isCalledOnce()
+        advanceTimeBy(FIRST_RESTART_DELAY)
+        startSyncRecorder.assertions().isCalledExactly(2)
+    }
+
+    @Test
+    fun `when the sync service is in error but the app is in background, it is not restarted`() = runTest {
+        val startSyncRecorder = lambdaRecorder<Result<Unit>> { Result.success(Unit) }
+        val syncService = FakeSyncService(initialSyncState = SyncState.Error).apply {
+            startSyncLambda = startSyncRecorder
+        }
+        val syncOrchestrator = createSyncOrchestrator(
+            matrixClient = FakeMatrixClient(syncService = syncService),
+            networkMonitor = FakeNetworkMonitor(initialStatus = NetworkStatus.Connected),
+            appForegroundStateService = FakeAppForegroundStateService(initialForegroundValue = false),
+        )
+
+        // We start observing
+        syncOrchestrator.observeStates()
+
+        advanceTimeBy(RESTART_BACKOFF_DELAYS.last() * 2)
+        startSyncRecorder.assertions().isNeverCalled()
+    }
+
+    @Test
+    fun `when the sync service is in error but there is no network, it is not restarted`() = runTest {
+        val startSyncRecorder = lambdaRecorder<Result<Unit>> { Result.success(Unit) }
+        val syncService = FakeSyncService(initialSyncState = SyncState.Error).apply {
+            startSyncLambda = startSyncRecorder
+        }
+        val syncOrchestrator = createSyncOrchestrator(
+            matrixClient = FakeMatrixClient(syncService = syncService),
+            networkMonitor = FakeNetworkMonitor(initialStatus = NetworkStatus.Disconnected),
+            appForegroundStateService = FakeAppForegroundStateService(initialForegroundValue = true),
+        )
+
+        // We start observing
+        syncOrchestrator.observeStates()
+
+        advanceTimeBy(RESTART_BACKOFF_DELAYS.last() * 2)
+        startSyncRecorder.assertions().isNeverCalled()
+    }
+
     private fun TestScope.createSyncOrchestrator(
         matrixClient: FakeMatrixClient = FakeMatrixClient(),
         networkMonitor: FakeNetworkMonitor = FakeNetworkMonitor(),

features/ftue/impl/src/main/kotlin/io/element/android/features/ftue/impl/FtueFlowNode.kt

@@ -32,11 +32,13 @@ import io.element.android.libraries.architecture.BackstackView
 import io.element.android.libraries.architecture.BaseFlowNode
 import io.element.android.libraries.architecture.createNode
 import io.element.android.libraries.di.SessionScope
+import io.element.android.libraries.ui.common.nodes.EmptyState
 import io.element.android.libraries.ui.common.nodes.emptyNode
 import kotlinx.coroutines.flow.filterIsInstance
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
 import kotlinx.parcelize.Parcelize
+import kotlin.time.Duration.Companion.milliseconds
 
 @ContributesNode(SessionScope::class)
 @AssistedInject
@@ -84,7 +86,10 @@ class FtueFlowNode(
     override fun resolve(navTarget: NavTarget, buildContext: BuildContext): Node {
         return when (navTarget) {
             NavTarget.Placeholder -> {
-                emptyNode(buildContext)
+                // Rendered while waiting for the initial state (see FtueStep.WaitingForInitialState).
+                // This can last for a while, or forever if the sync service never manages to run, so
+                // render a loading state rather than nothing at all.
+                emptyNode(buildContext, EmptyState(delayBeforeShowingContent = 500.milliseconds))
             }
             is NavTarget.SessionVerification -> {
                 val callback = object : FtueSessionVerificationFlowNode.Callback {

libraries/ui-common/build.gradle.kts

@@ -18,4 +18,5 @@ dependencies {
     implementation(libs.appyx.core)
     implementation(projects.libraries.core)
     implementation(projects.libraries.designsystem)
+    implementation(projects.libraries.uiStrings)
 }

libraries/ui-common/src/main/kotlin/io/element/android/libraries/ui/common/nodes/EmptyNode.kt

@@ -8,38 +8,23 @@
 
 package io.element.android.libraries.ui.common.nodes
 
-import androidx.compose.foundation.background
-import androidx.compose.foundation.layout.Box
-import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.runtime.Composable
-import androidx.compose.ui.Modifier
 import com.bumble.appyx.core.modality.BuildContext
 import com.bumble.appyx.core.node.Node
 import com.bumble.appyx.core.node.node
-import io.element.android.compound.theme.ElementTheme
+import kotlin.time.Duration
-import io.element.android.libraries.designsystem.preview.ElementPreview
-import io.element.android.libraries.designsystem.preview.PreviewsDayNight
 
 /**
  * Ref: https://www.figma.com/design/0MMNu7cTOzLOlWb7ctTkv3/Element-X?node-id=1518-85323
+ *
+ * @param buildContext the [BuildContext] of the Node.
+ * @param state the [EmptyState] of the Node. Default value will render an empty node forever.
  */
 fun emptyNode(
     buildContext: BuildContext,
+    state: EmptyState = EmptyState(Duration.INFINITE),
 ): Node = node(buildContext) { modifier ->
-    EmptyView(modifier)
+    EmptyView(
-}
+        state = state,
-
+        modifier = modifier,
-@Composable
-private fun EmptyView(
-    modifier: Modifier = Modifier,
-) = Box(
-    modifier = modifier
-        .fillMaxSize()
-        .background(ElementTheme.colors.bgCanvasDefault),
     )
-
-@PreviewsDayNight
-@Composable
-internal fun EmptyViewPreview() = ElementPreview {
-    EmptyView(Modifier)
 }

libraries/ui-common/src/main/kotlin/io/element/android/libraries/ui/common/nodes/EmptyState.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.ui.common.nodes
+
+import kotlin.time.Duration
+
+/**
+ * @property delayBeforeShowingContent when finite, a loading indicator fades in after this delay. Use it for
+ * placeholders which can stay on screen for a long time, or forever, else the application looks frozen.
+ */
+data class EmptyState(
+    val delayBeforeShowingContent: Duration,
+)

libraries/ui-common/src/main/kotlin/io/element/android/libraries/ui/common/nodes/EmptyStatePreviewParam.kt

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.ui.common.nodes
+
+import androidx.compose.ui.tooling.preview.PreviewParameterProvider
+import kotlin.time.Duration
+import kotlin.time.Duration.Companion.seconds
+
+open class EmptyStatePreviewParam : PreviewParameterProvider<EmptyState> {
+    override val values: Sequence<EmptyState>
+        get() = sequenceOf(
+            EmptyState(Duration.INFINITE),
+            EmptyState(1.seconds),
+        )
+}

libraries/ui-common/src/main/kotlin/io/element/android/libraries/ui/common/nodes/EmptyView.kt

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.ui.common.nodes
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.tooling.preview.PreviewParameter
+import androidx.compose.ui.unit.dp
+import io.element.android.compound.theme.ElementTheme
+import io.element.android.libraries.designsystem.preview.ElementPreview
+import io.element.android.libraries.designsystem.preview.PreviewsDayNight
+import io.element.android.libraries.designsystem.theme.components.CircularProgressIndicator
+import io.element.android.libraries.designsystem.theme.components.Text
+import io.element.android.libraries.designsystem.utils.DelayedVisibility
+import io.element.android.libraries.ui.strings.CommonStrings
+
+@Composable
+fun EmptyView(
+    state: EmptyState,
+    modifier: Modifier = Modifier,
+) = Box(
+    modifier = modifier
+        .fillMaxSize()
+        .background(ElementTheme.colors.bgCanvasDefault),
+    contentAlignment = Alignment.Center,
+) {
+    if (state.delayBeforeShowingContent.isFinite()) {
+        DelayedVisibility(duration = state.delayBeforeShowingContent) {
+            LoadingContent()
+        }
+    }
+}
+
+@Composable
+private fun LoadingContent() = Column(
+    modifier = Modifier.padding(16.dp),
+    horizontalAlignment = Alignment.CenterHorizontally,
+    verticalArrangement = Arrangement.spacedBy(16.dp),
+) {
+    CircularProgressIndicator()
+    Text(
+        text = stringResource(CommonStrings.common_loading),
+        color = ElementTheme.colors.textSecondary,
+        style = ElementTheme.typography.fontBodyMdRegular,
+    )
+}
+
+@PreviewsDayNight
+@Composable
+internal fun EmptyViewPreview(
+    @PreviewParameter(EmptyStatePreviewParam::class) state: EmptyState
+) = ElementPreview {
+    EmptyView(state = state)
+}