Clean up back-pressed behavior which could result in an empty backstack and crash.

Co-authored-by: Greyson Parrelli <greyson@signal.org>
Co-authored-by: jeffrey-signal <jeffrey@signal.org>

app/src/main/java/org/thoughtcrime/securesms/registration/olddevice/QuickTransferOldDeviceNavigation.kt

@@ -6,6 +6,7 @@
 package org.thoughtcrime.securesms.registration.olddevice
 
 import android.os.Parcelable
+import androidx.activity.compose.LocalOnBackPressedDispatcherOwner
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
@@ -49,6 +50,13 @@ fun TransferAccountNavHost(
 ) {
   val backStack by viewModel.backStack.collectAsStateWithLifecycle()
 
+  val backDispatcher = LocalOnBackPressedDispatcherOwner.current?.onBackPressedDispatcher
+  LaunchedEffect(viewModel, backDispatcher) {
+    viewModel.finishRequests.collect {
+      backDispatcher?.onBackPressed()
+    }
+  }
+
   val entryProvider = entryProvider {
     navigationEntries(
       viewModel = viewModel,

app/src/main/java/org/thoughtcrime/securesms/registration/olddevice/QuickTransferOldDeviceViewModel.kt

@@ -8,8 +8,12 @@ package org.thoughtcrime.securesms.registration.olddevice
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.channels.BufferOverflow
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.receiveAsFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import org.signal.core.util.logging.Log
@@ -41,8 +45,11 @@ class QuickTransferOldDeviceViewModel(reRegisterUri: String) : ViewModel() {
   private val _backStack: MutableStateFlow<List<TransferAccountRoute>> = MutableStateFlow(listOf(TransferAccountRoute.Transfer))
   val backStack: StateFlow<List<TransferAccountRoute>> = _backStack
 
+  private val finishChannel = Channel<Unit>(capacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
+  val finishRequests: Flow<Unit> = finishChannel.receiveAsFlow()
+
   fun goBack() {
-    _backStack.update { it.dropLast(1) }
+    popOrFinish()
   }
 
   fun onEvent(event: PrepareDeviceScreenEvents) {
@@ -51,7 +58,7 @@ class QuickTransferOldDeviceViewModel(reRegisterUri: String) : ViewModel() {
         store.update { it.copy(navigateToBackupCreation = true) }
       }
       PrepareDeviceScreenEvents.NavigateBack -> {
-        _backStack.update { it.dropLast(1) }
+        popOrFinish()
       }
       PrepareDeviceScreenEvents.SkipAndContinue -> {
         _backStack.update { listOf(TransferAccountRoute.Transfer) }
@@ -96,6 +103,14 @@ class QuickTransferOldDeviceViewModel(reRegisterUri: String) : ViewModel() {
     store.update { it.copy(navigateToBackupCreation = false) }
   }
 
+  private fun popOrFinish() {
+    if (_backStack.value.size > 1) {
+      _backStack.update { it.dropLast(1) }
+    } else {
+      finishChannel.trySend(Unit)
+    }
+  }
+
   private fun transferAccount() {
     viewModelScope.launch(Dispatchers.IO) {
       val restoreMethodToken = UUID.randomUUID().toString()

feature/registration/src/main/java/org/signal/registration/RegistrationNavigation.kt

@@ -8,6 +8,7 @@
 package org.signal.registration
 
 import android.os.Parcelable
+import androidx.activity.compose.LocalOnBackPressedDispatcherOwner
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.material3.CircularProgressIndicator
@@ -231,6 +232,13 @@ fun RegistrationNavHost(
     return
   }
 
+  val backDispatcher = LocalOnBackPressedDispatcherOwner.current?.onBackPressedDispatcher
+  LaunchedEffect(viewModel, backDispatcher) {
+    viewModel.finishRequests.collect {
+      backDispatcher?.onBackPressed()
+    }
+  }
+
   val entryProvider = entryProvider {
     navigationEntries(
       registrationRepository = registrationRepository,

feature/registration/src/main/java/org/signal/registration/RegistrationViewModel.kt

@@ -13,9 +13,13 @@ import androidx.lifecycle.ViewModelProvider
 import androidx.lifecycle.createSavedStateHandle
 import androidx.lifecycle.viewModelScope
 import androidx.lifecycle.viewmodel.CreationExtras
+import kotlinx.coroutines.channels.BufferOverflow
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.receiveAsFlow
 import kotlinx.coroutines.launch
 import org.signal.core.ui.navigation.ResultEventBus
 import org.signal.core.util.logging.Log
@@ -35,6 +39,9 @@ class RegistrationViewModel(private val repository: RegistrationRepository, save
   private var _state: MutableStateFlow<RegistrationFlowState> = savedStateHandle.getMutableStateFlow("registration_state", initialValue = RegistrationFlowState())
   val state: StateFlow<RegistrationFlowState> = _state.asStateFlow()
 
+  private val finishChannel = Channel<Unit>(capacity = 1, onBufferOverflow = BufferOverflow.DROP_OLDEST)
+  val finishRequests: Flow<Unit> = finishChannel.receiveAsFlow()
+
   val resultBus = ResultEventBus()
 
   init {
@@ -66,7 +73,14 @@ class RegistrationViewModel(private val repository: RegistrationRepository, save
       is RegistrationFlowEvent.Registered -> state.copy(accountEntropyPool = event.accountEntropyPool)
       is RegistrationFlowEvent.MasterKeyRestoredFromSvr -> state.copy(temporaryMasterKey = event.masterKey)
       is RegistrationFlowEvent.NavigateToScreen -> applyNavigationToScreenEvent(state, event)
-      is RegistrationFlowEvent.NavigateBack -> state.copy(backStack = state.backStack.dropLast(1))
+      is RegistrationFlowEvent.NavigateBack -> {
+        if (state.backStack.size > 1) {
+          state.copy(backStack = state.backStack.dropLast(1))
+        } else {
+          finishChannel.trySend(Unit)
+          state
+        }
+      }
       is RegistrationFlowEvent.RecoveryPasswordInvalid -> state.copy(doNotAttemptRecoveryPassword = true)
       is RegistrationFlowEvent.PendingRestoreOptionSelected -> state.copy(pendingRestoreOption = event.option)
       is RegistrationFlowEvent.UserSuppliedAepSubmitted -> state.copy(unverifiedRestoredAep = event.aep)