feat(reviewers): use obscure fixed port with setting

1. Put the fixed port under an opt-in developer option due to uncertainty about the security until someone takes a deeper look in the matter
2. Randomize the initial value of the port so the port isn't publicly known

Author: BrayanDSO

AnkiDroid/src/main/java/com/ichi2/anki/AbstractFlashcardViewer.kt

@@ -127,7 +127,7 @@ import com.ichi2.anki.settings.Prefs
 import com.ichi2.anki.snackbar.BaseSnackbarBuilderProvider
 import com.ichi2.anki.snackbar.SnackbarBuilder
 import com.ichi2.anki.snackbar.showSnackbar
-import com.ichi2.anki.ui.windows.reviewer.ReviewerViewModel
+import com.ichi2.anki.ui.windows.reviewer.ReviewerFragment
 import com.ichi2.anki.utils.OnlyOnce.Method.ANSWER_CARD
 import com.ichi2.anki.utils.OnlyOnce.preventSimultaneousExecutions
 import com.ichi2.anki.utils.ext.showDialogFragment
@@ -163,8 +163,6 @@ import kotlinx.coroutines.runBlocking
 import timber.log.Timber
 import java.io.File
 import java.io.UnsupportedEncodingException
-import java.net.BindException
-import java.net.ServerSocket
 import java.net.URLDecoder
 import java.util.concurrent.locks.Lock
 import java.util.concurrent.locks.ReadWriteLock
@@ -272,7 +270,7 @@ abstract class AbstractFlashcardViewer :
 
     // needs to be lateinit due to a reliance on Context
 
-    val server: AnkiServer
+    lateinit var server: AnkiServer
 
     @get:VisibleForTesting
     var cardContent: String? = null
@@ -363,16 +361,6 @@ abstract class AbstractFlashcardViewer :
     }
 
     init {
-        val port =
-            try {
-                ServerSocket(ReviewerViewModel.DEFAULT_PORT).use {
-                    it.reuseAddress = true
-                    it.localPort
-                }
-            } catch (_: BindException) {
-                0
-            }
-        server = AnkiServer(this, port).also { it.start() }
         ChangeManager.subscribe(this)
     }
 
@@ -561,6 +549,8 @@ abstract class AbstractFlashcardViewer :
 
         setContentView(getContentViewAttr(fullscreenMode))
 
+        val port = ReviewerFragment.getServerPort()
+        server = AnkiServer(this, port).also { it.start() }
         // Make ACTION_PROCESS_TEXT for in-app searching possible on > Android 4.0
         delegate.isHandleNativeActionModesEnabled = true
         val mainView = findViewById<View>(android.R.id.content)
@@ -655,7 +645,9 @@ abstract class AbstractFlashcardViewer :
 
     override fun onDestroy() {
         super.onDestroy()
-        server.stop()
+        if (this::server.isInitialized) {
+            server.stop()
+        }
         tts.releaseTts(this)
         // WebView.destroy() should be called after the end of use
         // http://developer.android.com/reference/android/webkit/WebView.html#destroy()

AnkiDroid/src/main/java/com/ichi2/anki/settings/Prefs.kt

@@ -198,4 +198,8 @@ object Prefs {
     var isDevOptionsEnabled: Boolean
         get() = getBoolean(R.string.dev_options_enabled_by_user_key, false) || BuildConfig.DEBUG
         set(value) = putBoolean(R.string.dev_options_enabled_by_user_key, value)
+
+    var useFixedPortInReviewer by booleanPref(R.string.use_fixed_port_pref_key, false)
+
+    var reviewerPort by intPref(R.string.reviewer_port_pref_key, defaultValue = 0)
 }

AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt

@@ -93,6 +93,9 @@ import com.ichi2.anki.utils.ext.window
 import com.ichi2.anki.utils.isWindowCompact
 import com.ichi2.libanki.sched.Counts
 import com.ichi2.utils.dp
+import timber.log.Timber
+import java.net.BindException
+import java.net.ServerSocket
 
 class ReviewerFragment :
     CardViewerFragment(R.layout.reviewer2),
@@ -101,7 +104,8 @@ class ReviewerFragment :
     DispatchKeyEventListener,
     TagsDialogListener {
     override val viewModel: ReviewerViewModel by viewModels {
-        ReviewerViewModel.factory(CardMediaPlayer(), BindingMap(sharedPrefs(), ViewerAction.entries))
+        val bindingMap = BindingMap(sharedPrefs(), ViewerAction.entries)
+        ReviewerViewModel.factory(CardMediaPlayer(), bindingMap, getServerPort())
     }
 
     override val webView: WebView
@@ -539,5 +543,23 @@ class ReviewerFragment :
 
     companion object {
         fun getIntent(context: Context): Intent = CardViewerActivity.getIntent(context, ReviewerFragment::class)
+
+        fun getServerPort(): Int {
+            if (!Prefs.useFixedPortInReviewer) return 0
+            return try {
+                ServerSocket(Prefs.reviewerPort)
+                    .use {
+                        it.reuseAddress = true
+                        it.localPort
+                    }.also {
+                        if (Prefs.reviewerPort == 0) {
+                            Prefs.reviewerPort = it
+                        }
+                    }
+            } catch (_: BindException) {
+                Timber.w("Fixed port %d under use. Using dynamic port", Prefs.reviewerPort)
+                0
+            }
+        }
     }
 }

AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerViewModel.kt

@@ -71,12 +71,11 @@ import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
 import timber.log.Timber
-import java.net.BindException
-import java.net.ServerSocket
 
 class ReviewerViewModel(
     cardMediaPlayer: CardMediaPlayer,
     private val bindingMap: BindingMap<ReviewerBinding, ViewerAction>,
+    serverPort: Int = 0,
 ) : CardViewerViewModel(cardMediaPlayer),
     ChangeManager.Subscriber,
     BindingProcessor<ReviewerBinding, ViewerAction> {
@@ -103,7 +102,7 @@ class ReviewerViewModel(
     val editNoteTagsFlow = MutableSharedFlow<NoteId>()
     val setDueDateFlow = MutableSharedFlow<CardId>()
 
-    override val server: AnkiServer
+    override val server: AnkiServer = AnkiServer(this, serverPort).also { it.start() }
     private val stateMutationKey = TimeManager.time.intTimeMS().toString()
     val statesMutationEval = MutableSharedFlow<String>()
 
@@ -131,17 +130,6 @@ class ReviewerViewModel(
         }
 
     init {
-        val port =
-            try {
-                ServerSocket(DEFAULT_PORT).use {
-                    it.reuseAddress = true
-                    it.localPort
-                }
-            } catch (_: BindException) {
-                0
-            }
-        server = AnkiServer(this, port).also { it.start() }
-
         bindingMap.setProcessor(this)
         ChangeManager.subscribe(this)
         launchCatchingIO {
@@ -662,20 +650,14 @@ class ReviewerViewModel(
     }
 
     companion object {
-        /**
-         * Default port of the Reviewer's [AnkiServer].
-         * Using a static port makes the URL constant, and that
-         * makes possible to use JavaScript's `localStorage`.
-         */
-        const val DEFAULT_PORT = 40001
-
         fun factory(
             soundPlayer: CardMediaPlayer,
             bindingMap: BindingMap<ReviewerBinding, ViewerAction>,
+            serverPort: Int,
         ): ViewModelProvider.Factory =
             viewModelFactory {
                 initializer {
-                    ReviewerViewModel(soundPlayer, bindingMap)
+                    ReviewerViewModel(soundPlayer, bindingMap, serverPort)
                 }
             }
 

AnkiDroid/src/main/res/values/preferences.xml

@@ -204,6 +204,8 @@
     <string name="pref_new_notifications">newNotifications</string>
     <string name="dev_options_enabled_by_user_key">devOptionsEnabledByUser</string>
     <string name="pref_cat_wip_key">workInProgressDevOptions</string>
+    <string name="reviewer_port_pref_key">reviewerPort</string>
+    <string name="use_fixed_port_pref_key">useFixedPort</string>
     <!-- Developer options > Create fake notes -->
     <string name="pref_fill_default_deck_number_key">FillDefaultNumberDeck</string>
     <string name="pref_fill_default_deck_key">FillDefaultDeck</string>

AnkiDroid/src/main/res/xml/preferences_dev_options.xml

@@ -48,6 +48,10 @@
         android:title="Set Database to pre-Scoped Storage default"
         android:summary="Touch here to set the database path to /storage/emulated/0/AnkiDroid"
         android:key="@string/pref_set_database_path_debug_key"/>
+    <SwitchPreferenceCompat
+        android:title="Use fixed port in the Reviewer"
+        android:key="@string/use_fixed_port_pref_key"
+        android:defaultValue="false"/>
     <SwitchPreferenceCompat
         android:title="New congrats screen"
         android:key="@string/new_congrats_screen_pref_key"