From 25da95364f62624be98404df64afae4d699f134c Mon Sep 17 00:00:00 2001
From: Ashish Yadav <48384865+criticalAY@users.noreply.github.com>
Date: Thu, 2 Jul 2026 04:14:56 +0530
Subject: [PATCH] fix(deck-picker): do not block the main thread on startup
 during sync

DeckPicker recreation (for example a light/dark theme change) ran
InitialActivity.getStartupFailureType on the main thread, which waits on
the collection queue. A sync stuck on an unresponsive server holds that
queue for the entire network call, so the recreated activity froze and
ANRed after 5 seconds. Run the check on Dispatchers.IO and deliver the
result through flowOfStartupResponse instead.

Assisted-by: Claude Opus 4.8 (some part of the PR)
---
 .../anki/deckpicker/DeckPickerViewModel.kt    | 24 +++++---
 .../deckpicker/DeckPickerViewModelTest.kt     | 58 +++++++++++++++++++
 2 files changed, 74 insertions(+), 8 deletions(-)

diff --git a/AnkiDroid/src/main/java/com/ichi2/anki/deckpicker/DeckPickerViewModel.kt b/AnkiDroid/src/main/java/com/ichi2/anki/deckpicker/DeckPickerViewModel.kt
index 182c2ac6b519..ad171606af5d 100644
--- a/AnkiDroid/src/main/java/com/ichi2/anki/deckpicker/DeckPickerViewModel.kt
+++ b/AnkiDroid/src/main/java/com/ichi2/anki/deckpicker/DeckPickerViewModel.kt
@@ -522,17 +522,25 @@ class DeckPickerViewModel :
         }
 
         Timber.d("handleStartup: Continuing after permission granted")
-        val failure = InitialActivity.getStartupFailureType(environment::initializeAnkiDroidFolder)
-        if (failure != null) {
-            flowOfStartupResponse.value = StartupResponse.FatalError(failure)
-            return
-        }
+        viewModelScope.launch {
+            // opening the collection waits on the collection queue, which a sync stuck on an
+            // unresponsive server can hold for a long time. Waiting on the main thread here
+            // froze the DeckPicker when it was recreated
+            val failure =
+                withContext(Dispatchers.IO) {
+                    InitialActivity.getStartupFailureType(environment::initializeAnkiDroidFolder)
+                }
+            if (failure != null) {
+                flowOfStartupResponse.value = StartupResponse.FatalError(failure)
+                return@launch
+            }
 
-        // successful startup
+            // successful startup
 
-        configureRenderingMode()
+            configureRenderingMode()
 
-        flowOfStartupResponse.value = StartupResponse.Success
+            flowOfStartupResponse.value = StartupResponse.Success
+        }
     }
 
     interface AnkiDroidEnvironment {
diff --git a/AnkiDroid/src/test/java/com/ichi2/anki/deckpicker/DeckPickerViewModelTest.kt b/AnkiDroid/src/test/java/com/ichi2/anki/deckpicker/DeckPickerViewModelTest.kt
index a32f9c861bde..387ee3cdae7d 100644
--- a/AnkiDroid/src/test/java/com/ichi2/anki/deckpicker/DeckPickerViewModelTest.kt
+++ b/AnkiDroid/src/test/java/com/ichi2/anki/deckpicker/DeckPickerViewModelTest.kt
@@ -23,18 +23,25 @@ import anki.card_rendering.EmptyCardsReport
 import anki.card_rendering.emptyCardsReport
 import app.cash.turbine.test
 import com.ichi2.anki.CollectionManager.withCol
+import com.ichi2.anki.PermissionSet
 import com.ichi2.anki.RobolectricTest
 import com.ichi2.anki.libanki.Consts
 import com.ichi2.anki.libanki.DeckId
 import com.ichi2.anki.libanki.Note
 import com.ichi2.anki.libanki.emptyCids
 import com.ichi2.testutils.ensureOpsExecuted
+import kotlinx.coroutines.runBlocking
 import org.hamcrest.CoreMatchers.not
 import org.hamcrest.MatcherAssert.assertThat
 import org.hamcrest.Matchers.equalTo
+import org.hamcrest.Matchers.lessThan
 import org.junit.Test
 import org.junit.runner.RunWith
 import timber.log.Timber
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.TimeUnit
+import kotlin.concurrent.thread
+import kotlin.system.measureTimeMillis
 import kotlin.test.assertEquals
 
 /** Test of [DeckPickerViewModel] */
@@ -239,4 +246,55 @@ class DeckPickerViewModelTest : RobolectricTest() {
             }
         }
     }
+
+    @Test
+    fun `handleStartup does not block while a hung sync holds the collection queue`() {
+        col
+        val queueHeld = CountDownLatch(1)
+        val releaseQueue = CountDownLatch(1)
+        // a sync against an unresponsive server runs `withCol { syncCollection(...) }`,
+        // holding the collection queue for the whole network call
+        val hungSync =
+            thread(name = "hung-sync") {
+                runBlocking {
+                    withCol {
+                        queueHeld.countDown()
+                        releaseQueue.await()
+                    }
+                }
+            }
+        try {
+            assertThat("sync is holding the collection queue", queueHeld.await(5, TimeUnit.SECONDS), equalTo(true))
+
+            // if handleStartup blocks on the queue, free it after a delay so the test fails with a message instead of hanging
+            thread(name = "watchdog") {
+                if (!releaseQueue.await(WATCHDOG_MS, TimeUnit.MILLISECONDS)) {
+                    releaseQueue.countDown()
+                }
+            }
+
+            val elapsedMillis = measureTimeMillis { viewModel.handleStartup(grantedPermissionsEnvironment) }
+
+            assertThat(
+                "handleStartup waited ${elapsedMillis}ms for the collection queue. On a device this ANRs when DeckPicker is recreated while a sync is stuck on an unresponsive server",
+                elapsedMillis,
+                lessThan(WATCHDOG_MS),
+            )
+        } finally {
+            releaseQueue.countDown()
+            hungSync.join()
+        }
+    }
+
+    private val grantedPermissionsEnvironment =
+        object : DeckPickerViewModel.AnkiDroidEnvironment {
+            override fun hasRequiredPermissions() = true
+
+            override val requiredPermissions: PermissionSet
+                get() = error("unused: permissions are granted")
+
+            override fun initializeAnkiDroidFolder() = true
+        }
 }
+
+private const val WATCHDOG_MS = 2000L