Prevent backups from being scheduled twice within the jitter window.

Fixes #13559.

app/src/main/java/org/thoughtcrime/securesms/service/LocalBackupListener.java

@@ -47,7 +47,7 @@ public class LocalBackupListener extends PersistentAlarmManagerListener {
     LocalDateTime now    = LocalDateTime.now();
     int           hour   = SignalStore.settings().getBackupHour();
     int           minute = SignalStore.settings().getBackupMinute();
-    LocalDateTime next   = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, hour, minute, BACKUP_JITTER_WINDOW_SECONDS);
+    LocalDateTime next   = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, hour, minute, BACKUP_JITTER_WINDOW_SECONDS, new Random());
 
     long nextTime = JavaTimeExtensionsKt.toMillis(next);
 

app/src/main/java/org/thoughtcrime/securesms/service/MessageBackupListener.kt

@@ -13,7 +13,8 @@ import org.thoughtcrime.securesms.util.RemoteConfig
 import org.thoughtcrime.securesms.util.toMillis
 import java.time.LocalDateTime
 import java.util.Random
-import java.util.concurrent.TimeUnit
+import kotlin.time.Duration.Companion.days
+import kotlin.time.Duration.Companion.minutes
 
 class MessageBackupListener : PersistentAlarmManagerListener() {
   override fun shouldScheduleExact(): Boolean {
@@ -33,8 +34,8 @@ class MessageBackupListener : PersistentAlarmManagerListener() {
   }
 
   companion object {
-    private val BACKUP_JITTER_WINDOW_SECONDS = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    private val BACKUP_JITTER_WINDOW_SECONDS = 10.minutes.inWholeSeconds.toInt()
-    private val BACKUP_MEDIA_SYNC_INTERVAL = TimeUnit.DAYS.toMillis(7)
+    private val BACKUP_MEDIA_SYNC_INTERVAL = 7.days.inWholeMilliseconds
 
     @JvmStatic
     fun schedule(context: Context?) {
@@ -44,22 +45,23 @@ class MessageBackupListener : PersistentAlarmManagerListener() {
     }
 
     @JvmStatic
-    fun getNextDailyBackupTimeFromNowWithJitter(now: LocalDateTime, hour: Int, minute: Int, maxJitterSeconds: Int): LocalDateTime {
+    fun getNextDailyBackupTimeFromNowWithJitter(now: LocalDateTime, hour: Int, minute: Int, maxJitterSeconds: Int, randomSource: Random = Random()): LocalDateTime {
       var next = now.withHour(hour).withMinute(minute).withSecond(0)
 
-      if (!now.plusSeconds(maxJitterSeconds.toLong() / 2).isBefore(next)) {
+      val endOfJitterWindowForNow = now.plusSeconds(maxJitterSeconds.toLong() / 2)
+      while (!endOfJitterWindowForNow.isBefore(next)) {
         next = next.plusDays(1)
       }
 
-      val jitter = Random().nextInt(BACKUP_JITTER_WINDOW_SECONDS) - BACKUP_JITTER_WINDOW_SECONDS / 2
+      val jitter = randomSource.nextInt(maxJitterSeconds) - maxJitterSeconds / 2
       return next.plusSeconds(jitter.toLong())
     }
 
-    fun setNextBackupTimeToIntervalFromNow(): Long {
+    fun setNextBackupTimeToIntervalFromNow(maxJitterSeconds: Int = BACKUP_JITTER_WINDOW_SECONDS): Long {
       val now = LocalDateTime.now()
       val hour = SignalStore.settings.backupHour
       val minute = SignalStore.settings.backupMinute
-      var next = getNextDailyBackupTimeFromNowWithJitter(now, hour, minute, BACKUP_JITTER_WINDOW_SECONDS)
+      var next = getNextDailyBackupTimeFromNowWithJitter(now, hour, minute, maxJitterSeconds)
       next = when (SignalStore.backup.backupFrequency) {
         BackupFrequency.MANUAL -> next.plusDays(364)
         BackupFrequency.MONTHLY -> next.plusDays(29)

app/src/test/java/org/thoughtcrime/securesms/service/BackListenerTest.kt

@@ -8,8 +8,12 @@ package org.thoughtcrime.securesms.service
 import org.junit.Assert
 import org.junit.Test
 import org.thoughtcrime.securesms.BaseUnitTest
+import org.thoughtcrime.securesms.testutil.MockRandom
+import java.time.Duration
 import java.time.LocalDateTime
 import java.util.concurrent.TimeUnit
+import kotlin.time.Duration.Companion.days
+import kotlin.time.Duration.Companion.minutes
 
 class BackListenerTest : BaseUnitTest() {
 
@@ -44,4 +48,14 @@ class BackListenerTest : BaseUnitTest() {
     val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
     Assert.assertEquals(8, next.dayOfMonth)
   }
+
+  @Test
+  fun testBackupJitterWhenScheduledForMidnightButJitterMakesItRunJustBefore() {
+    val mockRandom = MockRandom(listOf(1.minutes.inWholeSeconds.toInt()))
+    val jitterWindowSeconds = 10.minutes.inWholeSeconds.toInt()
+    val now: LocalDateTime = LocalDateTime.of(2024, 6, 27, 23, 57, 0)
+    val next: LocalDateTime = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 0, 0, jitterWindowSeconds, mockRandom)
+
+    Assert.assertTrue(Duration.between(now, next).toSeconds() > (1.days.inWholeSeconds - jitterWindowSeconds))
+  }
 }

app/src/test/java/org/thoughtcrime/securesms/testutil/MockRandom.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.testutil
+
+import java.util.LinkedList
+import java.util.Random
+
+class MockRandom(initialInts: List<Int>) : Random() {
+
+  val nextInts = LinkedList(initialInts)
+
+  override fun nextInt(): Int {
+    return nextInts.remove()
+  }
+}