Prevent backups from being scheduled twice within the jitter window.

Fixes #13559.

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()
+  }
+}