Fix backup jitter and add unit tests.

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

@@ -47,14 +47,7 @@ public class LocalBackupListener extends PersistentAlarmManagerListener {
     LocalDateTime now    = LocalDateTime.now();
     int           hour   = SignalStore.settings().getBackupHour();
     int           minute = SignalStore.settings().getBackupMinute();
-    LocalDateTime next   = now.withHour(hour).withMinute(minute).withSecond(0);
+    LocalDateTime next   = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, hour, minute, BACKUP_JITTER_WINDOW_SECONDS);
-
-    if (now.isAfter(next)) {
-      next = next.plusDays(1);
-    }
-
-    int jitter = (new Random().nextInt(BACKUP_JITTER_WINDOW_SECONDS)) - (BACKUP_JITTER_WINDOW_SECONDS / 2);
-    next = next.plusSeconds(jitter);
 
     long nextTime = JavaTimeExtensionsKt.toMillis(next);
 

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

@@ -41,21 +41,28 @@ class MessageBackupListener : PersistentAlarmManagerListener() {
       }
     }
 
+    @JvmStatic
+    fun getNextDailyBackupTimeFromNowWithJitter(now: LocalDateTime, hour: Int, minute: Int, maxJitterSeconds: Int): LocalDateTime {
+      var next = now.withHour(hour).withMinute(minute).withSecond(0)
+
+      if (!now.plusSeconds(maxJitterSeconds.toLong() / 2).isBefore(next)) {
+        next = next.plusDays(1)
+      }
+
+      val jitter = Random().nextInt(BACKUP_JITTER_WINDOW_SECONDS) - BACKUP_JITTER_WINDOW_SECONDS / 2
+      return next.plusSeconds(jitter.toLong())
+    }
+
     fun setNextBackupTimeToIntervalFromNow(): Long {
       val now = LocalDateTime.now()
       val hour = SignalStore.settings().backupHour
       val minute = SignalStore.settings().backupMinute
-      var next = now.withHour(hour).withMinute(minute).withSecond(0)
+      var next = getNextDailyBackupTimeFromNowWithJitter(now, hour, minute, BACKUP_JITTER_WINDOW_SECONDS)
-      val jitter = Random().nextInt(BACKUP_JITTER_WINDOW_SECONDS) - BACKUP_JITTER_WINDOW_SECONDS / 2
-      next.plusSeconds(jitter.toLong())
       next = when (SignalStore.backup().backupFrequency) {
-        BackupFrequency.DAILY -> next.plusDays(1)
+        BackupFrequency.MANUAL -> next.plusDays(364)
-        BackupFrequency.MANUAL -> next.plusDays(365)
+        BackupFrequency.MONTHLY -> next.plusDays(29)
-        BackupFrequency.MONTHLY -> next.plusDays(30)
+        BackupFrequency.WEEKLY -> next.plusDays(6)
-        BackupFrequency.WEEKLY -> next.plusDays(7)
+        else -> next
-      }
-      if (now.isAfter(next)) {
-        next = next.plusDays(1)
       }
       val nextTime = next.toMillis()
       SignalStore.backup().nextBackupTime = nextTime

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

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.service
+
+import org.junit.Assert
+import org.junit.Test
+import org.thoughtcrime.securesms.BaseUnitTest
+import java.time.LocalDateTime
+import java.util.concurrent.TimeUnit
+
+class BackListenerTest : BaseUnitTest() {
+
+  @Test
+  fun testBackupJitterExactlyWithinJitterWindow() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 2, 55)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(8, next.dayOfMonth)
+  }
+
+  @Test
+  fun testBackupJitterWithinJitterWindow() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 2, 58)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(8, next.dayOfMonth)
+  }
+
+  @Test
+  fun testBackupJitterJustOutsideOfWindow() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 2, 54, 59)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(7, next.dayOfMonth)
+  }
+
+  @Test
+  fun testBackupJitter() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 3, 15, 0)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(8, next.dayOfMonth)
+  }
+}