From da572df89ad31c7866ceefe98cf036784ebb8bca Mon Sep 17 00:00:00 2001
From: Jonas Lochmann <git@inkompetenz.org>
Date: Mon, 17 May 2021 02:00:00 +0200
Subject: [PATCH] Handle race condition during sync

---
 .../android/data/dao/PendingSyncActionDao.kt  |  5 ++-
 .../android/sync/ApplyServerDataStatus.kt     | 13 +++++++-
 .../io/timelimit/android/sync/SyncUtil.kt     | 33 +++++++++++++++++--
 3 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/app/src/main/java/io/timelimit/android/data/dao/PendingSyncActionDao.kt b/app/src/main/java/io/timelimit/android/data/dao/PendingSyncActionDao.kt
index bbd3723..a49e35c 100644
--- a/app/src/main/java/io/timelimit/android/data/dao/PendingSyncActionDao.kt
+++ b/app/src/main/java/io/timelimit/android/data/dao/PendingSyncActionDao.kt
@@ -1,5 +1,5 @@
 /*
- * TimeLimit Copyright <C> 2019 Jonas Lochmann
+ * TimeLimit Copyright <C> 2019 - 2021 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -51,6 +51,9 @@ interface PendingSyncActionDao {
     @Query("SELECT COUNT(*) FROM pending_sync_action")
     fun countAllActionsLive(): LiveData<Long>
 
+    @Query("SELECT COUNT(*) FROM pending_sync_action")
+    fun countAllActions(): Long
+
     @Query("SELECT * FROM pending_sync_action WHERE scheduled_for_upload = 0 ORDER BY sequence_number DESC")
     fun getLatestUnscheduledActionSync(): PendingSyncAction?
 
diff --git a/app/src/main/java/io/timelimit/android/sync/ApplyServerDataStatus.kt b/app/src/main/java/io/timelimit/android/sync/ApplyServerDataStatus.kt
index 2f9b96e..e1fd438 100644
--- a/app/src/main/java/io/timelimit/android/sync/ApplyServerDataStatus.kt
+++ b/app/src/main/java/io/timelimit/android/sync/ApplyServerDataStatus.kt
@@ -1,5 +1,5 @@
 /*
- * TimeLimit Copyright <C> 2019 - 2020 Jonas Lochmann
+ * TimeLimit Copyright <C> 2019 - 2021 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -36,6 +36,15 @@ object ApplyServerDataStatus {
 
     fun applyServerDataStatusSync(status: ServerDataStatus, database: Database, platformIntegration: PlatformIntegration) {
         database.runInTransaction {
+            // this would override some local data which was not sent yet
+            // so it's better to cancel in this case (or, more complicated,
+            // apply the delta which was not sent locally)
+            //
+            // locking the database during a sync is no option as the network
+            // connection could be really bad
+            if (database.pendingSyncAction().countAllActions() > 0)
+                throw PendingSyncActionException()
+
             run {
                 // apply ful version until and message
 
@@ -539,4 +548,6 @@ object ApplyServerDataStatus {
             }
         }
     }
+
+    class PendingSyncActionException: RuntimeException()
 }
diff --git a/app/src/main/java/io/timelimit/android/sync/SyncUtil.kt b/app/src/main/java/io/timelimit/android/sync/SyncUtil.kt
index 38474a0..b9feea9 100644
--- a/app/src/main/java/io/timelimit/android/sync/SyncUtil.kt
+++ b/app/src/main/java/io/timelimit/android/sync/SyncUtil.kt
@@ -1,5 +1,5 @@
 /*
- * TimeLimit Copyright <C> 2019 - 2020 Jonas Lochmann
+ * TimeLimit Copyright <C> 2019 - 2021 Jonas Lochmann
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -33,6 +33,7 @@ import io.timelimit.android.sync.network.api.UnauthorizedHttpError
 import io.timelimit.android.ui.IsAppInForeground
 import io.timelimit.android.work.SyncInBackgroundWorker
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
 import kotlinx.coroutines.withContext
@@ -190,8 +191,7 @@ class SyncUtil (private val logic: AppLogic) {
             }
 
             try {
-                pushActions(server = server)
-                pullStatus(server = server)
+                doSyncRoundRetryOnNewActions(server)
 
                 logic.syncNotificationLogic.sync(forceUiSync = false)
             } catch (ex: UnauthorizedHttpError) {
@@ -214,6 +214,33 @@ class SyncUtil (private val logic: AppLogic) {
         }
     }
 
+    private suspend fun doSyncRoundRetryOnNewActions(server: ServerLogic.ServerConfig) {
+        // retry two times
+        for (i in 1..2) {
+            try {
+                doSyncRound(server = server)
+
+                return
+            } catch (ex: ApplyServerDataStatus.PendingSyncActionException) {
+                if (BuildConfig.DEBUG) {
+                    Log.d(LOG_TAG, "got new actions while receiving the status => retry")
+                }
+
+                delay(250)
+
+                continue
+            }
+        }
+
+        // and do not catch at the last time
+        doSyncRound(server = server)
+    }
+
+    private suspend fun doSyncRound(server: ServerLogic.ServerConfig) {
+        pushActions(server = server)
+        pullStatus(server = server)
+    }
+
     private suspend fun pushActions(server: ServerLogic.ServerConfig) {
         upload.uploadActions(server)
     }