feat: Implemented Lock Mechanism for Synchronization.

- Refactored beforeSync() to incorporate a robust locking mechanism.
- Added backoff strategy to handle race conditions during concurrent sync attempts.
- Ensured single instance of sync data file on Google Drive to prevent duplication.
- Optimized lock file checking and handling for efficient synchronization process.

This update significantly improves the reliability and efficiency of the syncing process by preventing race conditions and ensuring that synchronization actions are atomic.

Signed-off-by: KaiserBh <kaiserbh@proton.me>

app/src/main/java/eu/kanade/tachiyomi/data/sync/service/GoogleDriveSyncService.kt

@@ -20,6 +20,7 @@ import com.google.api.services.drive.DriveScopes
 import com.google.api.services.drive.model.File
 import eu.kanade.tachiyomi.R
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.withContext
 import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
@@ -32,6 +33,7 @@ import java.io.ByteArrayInputStream
 import java.io.ByteArrayOutputStream
 import java.io.IOException
 import java.io.InputStreamReader
+import java.time.Instant
 import java.util.zip.GZIPInputStream
 import java.util.zip.GZIPOutputStream
 
@@ -57,12 +59,50 @@ class GoogleDriveSyncService(context: Context, json: Json, syncPreferences: Sync
 
     private val remoteFileName = "tachiyomi_sync_data.gz"
 
+    private val lockFileName = "tachiyomi_sync.lock"
+
     private val googleDriveService = GoogleDriveService(context)
 
-    override suspend fun beforeSync() = googleDriveService.refreshToken()
+    override suspend fun beforeSync() {
+        googleDriveService.refreshToken()
+        val drive = googleDriveService.driveService ?: throw Exception("Google Drive service not initialized")
+
+        var backoff = 2000L // Start with 2 seconds
+
+        while (true) {
+            val lockFiles = findLockFile(drive) // Fetch the current list of lock files
+
+            when {
+                lockFiles.isEmpty() -> {
+                    // No lock file exists, try to create a new one
+                    createLockFile(drive)
+                }
+                lockFiles.size == 1 -> {
+                    // Exactly one lock file exists
+                    val lockFile = lockFiles.first()
+                    val createdTime = Instant.parse(lockFile.createdTime.toString())
+                    val ageMinutes = java.time.Duration.between(createdTime, Instant.now()).toMinutes()
+                    if (ageMinutes <= 3) {
+                        // Lock file is new and presumably held by this process, break the loop to proceed
+                        break
+                    } else {
+                        // Lock file is old, delete and attempt to create a new one
+                        deleteLockFile(drive)
+                        createLockFile(drive)
+                    }
+                }
+                else -> {
+                    // More than one lock file exists, likely due to a race condition
+                    delay(backoff) // Apply backoff strategy
+                    backoff = (backoff * 2).coerceAtMost(32000L) // Max backoff of 32 seconds
+                }
+            }
+            // The loop continues until it can confirm that there's exactly one new lock file.
+        }
+    }
 
     override suspend fun pullSyncData(): SyncData? {
-        val drive = googleDriveService.googleDriveService
+        val drive = googleDriveService.driveService
 
         // Check if the Google Drive service is initialized
         if (drive == null) {
@@ -89,27 +129,11 @@ class GoogleDriveSyncService(context: Context, json: Json, syncPreferences: Sync
 
     override suspend fun pushSyncData(syncData: SyncData) {
         val jsonData = json.encodeToString(syncData)
+        val drive = googleDriveService.driveService ?: throw Exception(context.getString(R.string.google_drive_not_signed_in))
 
-        val drive = googleDriveService.googleDriveService
-
-        // Check if the Google Drive service is initialized
-        if (drive == null) {
-            logcat(LogPriority.ERROR) { "Google Drive service not initialized" }
-            throw Exception(context.getString(R.string.google_drive_not_signed_in))
-        }
-
-        // delete file if exists
         val fileList = getFileList(drive)
-        if (fileList.isNotEmpty()) {
-            drive.files().delete(fileList[0].id).execute()
-        }
-
-        val fileMetadata = File()
-        fileMetadata.name = remoteFileName
-        fileMetadata.mimeType = "application/gzip"
 
         val byteArrayOutputStream = ByteArrayOutputStream()
-
         withContext(Dispatchers.IO) {
             val gzipOutputStream = GZIPOutputStream(byteArrayOutputStream)
             gzipOutputStream.write(jsonData.toByteArray(Charsets.UTF_8))
@@ -117,24 +141,98 @@ class GoogleDriveSyncService(context: Context, json: Json, syncPreferences: Sync
         }
 
         val byteArrayContent = ByteArrayContent("application/octet-stream", byteArrayOutputStream.toByteArray())
-        val uploadedFile = drive.files().create(fileMetadata, byteArrayContent)
-            .setFields("id")
-            .execute()
 
-        logcat(LogPriority.DEBUG) { "Created sync data file in Google Drive with file ID: ${uploadedFile.id}" }
+        try {
+            if (fileList.isNotEmpty()) {
+                // File exists, so update it
+                val fileId = fileList[0].id
+                drive.files().update(fileId, null, byteArrayContent).execute()
+                logcat(LogPriority.DEBUG) { "Updated existing sync data file in Google Drive with file ID: $fileId" }
+            } else {
+                // File doesn't exist, so create it
+                val fileMetadata = File().apply {
+                    name = remoteFileName
+                    mimeType = "application/gzip"
+                }
+                val uploadedFile = drive.files().create(fileMetadata, byteArrayContent)
+                    .setFields("id")
+                    .execute()
+
+                logcat(LogPriority.DEBUG) { "Created new sync data file in Google Drive with file ID: ${uploadedFile.id}" }
+            }
+
+            // Data has been successfully pushed or updated, delete the lock file
+            deleteLockFile(drive)
+        } catch (e: Exception) {
+            logcat(LogPriority.ERROR) { "Failed to push or update sync data: ${e.message}" }
+        }
     }
 
     private fun getFileList(drive: Drive): MutableList<File> {
-        // Search for the existing file by name
-        val query = "mimeType='application/gzip' and trashed = false and name = '$remoteFileName'"
-        val fileList = drive.files().list().setQ(query).execute().files
-        Log.d("GoogleDrive", "File list: $fileList")
+        try {
+            // Search for the existing file by name
+            val query = "mimeType='application/gzip' and trashed = false and name = '$remoteFileName'"
+            val fileList = drive.files().list().setQ(query).execute().files
+            Log.d("GoogleDrive", "File list: $fileList")
 
-        return fileList
+            return fileList
+        } catch (e: Exception) {
+            Log.e("GoogleDrive", "Error no sync data found: ${e.message}")
+            return mutableListOf()
+        }
+    }
+
+    private fun createLockFile(drive: Drive) {
+        try {
+            val fileMetadata = File()
+            fileMetadata.name = lockFileName
+            fileMetadata.mimeType = "text/plain"
+
+            // Create an empty content to upload as the lock file
+            val emptyContent = ByteArrayContent.fromString("text/plain", "")
+
+            val file = drive.files().create(fileMetadata, emptyContent)
+                .setFields("id, name, createdTime")
+                .execute()
+
+            Log.d("GoogleDrive", "Created lock file with ID: ${file.id}")
+        } catch (e: Exception) {
+            Log.e("GoogleDrive", "Error creating lock file: ${e.message}")
+        }
+    }
+
+    private fun findLockFile(drive: Drive): MutableList<File> {
+        try {
+            val query = "mimeType='text/plain' and trashed = false and name = '$lockFileName'"
+            val fileList = drive.files().list().setQ(query).setFields("files(id, name, createdTime)").execute().files
+            Log.d("GoogleDrive", "Lock file search result: $fileList")
+            return fileList
+        } catch (e: Exception) {
+            Log.e("GoogleDrive", "Error finding lock file: ${e.message}")
+            return mutableListOf()
+        }
+    }
+
+    private fun deleteLockFile(drive: Drive) {
+        try {
+            val lockFiles = findLockFile(drive)
+
+            if (lockFiles.isNotEmpty()) {
+                for (file in lockFiles) {
+                    drive.files().delete(file.id).execute()
+                    Log.d("GoogleDrive", "Deleted lock file with ID: ${file.id}")
+                }
+            } else {
+                Log.d("GoogleDrive", "No lock file found to delete.")
+            }
+        } catch (e: Exception) {
+            Log.e("GoogleDrive", "Error deleting lock file: ${e.message}")
+            throw Exception(context.getString(R.string.error_deleting_google_drive_lock_file))
+        }
     }
 
     suspend fun deleteSyncDataFromGoogleDrive(): DeleteSyncDataStatus {
-        val drive = googleDriveService.googleDriveService
+        val drive = googleDriveService.driveService
 
         if (drive == null) {
             logcat(LogPriority.ERROR) { "Google Drive service not initialized" }
@@ -160,7 +258,7 @@ class GoogleDriveSyncService(context: Context, json: Json, syncPreferences: Sync
 }
 
 class GoogleDriveService(private val context: Context) {
-    var googleDriveService: Drive? = null
+    var driveService: Drive? = null
     companion object {
         const val REDIRECT_URI = "eu.kanade.google.oauth:/oauth2redirect"
     }
@@ -179,7 +277,7 @@ class GoogleDriveService(private val context: Context) {
         val refreshToken = syncPreferences.googleDriveRefreshToken().get()
 
         if (accessToken == "" || refreshToken == "") {
-            googleDriveService = null
+            driveService = null
             return
         }
 
@@ -296,7 +394,7 @@ class GoogleDriveService(private val context: Context) {
         credential.accessToken = accessToken
         credential.refreshToken = refreshToken
 
-        googleDriveService = Drive.Builder(
+        driveService = Drive.Builder(
             NetHttpTransport(),
             jsonFactory,
             credential,

i18n/src/commonMain/resources/MR/base/strings.xml

@@ -577,6 +577,7 @@
     <string name="google_drive_login_success">Logged in to Google Drive</string>
     <string name="google_drive_login_failed">Failed to log in to Google Drive: %s</string>
     <string name="google_drive_not_signed_in">Not signed in to Google Drive</string>
+    <string name="error_deleting_google_drive_lock_file">Error Deleting Google Drive Lock File</string>
     <string name="pref_purge_confirmation_title">Purge confirmation</string>
     <string name="pref_purge_confirmation_message">Purging sync data will delete all your sync data from Google Drive. Are you sure you want to continue?</string>