Add mutex to "updateExtensionDatabase" (#829)

If called in quick succession it is possible that duplicated extensions get inserted to the database, because it has not yet been updated by the first call

server/src/main/kotlin/suwayomi/tachidesk/manga/impl/extension/ExtensionsList.kt

@@ -8,6 +8,8 @@ package suwayomi.tachidesk.manga.impl.extension
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 import eu.kanade.tachiyomi.source.local.LocalSource
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import mu.KotlinLogging
 import org.jetbrains.exposed.dao.id.EntityID
 import org.jetbrains.exposed.sql.ResultRow
@@ -86,7 +88,10 @@ object ExtensionsList {
             }
         }
 
-    private fun updateExtensionDatabase(foundExtensions: List<OnlineExtension>) {
+    private val updateExtensionDatabaseMutex = Mutex()
+
+    private suspend fun updateExtensionDatabase(foundExtensions: List<OnlineExtension>) {
+        updateExtensionDatabaseMutex.withLock {
             transaction {
                 val uniqueExtensions =
                     foundExtensions.groupBy { it.pkgName }.mapValues {
@@ -200,6 +205,7 @@ object ExtensionsList {
                 }
             }
         }
+    }
 
     private fun String.repoUrlReplace(): String {
         return if (contains("github")) {