Prevent duplicated chapter pages (#1353)

In case "ChapterForDownload#asDownloadReady" was called in quick succession, the page list got inserted twice.

This caused problems with getting the images from the rest endpoint, because they are selected by sorting them by asc index and selecting the page by using the provided index as an offset.

This, however, only works as long as there are no duplicates, otherwise, page indexes 1, 2; 3, 4; 5, 6; ... will just return the same page.

server/src/main/kotlin/suwayomi/tachidesk/manga/impl/chapter/ChapterForDownload.kt

@@ -11,6 +11,9 @@ import eu.kanade.tachiyomi.source.model.Page
 import eu.kanade.tachiyomi.source.model.SChapter
 import io.github.oshai.kotlinlogging.KLogger
 import io.github.oshai.kotlinlogging.KotlinLogging
+import io.github.reactivecircus.cache4k.Cache
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import org.jetbrains.exposed.sql.ResultRow
 import org.jetbrains.exposed.sql.SqlExpressionBuilder.eq
 import org.jetbrains.exposed.sql.and
@@ -26,6 +29,7 @@ import suwayomi.tachidesk.manga.model.table.ChapterTable
 import suwayomi.tachidesk.manga.model.table.MangaTable
 import suwayomi.tachidesk.manga.model.table.PageTable
 import suwayomi.tachidesk.manga.model.table.toDataClass
+import kotlin.time.Duration.Companion.minutes
 
 suspend fun getChapterDownloadReady(
     chapterId: Int? = null,
@@ -43,6 +47,12 @@ suspend fun getChapterDownloadReadyByIndex(
     mangaId: Int,
 ): ChapterDataClass = getChapterDownloadReady(chapterIndex = chapterIndex, mangaId = mangaId)
 
+private val mutexByChapterId: Cache<Int, Mutex> =
+    Cache
+        .Builder<Int, Mutex>()
+        .expireAfterAccess(10.minutes)
+        .build()
+
 private class ChapterForDownload(
     optChapterId: Int? = null,
     optChapterIndex: Int? = null,
@@ -69,9 +79,7 @@ private class ChapterForDownload(
 
             markAsNotDownloaded()
 
-            val pageList = fetchPageList()
-
-            updateDatabasePages(pageList)
+            updatePageList()
         }
 
         return asDataClass()
@@ -109,6 +117,14 @@ private class ChapterForDownload(
             }.first()
     }
 
+    private suspend fun updatePageList() {
+        val mutex = mutexByChapterId.get(chapterId) { Mutex() }
+        mutex.withLock {
+            val pageList = fetchPageList()
+            updateDatabasePages(pageList)
+        }
+    }
+
     private suspend fun fetchPageList(): List<Page> {
         val mangaEntry = transaction { MangaTable.selectAll().where { MangaTable.id eq mangaId }.first() }
         val source = getCatalogueSourceOrStub(mangaEntry[MangaTable.sourceReference])