Fix delay between URL fetch and image download (#9452)

Fetch each source image URL immediately before downloading each image
instead of fetching all URLs and then downloading all images.

Source image URLs may change, so the downloader may fail if there is
too long a delay between fetching the image URL and downloading the
image.

app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt

@@ -338,21 +338,21 @@ class Downloader(
 
             download.status = Download.State.DOWNLOADING
 
-            // Get all the URLs to the source images, fetch pages if necessary
-            pageList.filter { it.imageUrl.isNullOrEmpty() }.forEach { page ->
-                page.status = Page.State.LOAD_PAGE
-                try {
-                    page.imageUrl = download.source.fetchImageUrl(page).awaitSingle()
-                } catch (e: Throwable) {
-                    page.status = Page.State.ERROR
-                }
-            }
-
             // Start downloading images, consider we can have downloaded images already
             // Concurrently do 2 pages at a time
             pageList.asFlow()
                 .flatMapMerge(concurrency = 2) { page ->
                     flow {
+                        // Fetch image URL if necessary
+                        if (page.imageUrl.isNullOrEmpty()) {
+                            page.status = Page.State.LOAD_PAGE
+                            try {
+                                page.imageUrl = download.source.fetchImageUrl(page).awaitSingle()
+                            } catch (e: Throwable) {
+                                page.status = Page.State.ERROR
+                            }
+                        }
+
                         withIOContext { getOrDownloadImage(page, download, tmpDir) }
                         emit(page)
                     }.flowOn(Dispatchers.IO)