Fixup HttpPageLoader _loadPage (#8984)

Fixup for e4bc8990 (#8955)

HttpSource.fetchImage() uses Call.asObservableSuccess(), which
cancels the call on unsubscribe. This causes the call to be cancelled
before it is used, leading to a "java.net.SocketException: Socket is
closed" when trying to use the response in putImageToCache().

To fix this, use Call.awaitSuccess() via a new HttpSource.getImage()
suspending function. This addition to source-api is only intended for
app use, so it will not be added to the extensions-api stubs.

app/src/main/java/eu/kanade/tachiyomi/ui/reader/loader/HttpPageLoader.kt

@@ -200,7 +200,7 @@ class HttpPageLoader(
 
             if (!chapterCache.isImageInCache(imageUrl)) {
                 page.status = Page.State.DOWNLOAD_IMAGE
-                val imageResponse = source.fetchImage(page).awaitSingle()
+                val imageResponse = source.getImage(page)
                 chapterCache.putImageToCache(imageUrl, imageResponse)
             }
 

source-api/src/main/java/eu/kanade/tachiyomi/source/online/HttpSource.kt

@@ -3,6 +3,7 @@ package eu.kanade.tachiyomi.source.online
 import eu.kanade.tachiyomi.network.GET
 import eu.kanade.tachiyomi.network.NetworkHelper
 import eu.kanade.tachiyomi.network.asObservableSuccess
+import eu.kanade.tachiyomi.network.awaitSuccess
 import eu.kanade.tachiyomi.network.newCachelessCallWithProgress
 import eu.kanade.tachiyomi.source.CatalogueSource
 import eu.kanade.tachiyomi.source.model.FilterList
@@ -305,6 +306,17 @@ abstract class HttpSource : CatalogueSource {
             .asObservableSuccess()
     }
 
+    /**
+     * Returns the response of the source image.
+     *
+     * @param page the page whose source image has to be downloaded.
+     */
+    suspend fun getImage(page: Page): Response {
+        // images will be cached or saved manually, so don't take up network cache
+        return client.newCachelessCallWithProgress(imageRequest(page), page)
+            .awaitSuccess()
+    }
+
     /**
      * Returns the request for getting the source image. Override only if it's needed to override
      * the url, send different headers or request method like POST.