Prevent downloads when less than 50MB of disk space is available (closes #1018)

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

@@ -272,6 +272,13 @@ class Downloader(
     private fun downloadChapter(download: Download): Observable<Download> = Observable.defer {
         val chapterDirname = provider.getChapterDirName(download.chapter)
         val mangaDir = provider.getMangaDir(download.manga, download.source)
+
+        if (DiskUtil.getAvailableStorageSpace(mangaDir) < MIN_DISK_SPACE) {
+            download.status = Download.ERROR
+            notifier.onError(context.getString(R.string.download_insufficient_space), download.chapter.name)
+            return@defer Observable.just(download)
+        }
+
         val tmpDir = mangaDir.createDirectory(chapterDirname + TMP_DIR_SUFFIX)
 
         val pageListObservable = if (download.pages == null) {
@@ -489,5 +496,8 @@ class Downloader(
 
     companion object {
         const val TMP_DIR_SUFFIX = "_tmp"
+
+        // Arbitrary minimum required space to start a download: 50 MB
+        const val MIN_DISK_SPACE = 50 * 1024 * 1024
     }
 }