Resume failed downloads when possible (#634)

lib/providers/apps_provider.dart

@@ -202,14 +202,18 @@ Future<String> checkPartialDownloadHash(String url, int bytesToGrab,
 Future<File> downloadFile(
     String url, String fileNameNoExt, Function? onProgress, String destDir,
     {bool useExisting = true, Map<String, String>? headers}) async {
+  // Send the initial request but cancel it as soon as you have the headers
+  var reqHeaders = headers ?? {};
   var req = Request('GET', Uri.parse(url));
-  if (headers != null) {
-    req.headers.addAll(headers);
-  }
+  req.headers.addAll(reqHeaders);
   var client = http.Client();
   StreamedResponse response = await client.send(req);
-  String ext =
-      response.headers['content-disposition']?.split('.').last ?? 'apk';
+  var resHeaders = response.headers;
+
+  // Use the headers to decide what the file extension is, and
+  // whether it supports partial downloads (range request), and
+  // what the total size of the file is (if provided)
+  String ext = resHeaders['content-disposition']?.split('.').last ?? 'apk';
   if (ext.endsWith('"') || ext.endsWith("other")) {
     ext = ext.substring(0, ext.length - 1);
   }
@@ -217,18 +221,68 @@ Future<File> downloadFile(
     ext = 'apk';
   }
   File downloadedFile = File('$destDir/$fileNameNoExt.$ext');
-  if (!(downloadedFile.existsSync() && useExisting)) {
+
+  bool rangeFeatureEnabled = false;
+  if (resHeaders['accept-ranges']?.isNotEmpty == true) {
+    rangeFeatureEnabled =
+        resHeaders['accept-ranges']?.trim().toLowerCase() == 'bytes';
+  }
+
+  // If you have an existing file that is usable,
+  // decide whether you can use it (either return full or resume partial)
+  var fullContentLength = response.contentLength;
+  if (useExisting && downloadedFile.existsSync()) {
+    var length = downloadedFile.lengthSync();
+    if (fullContentLength == null) {
+      // Assume full
+      client.close();
+      return downloadedFile;
+    } else {
+      // Check if resume needed/possible
+      if (length == fullContentLength) {
+        client.close();
+        return downloadedFile;
+      }
+      if (length > fullContentLength) {
+        useExisting = false;
+      }
+    }
+  }
+
+  // Download to a '.temp' file (to distinguish btn. complete/incomplete files)
   File tempDownloadedFile = File('${downloadedFile.path}.part');
-    if (tempDownloadedFile.existsSync()) {
+
+  // If the range feature is not available (or you need to start a ranged req from 0),
+  // complete the already-started request, else cancel it and start a ranged request,
+  // and open the file for writing in the appropriate mode
+  var targetFileLength = useExisting && tempDownloadedFile.existsSync()
+      ? tempDownloadedFile.lengthSync()
+      : null;
+  int rangeStart = targetFileLength ?? 0;
+  IOSink? sink;
+  if (rangeFeatureEnabled && fullContentLength != null && rangeStart > 0) {
+    client.close();
+    client = http.Client();
+    req = Request('GET', Uri.parse(url));
+    req.headers.addAll(reqHeaders);
+    req.headers.addAll({'range': 'bytes=$rangeStart-${fullContentLength - 1}'});
+    response = await client.send(req);
+    sink = tempDownloadedFile.openWrite(mode: FileMode.writeOnlyAppend);
+  } else if (tempDownloadedFile.existsSync()) {
     tempDownloadedFile.deleteSync(recursive: true);
   }
-    var length = response.contentLength;
+  sink ??= tempDownloadedFile.openWrite(mode: FileMode.writeOnly);
+
+  // Perform the download
   var received = 0;
   double? progress;
-    var sink = tempDownloadedFile.openWrite();
+  if (rangeStart > 0 && fullContentLength != null) {
+    received = rangeStart;
+  }
   await response.stream.map((s) {
     received += s.length;
-      progress = (length != null ? received / length * 100 : 30);
+    progress =
+        (fullContentLength != null ? (received / fullContentLength) * 100 : 30);
     if (onProgress != null) {
       onProgress(progress);
     }
@@ -239,19 +293,35 @@ Future<File> downloadFile(
   if (onProgress != null) {
     onProgress(progress);
   }
-    if (response.statusCode != 200) {
+  if (response.statusCode < 200 || response.statusCode > 299) {
     tempDownloadedFile.deleteSync(recursive: true);
     throw response.reasonPhrase ?? tr('unexpectedError');
   }
+  print(tempDownloadedFile.lengthSync());
+  print(fullContentLength);
   if (tempDownloadedFile.existsSync()) {
     tempDownloadedFile.renameSync(downloadedFile.path);
   }
-  } else {
   client.close();
-  }
   return downloadedFile;
 }
 
+Future<Map<String, String>> getHeaders(String url,
+    {Map<String, String>? headers}) async {
+  var req = http.Request('GET', Uri.parse(url));
+  if (headers != null) {
+    req.headers.addAll(headers);
+  }
+  var client = http.Client();
+  var response = await client.send(req);
+  if (response.statusCode < 200 || response.statusCode > 299) {
+    throw ObtainiumError(response.reasonPhrase ?? tr('unexpectedError'));
+  }
+  var returnHeaders = response.headers;
+  client.close();
+  return returnHeaders;
+}
+
 Future<PackageInfo?> getInstalledInfo(String? packageName,
     {bool printErr = true}) async {
   if (packageName != null) {