feat(resume download): implement range header check

Signed-off-by: Saarthak Verma <saarthakverma739@gmail.com>

Author: Saavrm26

pkg/downloader/uri.go

@@ -215,6 +215,16 @@ func calculateHashForPartialFile(file *os.File) (hash.Hash, error) {
 	return hash, nil
 }
 
+func (uri URI) checkSeverSupportsRangeHeader() (bool, error) {
+	url := uri.ResolveURL()
+	resp, err := http.Head(url)
+	if err != nil {
+		return false, err
+	}
+	defer resp.Body.Close()
+	return resp.Header.Get("Accept-Ranges") == "bytes", nil
+}
+
 func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStatus func(string, string, string, float64)) error {
 	url := uri.ResolveURL()
 	if uri.LooksLikeOCI() {
@@ -282,21 +292,23 @@ func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStat
 		return fmt.Errorf("failed to create request for %q: %v", filePath, err)
 	}
 
-	/* TODO
-	 * 1. ~~Mock downloads~~
-	 * 2. Check if server supports partial downloads
-	 * 3. ~~Resume partial downloads~~
-	 * 4. Ensure progressWriter accurately reflects progress if a partial file is present
-	 * 5. MAYBE:
-	 * 	a. Delete file if calculatedSHA != sha
-	 */
-
 	// save partial download to dedicated file
 	tmpFilePath := filePath + ".partial"
 	tmpFileInfo, err := os.Stat(tmpFilePath)
 	if err == nil {
-		startPos := tmpFileInfo.Size()
-		req.Header.Set("Range", fmt.Sprintf("bytes=%d-", startPos))
+		support, err := uri.checkSeverSupportsRangeHeader()
+		if err != nil {
+			return fmt.Errorf("failed to check if uri server supports range header: %v", err)
+		}
+		if support {
+			startPos := tmpFileInfo.Size()
+			req.Header.Set("Range", fmt.Sprintf("bytes=%d-", startPos))
+		} else {
+			err := removePartialFile(tmpFilePath)
+			if err != nil {
+				return err
+			}
+		}
 	} else if !errors.Is(err, os.ErrNotExist) {
 		return fmt.Errorf("failed to check file %q existence: %v", filePath, err)
 	}
@@ -341,6 +353,11 @@ func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStat
 		return fmt.Errorf("failed to write file %q: %v", filePath, err)
 	}
 
+	err = os.Rename(tmpFilePath, filePath)
+	if err != nil {
+		return fmt.Errorf("failed to rename temporary file %s -> %s: %v", tmpFilePath, filePath, err)
+	}
+
 	if sha != "" {
 		// Verify SHA
 		calculatedSHA := fmt.Sprintf("%x", progress.hash.Sum(nil))
@@ -352,11 +369,6 @@ func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStat
 		log.Debug().Msgf("SHA missing for %q. Skipping validation", filePath)
 	}
 
-	err = os.Rename(tmpFilePath, filePath)
-	if err != nil {
-		return fmt.Errorf("failed to rename temporary file %s -> %s: %v", tmpFilePath, filePath, err)
-	}
-
 	log.Info().Msgf("File %q downloaded and verified", filePath)
 	if utils.IsArchive(filePath) {
 		basePath := filepath.Dir(filePath)