feat(resume downloads): implement resumable downloads for interrupted transfers

- Adds support for resuming partially downloaded files
- Uses HTTP Range header to continue from last byte position
- Maintains download progress across interruptions
- Preserves partial downloads with .partial extension
- Validates SHA256 checksum after completion

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

pkg/downloader/uri.go

@@ -2,7 +2,9 @@ package downloader
 
 import (
 	"crypto/sha256"
+	"errors"
 	"fmt"
+	"hash"
 	"io"
 	"net/http"
 	"net/url"
@@ -204,6 +206,15 @@ func removePartialFile(tmpFilePath string) error {
 	return nil
 }
 
+func calculateHashForPartialFile(file *os.File) (hash.Hash, error) {
+	hash := sha256.New()
+	_, err := io.Copy(hash, file)
+	if err != nil {
+		return nil, err
+	}
+	return hash, nil
+}
+
 func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStatus func(string, string, string, float64)) error {
 	url := uri.ResolveURL()
 	if uri.LooksLikeOCI() {
@@ -266,8 +277,32 @@ func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStat
 
 	log.Info().Msgf("Downloading %q", url)
 
-	// Download file
+	req, err := http.NewRequest("GET", url, nil)
-	resp, err := http.Get(url)
+	if err != nil {
+		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))
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("failed to check file %q existence: %v", filePath, err)
+	}
+
+	// Start the request
+	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return fmt.Errorf("failed to download file %q: %v", filePath, err)
 	}
@@ -282,33 +317,29 @@ func (uri URI) DownloadFile(filePath, sha string, fileN, total int, downloadStat
 	if err != nil {
 		return fmt.Errorf("failed to create parent directory for file %q: %v", filePath, err)
 	}
-	/** Enabling partial downloads
-	 * - Do I remove the partial file
-	 * -
-	 */
-	// save partial download to dedicated file
-	fmt.Printf("DELETEING PARTIAL FILE")
-	tmpFilePath := filePath + ".partial"
 
-	// remove tmp file
+	// Create and write file
-	err = removePartialFile(tmpFilePath)
+	outFile, err := os.OpenFile(tmpFilePath, os.O_APPEND|os.O_RDWR|os.O_CREATE, 0644)
 	if err != nil {
-		return err
+		return fmt.Errorf("failed to create / open file %q: %v", tmpFilePath, err)
-	}
-
-	// Create and write file content
-	outFile, err := os.Create(tmpFilePath)
-	if err != nil {
-		return fmt.Errorf("failed to create file %q: %v", tmpFilePath, err)
 	}
 	defer outFile.Close()
-
+	outFileInfo, err := outFile.Stat()
+	if err != nil {
+		return fmt.Errorf("failed to get file info: %v", err)
+	}
+	fileSize := outFileInfo.Size()
+	hash, err := calculateHashForPartialFile(outFile)
+	if err != nil {
+		return fmt.Errorf("failed to calculate hash for partial file")
+	}
 	progress := &progressWriter{
 		fileName:       tmpFilePath,
 		total:          resp.ContentLength,
-		hash:           sha256.New(),
+		hash:           hash,
 		fileNo:         fileN,
 		totalFiles:     total,
+		written:        fileSize,
 		downloadStatus: downloadStatus,
 	}
 	_, err = io.Copy(io.MultiWriter(outFile, progress), resp.Body)
@@ -316,11 +347,6 @@ 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))
@@ -332,6 +358,11 @@ 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)