feat: s3 download

internal/pkg/grib/downloader.go

@@ -24,28 +24,51 @@ func (d *Downloader) fileURL(run string, hour int, step int) string {
 	return fmt.Sprintf("%s/gfs.%s/%02d/atmos/gfs.t%02dz.pgrb2.0p50.f%03d", d.DatasetURL, run, hour, hour, step)
 }
 
-func (d *Downloader) fetch(ctx context.Context, url, dst string) error {
+func (d *Downloader) fetch(ctx context.Context, url, dst string) (err error) {
+	// Check if final file already exists
 	if _, err := os.Stat(dst); err == nil {
 		return nil
 	}
+
 	tmp := dst + ".part"
+
+	// Remove old .part file if it exists (fixes race condition)
+	os.Remove(tmp)
+
 	f, err := os.Create(tmp)
 	if err != nil {
 		return err
 	}
-	defer f.Close()
+
+	// Cleanup .part file on any error (using named return value)
+	defer func() {
+		f.Close()
+		if err != nil {
+			os.Remove(tmp)
+		}
+	}()
+
 	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	resp, err := d.Client.Do(req)
 	if err != nil {
 		return err
 	}
 	defer resp.Body.Close()
+
 	if resp.StatusCode != http.StatusOK {
 		return errcodes.Wrap(errcodes.ErrDownload, "bad status: "+resp.Status)
 	}
+
 	if _, err := io.Copy(f, resp.Body); err != nil {
 		return err
 	}
+
+	// Close file before rename
+	if err := f.Close(); err != nil {
+		return err
+	}
+
+	// If rename fails, err will be set and defer will cleanup .part file
 	return os.Rename(tmp, dst)
 }