Copy single file transfer to bundle

Fixes #746.

For a single file transfer copy the blob data from the local temp file
to the bundle in the transfer directory.  Renaming (moving) the file
does not work when the souce and destination are on different logical
devices.

Ref: https://groups.google.com/g/golang-dev/c/5w7Jmg_iCJQ?pli=1

internal/workflow/activities/bundle.go

@@ -98,6 +98,9 @@ func (a *BundleActivity) Execute(ctx context.Context, params *BundleActivityPara
 		unar := a.Unarchiver(params.Key, params.TempFile)
 		if unar == nil {
 			res.FullPath, err = a.SingleFile(ctx, params.TransferDir, params.Key, params.TempFile)
+			if err != nil {
+				err = fmt.Errorf("bundle single file: %v", err)
+			}
 			res.FullPathBeforeStrip = res.FullPath
 		} else {
 			res.FullPath, res.FullPathBeforeStrip, err = a.Bundle(ctx, unar, params.TransferDir, params.Key, params.TempFile, params.StripTopLevelDir)
@@ -150,25 +153,27 @@ func (a *BundleActivity) Unarchiver(key, filename string) archiver.Unarchiver {
 }
 
 // SingleFile bundles a transfer with the downloaded blob in it.
+//
+// TODO: Write metadata.csv and checksum files to the metadata dir.
 func (a *BundleActivity) SingleFile(ctx context.Context, transferDir, key, tempFile string) (string, error) {
 	b, err := bundler.NewBundlerWithTempDir(transferDir)
 	if err != nil {
-		return "", fmt.Errorf("error creating bundle: %v", err)
+		return "", fmt.Errorf("create bundler: %v", err)
 	}
 
-	dest, err := b.Create(filepath.Join("objects", key))
+	src, err := os.Open(tempFile)
 	if err != nil {
-		return "", fmt.Errorf("error creating file: %v", err)
+		return "", fmt.Errorf("open source file: %v", err)
 	}
-	defer dest.Close()
+	defer src.Close()
 
-	path, _ := securejoin.SecureJoin(transferDir, dest.Name())
-	if err := os.Rename(tempFile, path); err != nil {
-		return "", fmt.Errorf("error moving file (from %s to %s): %v", tempFile, path, err)
+	err = b.Write(filepath.Join("objects", key), src)
+	if err != nil {
+		return "", fmt.Errorf("write file: %v", err)
 	}
 
 	if err := b.Bundle(); err != nil {
-		return "", fmt.Errorf("error bundling the transfer: %v", err)
+		return "", fmt.Errorf("write bundle: %v", err)
 	}
 
 	return b.FullBaseFsPath(), nil