Use os.Copy over os.Rename to avoid os restrictions

bufisk.go

@@ -146,6 +146,12 @@ func downloadBufToFilePath(ctx context.Context, bufVersion string, bufFilePath s
 	if err != nil {
 		return fmt.Errorf("could not download buf (are you sure %q is a valid release version?): %w", bufVersion, err)
 	}
+	defer func() {
+		// Cleanup temp file, if it exists.
+		if err := os.Remove(tempFilePath); err != nil && !errors.Is(err, os.ErrNotExist) && retErr == nil {
+			retErr = fmt.Errorf("failed to cleanup temp file %q: %w", tempFilePath, err)
+		}
+	}()
 	// We could return the temporary *os.File from downloadTemp but oh well.
 	bufFileData, err := os.ReadFile(tempFilePath)
 	if err != nil {
@@ -271,8 +277,44 @@ func moveFileToPath(fromFilePath string, toFilePath string) error {
 	if err := os.MkdirAll(filepath.Dir(toFilePath), 0700); err != nil {
 		return err
 	}
+	// Try to use os.Rename if possible, but it doesn't work across devices.
 	// This will replace any other file atomically.
-	return os.Rename(fromFilePath, toFilePath)
+	if err := os.Rename(fromFilePath, toFilePath); err == nil {
+		return nil
+	}
+	// Otherwise, copy and remove.
+	if err := copyFileToPath(fromFilePath, toFilePath); err != nil {
+		return err
+	}
+	return os.Remove(fromFilePath)
+}
+
+func copyFileToPath(fromFilePath string, toFilePath string) (retErr error) {
+	if err := os.MkdirAll(filepath.Dir(toFilePath), 0700); err != nil {
+		return err
+	}
+	fromFile, err := os.Open(fromFilePath)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := fromFile.Close(); err != nil && retErr == nil {
+			retErr = err
+		}
+	}()
+	toFile, err := os.Create(toFilePath)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := toFile.Close(); err != nil && retErr == nil {
+			retErr = err
+		}
+	}()
+	if _, err := io.Copy(toFile, fromFile); err != nil {
+		return err
+	}
+	return nil
 }
 
 func getFileURL(bufVersion string, fileName string) string {