Function openFileOrSubstitute

cmd/tailscale/cli/file.go:649–689

(dir, base string, action onConflict)

}

func openFileOrSubstitute(dir, base string, action onConflict) (*os.File, error) {
	targetFile := filepath.Join(dir, base)
	f, err := os.OpenFile(targetFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644)
	if err == nil {
		return f, nil
	}
	// Something went wrong trying to open targetFile as a new file for writing.
	switch action {
	default:
		// This should not happen.
		return nil, fmt.Errorf("file issue. how to resolve this conflict? no one knows.")
	case skipOnExist:
		if _, statErr := os.Stat(targetFile); statErr == nil {
			// we can stat a file at that path: so it already exists.
			return nil, fmt.Errorf("refusing to overwrite file: %w", err)
		}
		return nil, fmt.Errorf("failed to write; %w", err)
	case overwriteExisting:
		// remove the target file and create it anew so we don't fall for an
		// attacker who symlinks a known target name to a file he wants changed.
		if err = os.Remove(targetFile); err != nil {
			return nil, fmt.Errorf("unable to remove target file: %w", err)
		}
		if f, err = os.OpenFile(targetFile, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644); err != nil {
			return nil, fmt.Errorf("unable to overwrite: %w", err)
		}
		return f, nil
	case createNumberedFiles:
		// It's possible the target directory or filesystem isn't writable by us,
		// not just that the target file(s) already exists.  For now, give up after
		// a limited number of attempts.  In future, maybe distinguish this case
		// and follow in the style of https://tinyurl.com/chromium100
		maxAttempts := 100
		for i := 1; i < maxAttempts; i++ {
			if f, err = os.OpenFile(numberedFileName(dir, base, i), os.O_RDWR|os.O_CREATE|os.O_EXCL, 0644); err == nil {
				return f, nil
			}
		}
		return nil, fmt.Errorf("unable to find a name for writing %v, final attempt: %w", targetFile, err)
	}
}

func receiveFile(ctx context.Context, wf apitype.WaitingFile, dir string) (targetFile string, size int64, err error) {
	rc, size, err := localClient.GetWaitingFile(ctx, wf.Name)