it sounds like an easy fix, but how would the rename operation below work on an open file?

TIL this actually works with the rename syscall on linux. It will fail on Windows with the classic "the process cannot access the file because it is being used by another process". Which is Support Tier 3, but the fix is simple: just wrap the file operations before the rename in a function and the defer within.

What do you mean by "rename operation"? I just moved f.Close() to defer, because file should be closed even if any error occured after it being opened. I can't see any logic changes.

This operation:

will fail on Windows, because defer closes the file after the function exits and you can't rename a still opened file.

MRE if you want to try this out: https://gist.github.com/tjungblu/7d4ff9bc88afe86004bf292fd9032966

Well, you are right

turned out to be slightly more verbose than I thought, but that could work:

	err = func() (err error) {
		var f *os.File
		f, err = os.OpenFile(partpath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, fileutil.PrivateFileMode)
		if err != nil {
			return fmt.Errorf("could not open %s (%v)", partpath, err)
		}
		defer func() {
			err = f.Close()
		}()
		
		lg.Info("created temporary db file", zap.String("path", partpath))

		var rd io.ReadCloser
		rd, err = cli.Snapshot(ctx)
		if err != nil {
			return
		}
		lg.Info("fetching snapshot", zap.String("endpoint", cfg.Endpoints[0]))
		var size int64
		size, err = io.Copy(f, rd)
		if err != nil {
			return
		}
		if !hasChecksum(size) {
			return fmt.Errorf("sha256 checksum not found [bytes: %d]", size)
		}
		if err = fileutil.Fsync(f); err != nil {
			return
		}

		lg.Info("fetched snapshot",
			zap.String("endpoint", cfg.Endpoints[0]),
			zap.String("size", humanize.Bytes(uint64(size))),
			zap.String("took", humanize.Time(now)),
		)

		return
	}()

	if err != nil {
		return err
	}

But err = f.Close() will rewrite any error which comes after defer func() {err = f.Close()}() inside that block, won't it?

Joining the errors should work :)

Ref: https://pkg.go.dev/errors#Join