func PruneEmptyConfigSections()

in internal/git/housekeeping/clean_stale_data.go [105:202]
69 lines of code
20 McCabe index (conditional complexity)

func PruneEmptyConfigSections(ctx context.Context, repo *localrepo.Repo) (_ int, returnedErr error) {
	repoPath, err := repo.Path(ctx)
	if err != nil {
		return 0, fmt.Errorf("getting repo path: %w", err)
	}
	configPath := filepath.Join(repoPath, "config")

	// The gitconfig shouldn't ever be big given that we nowadays don't write any unbounded
	// values into it anymore. Slurping it into memory should thus be fine.
	configContents, err := os.ReadFile(configPath)
	if err != nil {
		return 0, fmt.Errorf("reading config: %w", err)
	}
	configLines := strings.SplitAfter(string(configContents), "\n")
	if configLines[len(configLines)-1] == "" {
		// Strip the last line if it's empty.
		configLines = configLines[:len(configLines)-1]
	}

	skippedSections := 0

	// We now filter out any empty sections. A section is empty if the next line is a section
	// header as well, or if it is the last line in the gitconfig. This isn't quite the whole
	// story given that a section can also be empty if it just ain't got any keys, but only
	// comments or whitespace. But we only ever write the gitconfig programmatically, so we
	// shouldn't typically see any such cases at all.
	filteredLines := make([]string, 0, len(configLines))
	for i := 0; i < len(configLines)-1; i++ {
		// Skip if we have two consecutive section headers.
		if isSectionHeader(configLines[i]) && isSectionHeader(configLines[i+1]) {
			skippedSections++
			continue
		}
		filteredLines = append(filteredLines, configLines[i])
	}
	// The final line is always stripped in case it is a section header.
	if len(configLines) > 0 && !isSectionHeader(configLines[len(configLines)-1]) {
		skippedSections++
		filteredLines = append(filteredLines, configLines[len(configLines)-1])
	}

	// If we haven't filtered out anything then there is no need to update the target file.
	if len(configLines) == len(filteredLines) {
		return 0, nil
	}

	// Otherwise, we need to update the repository's configuration.
	configWriter, err := safe.NewLockingFileWriter(configPath)
	if err != nil {
		return 0, fmt.Errorf("creating config configWriter: %w", err)
	}
	defer func() {
		if err := configWriter.Close(); err != nil && returnedErr == nil {
			returnedErr = fmt.Errorf("closing config writer: %w", err)
		}
	}()

	for _, filteredLine := range filteredLines {
		if _, err := configWriter.Write([]byte(filteredLine)); err != nil {
			return 0, fmt.Errorf("writing filtered config: %w", err)
		}
	}

	// This is a sanity check to assert that we really didn't change anything as seen by
	// Git. We run `git config -l` on both old and new file and assert that they report
	// the same config entries. Because empty sections are never reported we shouldn't
	// see those, and as a result any difference in output is a difference we need to
	// worry about.
	var configOutputs []string
	for _, path := range []string{configPath, configWriter.Path()} {
		var configOutput bytes.Buffer
		if err := repo.ExecAndWait(ctx, gitcmd.Command{
			Name: "config",
			Flags: []gitcmd.Option{
				gitcmd.ValueFlag{Name: "-f", Value: path},
				gitcmd.Flag{Name: "-l"},
			},
		}, gitcmd.WithStdout(&configOutput)); err != nil {
			return 0, fmt.Errorf("listing config: %w", err)
		}

		configOutputs = append(configOutputs, configOutput.String())
	}
	if configOutputs[0] != configOutputs[1] {
		return 0, fmt.Errorf("section pruning has caused config change")
	}

	// We don't use transactional voting but commit the file directly -- we have asserted that
	// the change is idempotent anyway.
	if err := configWriter.Lock(); err != nil {
		return 0, fmt.Errorf("failed locking config: %w", err)
	}
	if err := configWriter.Commit(ctx); err != nil {
		return 0, fmt.Errorf("failed committing pruned config: %w", err)
	}

	return skippedSections, nil
}