Function migrationsFromFS

rivermigrate/river_migrate.go:776–862

Reads a series of migration bundles from a file system, which practically speaking will always be the embedded FS read from the contents of the `migration/ /` subdirectory.

(migrationFS fs.FS, line string)

// speaking will always be the embedded FS read from the contents of the
// `migration/<line>/` subdirectory.
func migrationsFromFS(migrationFS fs.FS, line string) ([]Migration, error) {
	const subdir = "migration"

	var (
		lastBundle *Migration
		migrations []Migration
	)

	err := fs.WalkDir(migrationFS, subdir, func(path string, entry fs.DirEntry, err error) error {
		if err != nil {
			return fmt.Errorf("error walking FS: %w", err)
		}

		// The WalkDir callback is invoked for each embedded subdirectory and
		// file. For our purposes here, we're only interested in files.
		if entry.IsDir() {
			return nil
		}

		filename := path

		// Invoked with the full path name. Strip `migration/` from the front so
		// we have a name that we can parse with.
		if !strings.HasPrefix(filename, subdir) {
			return fmt.Errorf("expected path %q to start with subdir %q", path, subdir)
		}
		filename = filename[len(subdir)+1:]

		// Ignore any migrations that don't belong to the line we're reading.
		if !strings.HasPrefix(filename, line) {
			return nil
		}
		filename = filename[len(line)+1:]

		versionStr, name, ok := strings.Cut(filename, "_")
		if !ok {
			return fmt.Errorf("expected name to start with version string like '001_': %q", filename)
		}

		version, err := strconv.Atoi(versionStr)
		if err != nil {
			return fmt.Errorf("error parsing version %q: %w", versionStr, err)
		}

		// Non-version name for the migration. So for `002_initial_schema` it
		// would be `initial schema`.
		name, _, _ = strings.Cut(name, ".")
		name = strings.ReplaceAll(name, "_", " ")

		// This works because `fs.WalkDir` guarantees lexical order, so all 001*
		// files always appear before all 002* files, etc.
		if lastBundle == nil || lastBundle.Version != version {
			migrations = append(migrations, Migration{Name: name, Version: version})
			lastBundle = &migrations[len(migrations)-1]
		}

		file, err := migrationFS.Open(path)
		if err != nil {