Add local file abstraction

ipblocklist/files/local.go

@@ -0,0 +1,30 @@
+package files
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+)
+
+type localFile struct {
+	root fs.FS
+	name string
+}
+
+func (l localFile) Open(ctx context.Context) (io.ReadCloser, error) {
+	return l.root.Open(l.name)
+}
+
+func NewLocal(path string) (File, error) {
+	if stat, err := os.Stat(path); os.IsNotExist(err) || stat.IsDir() || stat.Mode().Perm()&0o400 == 0 {
+		return nil, fmt.Errorf("%s is not a readable file", path)
+	}
+
+	return localFile{
+		root: os.DirFS(filepath.Dir(path)),
+		name: filepath.Base(path),
+	}, nil
+}