cmd/tailscaled,*: add start of configuration file support

Updates #1412

Co-authored-by: Maisem Ali <maisem@tailscale.com>
Change-Id: I38d559c1784d09fc804f521986c9b4b548718f7d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/conffile/conffile.go

@@ -0,0 +1,66 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package conffile contains code to load, manipulate, and access config file
+// settings.
+package conffile
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"github.com/tailscale/hujson"
+	"tailscale.com/ipn"
+)
+
+// Config describes a config file
+type Config struct {
+	Path    string // disk path of HuJSON
+	Raw     []byte // raw bytes from disk, in HuJSON form
+	Std     []byte // standardized JSON form
+	Version string // "alpha0" for now
+
+	// Parsed is the parsed config, converted from its on-disk version to the
+	// latest known format.
+	//
+	// As of 2023-10-15 there is exactly one format ("alpha0") so this is both
+	// the on-disk format and the in-memory upgraded format.
+	Parsed ipn.ConfigVAlpha
+}
+
+// Load reads and parses the config file at the provided path on disk.
+func Load(path string) (*Config, error) {
+	var c Config
+	c.Path = path
+
+	var err error
+	c.Raw, err = os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	c.Std, err = hujson.Standardize(c.Raw)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing config file %s HuJSON/JSON: %w", path, err)
+	}
+	var ver struct {
+		Version string `json:"version"`
+	}
+	if err := json.Unmarshal(c.Std, &ver); err != nil {
+		return nil, fmt.Errorf("error parsing config file %s: %w", path, err)
+	}
+	switch ver.Version {
+	case "":
+		return nil, fmt.Errorf("error parsing config file %s: no \"version\" field defined", path)
+	case "alpha0":
+	default:
+		return nil, fmt.Errorf("error parsing config file %s: unsupported \"version\" value %q; want \"alpha0\" for now", path, ver.Version)
+	}
+	c.Version = ver.Version
+
+	err = json.Unmarshal(c.Std, &c.Parsed)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing config file %s: %w", path, err)
+	}
+	return &c, nil
+}