posts/config.go

package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"log"
	"os"
	"strings"
	"sync"
)

// siteConfig holds the settings in site.json. Every field is optional.
type siteConfig struct {
	HomeText string `json:"homeText"` // text of the header link to the index
}

// withDefaults fills in any setting that is missing or blank.
func (c siteConfig) withDefaults() siteConfig {
	if strings.TrimSpace(c.HomeText) == "" {
		c.HomeText = "Home"
	}
	return c
}

// configFile reads site.json and re-reads it when its size or modification
// time changes. A missing file means all defaults; a broken file is logged
// and the last good settings stay in use.
type configFile struct {
	path string

	mu     sync.Mutex
	sig    string
	loaded bool
	cfg    siteConfig
}

func newConfigFile(path string) *configFile {
	return &configFile{path: path, cfg: siteConfig{}.withDefaults()}
}

func (c *configFile) Get() siteConfig {
	c.mu.Lock()
	defer c.mu.Unlock()

	sig := "missing"
	info, err := os.Stat(c.path)
	switch {
	case err == nil:
		sig = fmt.Sprintf("%d %d", info.Size(), info.ModTime().UnixNano())
	case !errors.Is(err, fs.ErrNotExist):
		log.Printf("%s: %v; keeping previous settings", c.path, err)
		return c.cfg
	}
	if c.loaded && sig == c.sig {
		return c.cfg
	}
	c.sig, c.loaded = sig, true

	if sig == "missing" {
		c.cfg = siteConfig{}.withDefaults()
		return c.cfg
	}
	cfg, err := readConfig(c.path)
	if err != nil {
		log.Printf("%s: %v; keeping previous settings", c.path, err)
		return c.cfg
	}
	c.cfg = cfg
	return c.cfg
}

func readConfig(path string) (siteConfig, error) {
	f, err := os.Open(path)
	if err != nil {
		return siteConfig{}, err
	}
	defer f.Close()
	dec := json.NewDecoder(f)
	dec.DisallowUnknownFields() // catch typos in setting names
	var cfg siteConfig
	if err := dec.Decode(&cfg); err != nil {
		return siteConfig{}, err
	}
	return cfg.withDefaults(), nil
}