docs: add history preprocessor directive

I was reading a post on [architecture decision
records](https://andydote.co.uk/2019/06/29/architecture-decision-records/),
and it pointed out that when processing documentation, one can leverage
the vcs to add in historical information.

This adds a tool that slip-streams the history of a file into the
rendered output.

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

Documentation/history.go

@@ -0,0 +1,154 @@
+//go:build ignore
+// +build ignore
+
+package main
+
+import (
+	"encoding/json"
+	"html/template"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+func main() {
+	log.SetFlags(log.LstdFlags | log.Lmsgprefix)
+	log.SetPrefix("history: ")
+
+	// Handle when called with "supports $renderer".
+	if len(os.Args) == 3 {
+		switch os.Args[1] {
+		case "supports":
+			switch os.Args[2] {
+			case "html":
+			default:
+				os.Exit(1)
+			}
+		default:
+			os.Exit(1)
+		}
+		os.Exit(0)
+	}
+
+	// Actual preprocessing mode.
+	log.Println("running preprocessor")
+
+	in := make([]json.RawMessage, 2)
+	dec := json.NewDecoder(os.Stdin)
+	if err := dec.Decode(&in); err != nil {
+		panic(err)
+	}
+	var cfg Config
+	if err := json.Unmarshal(in[0], &cfg); err != nil {
+		panic(err)
+	}
+	var book Book
+	if err := json.Unmarshal(in[1], &book); err != nil {
+		panic(err)
+	}
+
+	var b strings.Builder
+	for _, s := range book.Sections {
+		if err := s.Process(&b, &cfg, tmpl); err != nil {
+			panic(err)
+		}
+	}
+	if err := json.NewEncoder(os.Stdout).Encode(&book); err != nil {
+		panic(err)
+	}
+}
+
+// in: {"root":"/var/home/hank/work/clair/clair","config":{"book":{"authors":["Clair Authors"],"description":"Documentation for Clair.","language":"en","multilingual":false,"src":"Documentation","title":"Clair Documentation"},"output":{"html":{"git-repository-url":"https://github.com/quay/clair","preferred-dark-theme":"coal"}},"preprocessor":{"history":{"command":"go run Documentation/history.go"}}},"renderer":"html","mdbook_version":"0.4.13"}
+type Config struct {
+	Root     string `json:"root"`
+	Renderer string `json:"renderer"`
+	Version  string `json:"mdbook_version"`
+	Config   struct {
+		Book BookConfig `json:"book"`
+	} `json:"config"`
+}
+
+type BookConfig struct {
+	Source string `json:"src"`
+}
+
+type Book struct {
+	Sections []Section `json:"sections"`
+	X        *struct{} `json:"__non_exhaustive"`
+}
+
+type Section struct {
+	Chapter   *Chapter    `json:",omitempty"`
+	Separator interface{} `json:",omitempty"`
+	PartTitle string      `json:",omitempty"`
+}
+
+func (s *Section) Process(b *strings.Builder, cfg *Config, t *template.Template) error {
+	if s.Chapter != nil {
+		return s.Chapter.Process(b, cfg, t)
+	}
+	return nil
+}
+
+type Chapter struct {
+	Name        string    `json:"name"`
+	Content     string    `json:"content"`
+	Number      []int     `json:"number"`
+	SubItems    []Section `json:"sub_items"`
+	Path        *string   `json:"path"`
+	SourcePath  *string   `json:"source_path"`
+	ParentNames []string  `json:"parent_names"`
+}
+
+func (c *Chapter) Process(b *strings.Builder, cfg *Config, t *template.Template) error {
+	if marker.MatchString(c.Content) && c.Path != nil {
+		log.Println("inserting history into:", *c.Path)
+		cmd := exec.Command(`git`, `log`, `--reverse`, "--format=%cs\t%s", `--`, filepath.Join(cfg.Root, cfg.Config.Book.Source, *c.Path))
+		out, err := cmd.Output()
+		if err != nil {
+			return err
+		}
+		var ls []Line
+		for _, l := range strings.FieldsFunc(string(out), func(r rune) bool { return r == '\n' }) {
+			s := strings.SplitN(l, "\t", 2)
+			ls = append(ls, Line{Date: s[0], Summary: s[1]})
+		}
+		b.Reset()
+		if err := t.ExecuteTemplate(b, cfg.Renderer, ls); err != nil {
+			return err
+		}
+		c.Content = marker.ReplaceAllString(c.Content, b.String())
+	}
+	for _, s := range c.SubItems {
+		if err := s.Process(b, cfg, t); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type Line struct {
+	Date    string
+	Summary string
+}
+
+var (
+	tmpl   *template.Template
+	marker = regexp.MustCompile(`\{\{#\s*history\s*\}\}`)
+)
+
+func init() {
+	const (
+		html = `<details>
+<summary>Document History</summary>
+<p><ul>
+{{ range . }}<li><strong>{{.Date}}</strong> {{.Summary}}</li>
+{{ end }}</ul></p>
+</details>`
+	)
+	tmpl = template.New("")
+	template.Must(tmpl.New("html").Parse(html))
+}

book.toml

@@ -12,3 +12,6 @@ create-missing = true
 [output.html]
 git-repository-url= "https://github.com/quay/clair"
 preferred-dark-theme = "coal"
+
+[preprocessor.history]
+command = "go run Documentation/history.go"