refactor(examples): pre-parse frontmatter via scripts/examplegen (#9514)

* refactor(examples): pre-parse frontmatter via scripts/examplegen

This removes 2 MB from the slim binary.

Ref: #9380

scripts/examplegen/main.go

@@ -0,0 +1,144 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"github.com/gohugoio/hugo/parser/pageparser"
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/codersdk"
+)
+
+const (
+	examplesDir = "examples"
+	examplesSrc = "examples.go"
+)
+
+func main() {
+	if err := run(); err != nil {
+		panic(err)
+	}
+}
+
+func run() error {
+	fset := token.NewFileSet()
+	src, err := parser.ParseFile(fset, filepath.Join(examplesDir, examplesSrc), nil, parser.ParseComments)
+	if err != nil {
+		return err
+	}
+
+	var paths []string
+	for _, comment := range src.Comments {
+		for _, line := range comment.List {
+			if s, ok := parseEmbedTag(line.Text); ok && !strings.HasSuffix(s, ".json") {
+				paths = append(paths, s)
+			}
+		}
+	}
+
+	var examples []codersdk.TemplateExample
+	files := os.DirFS(examplesDir)
+	for _, name := range paths {
+		dir, err := fs.Stat(files, name)
+		if err != nil {
+			return err
+		}
+		if !dir.IsDir() {
+			continue
+		}
+		exampleID := dir.Name()
+		// Each one of these is a example!
+		readme, err := fs.ReadFile(files, path.Join(name, "README.md"))
+		if err != nil {
+			return xerrors.Errorf("example %q does not contain README.md", exampleID)
+		}
+
+		frontMatter, err := pageparser.ParseFrontMatterAndContent(bytes.NewReader(readme))
+		if err != nil {
+			return xerrors.Errorf("parse example %q front matter: %w", exampleID, err)
+		}
+
+		nameRaw, exists := frontMatter.FrontMatter["name"]
+		if !exists {
+			return xerrors.Errorf("example %q front matter does not contain name", exampleID)
+		}
+
+		name, valid := nameRaw.(string)
+		if !valid {
+			return xerrors.Errorf("example %q name isn't a string", exampleID)
+		}
+
+		descriptionRaw, exists := frontMatter.FrontMatter["description"]
+		if !exists {
+			return xerrors.Errorf("example %q front matter does not contain name", exampleID)
+		}
+
+		description, valid := descriptionRaw.(string)
+		if !valid {
+			return xerrors.Errorf("example %q description isn't a string", exampleID)
+		}
+
+		tags := []string{}
+		tagsRaw, exists := frontMatter.FrontMatter["tags"]
+		if exists {
+			tagsI, valid := tagsRaw.([]interface{})
+			if !valid {
+				return xerrors.Errorf("example %q tags isn't a slice: type %T", exampleID, tagsRaw)
+			}
+			for _, tagI := range tagsI {
+				tag, valid := tagI.(string)
+				if !valid {
+					return xerrors.Errorf("example %q tag isn't a string: type %T", exampleID, tagI)
+				}
+				tags = append(tags, tag)
+			}
+		}
+
+		var icon string
+		iconRaw, exists := frontMatter.FrontMatter["icon"]
+		if exists {
+			icon, valid = iconRaw.(string)
+			if !valid {
+				return xerrors.Errorf("example %q icon isn't a string", exampleID)
+			}
+		}
+
+		examples = append(examples, codersdk.TemplateExample{
+			ID:          exampleID,
+			Name:        name,
+			Description: description,
+			Icon:        icon,
+			Tags:        tags,
+			Markdown:    string(frontMatter.Content),
+
+			// URL is set by examples/examples.go.
+		})
+	}
+
+	w := os.Stdout
+
+	_, err = fmt.Fprint(w, "// Code generated by examplegen. DO NOT EDIT.\n")
+	if err != nil {
+		return err
+	}
+
+	enc := json.NewEncoder(os.Stdout)
+	enc.SetIndent("", "  ")
+	return enc.Encode(examples)
+}
+
+func parseEmbedTag(s string) (string, bool) {
+	if !strings.HasPrefix(s, "//go:embed") {
+		return "", false
+	}
+	return strings.TrimSpace(strings.TrimPrefix(s, "//go:embed")), true
+}