slidewalk/internal/watch/server.go

package watch

import (
	"bytes"
	"fmt"
	"io/fs"
	"net/http"
	"path/filepath"
	"strings"
	"sync"

	"github.com/fsnotify/fsnotify"

	"slidewalk/internal/parser"
	"slidewalk/internal/render"
	"slidewalk/web"
)

// Server serves a slide deck over HTTP and pushes browser reloads via
// Server-Sent Events whenever a slide file changes on disk.
type Server struct {
	dir     string
	watcher *fsnotify.Watcher
	broker  *broker

	mu   sync.RWMutex
	page []byte
}

// NewServer parses and renders the slide deck in dir, then starts watching
// dir for changes so the in-memory page stays up to date.
func NewServer(dir string) (*Server, error) {
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		return nil, fmt.Errorf("creating file watcher: %w", err)
	}
	if err := watcher.Add(dir); err != nil {
		watcher.Close()
		return nil, fmt.Errorf("watching %s: %w", dir, err)
	}

	s := &Server{
		dir:     dir,
		watcher: watcher,
		broker:  newBroker(),
	}

	if err := s.reload(); err != nil {
		watcher.Close()
		return nil, err
	}

	go s.watchLoop()

	return s, nil
}

// Close stops the file watcher.
func (s *Server) Close() error {
	return s.watcher.Close()
}

// reload re-parses and re-renders the slide deck, replacing the in-memory
// page.
func (s *Server) reload() error {
	slides, err := parser.ParseDir(s.dir)
	if err != nil {
		return fmt.Errorf("parsing slides in %s: %w", s.dir, err)
	}

	var buf bytes.Buffer
	page := render.Page{
		Title:     filepath.Base(filepath.Clean(s.dir)),
		Slides:    slides,
		DevReload: true,
	}
	if err := render.Render(&buf, page); err != nil {
		return fmt.Errorf("rendering page: %w", err)
	}

	s.mu.Lock()
	s.page = buf.Bytes()
	s.mu.Unlock()
	return nil
}

func (s *Server) watchLoop() {
	for {
		select {
		case event, ok := <-s.watcher.Events:
			if !ok {
				return
			}
			if event.Op&(fsnotify.Write|fsnotify.Create|fsnotify.Remove|fsnotify.Rename) == 0 {
				continue
			}
			if err := s.reload(); err != nil {
				// The edit left the deck in a state that fails to parse or
				// render (e.g. invalid frontmatter); keep serving the last
				// good version instead of crashing the dev server.
				continue
			}
			s.broker.notify()
		case _, ok := <-s.watcher.Errors:
			if !ok {
				return
			}
		}
	}
}

// Handler returns the HTTP handler serving the current rendered page, its
// static assets, and the SSE endpoint used for live reload.
func (s *Server) Handler() (http.Handler, error) {
	assets, err := fs.Sub(web.Assets, "assets")
	if err != nil {
		return nil, fmt.Errorf("preparing embedded assets: %w", err)
	}

	userAssets := http.FileServer(http.Dir(s.dir))

	mux := http.NewServeMux()
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path == "/" {
			s.mu.RLock()
			page := s.page
			s.mu.RUnlock()

			w.Header().Set("Content-Type", "text/html; charset=utf-8")
			_, _ = w.Write(page)
			return
		}

		if !isServableUserAsset(r.URL.Path) {
			http.NotFound(w, r)
			return
		}
		userAssets.ServeHTTP(w, r)
	})
	mux.HandleFunc("/events", s.handleEvents)
	mux.Handle("/vendor/", http.FileServer(http.FS(assets)))

	return mux, nil
}

// isServableUserAsset reports whether urlPath may be served from the slide
// source directory as a user asset (e.g. an image referenced by a slide).
// Slide Markdown files and dotfiles/dot-directories (".git", ".DS_Store",
// ...) are excluded.
func isServableUserAsset(urlPath string) bool {
	if strings.EqualFold(filepath.Ext(urlPath), ".md") {
		return false
	}
	for _, part := range strings.Split(urlPath, "/") {
		if strings.HasPrefix(part, ".") && part != "" {
			return false
		}
	}
	return true
}

// handleEvents serves the SSE endpoint that live-reload.js connects to. It
// pushes one event per change notification and otherwise blocks until the
// client disconnects.
func (s *Server) handleEvents(w http.ResponseWriter, r *http.Request) {
	flusher, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "streaming not supported", http.StatusInternalServerError)
		return
	}

	w.Header().Set("Content-Type", "text/event-stream")
	w.Header().Set("Cache-Control", "no-cache")
	w.Header().Set("Connection", "keep-alive")
	w.WriteHeader(http.StatusOK)
	flusher.Flush()

	ch := s.broker.subscribe()
	defer s.broker.unsubscribe(ch)

	for {
		select {
		case <-r.Context().Done():
			return
		case _, ok := <-ch:
			if !ok {
				return
			}
			fmt.Fprint(w, "data: reload\n\n")
			flusher.Flush()
		}
	}
}

// Serve starts an HTTP server on addr that serves the slide deck in dir,
// live-reloading connected browsers whenever a slide file changes.
func Serve(addr, dir string) error {
	srv, err := NewServer(dir)
	if err != nil {
		return err
	}
	defer srv.Close()

	handler, err := srv.Handler()
	if err != nil {
		return err
	}

	if err := http.ListenAndServe(addr, handler); err != nil {
		return fmt.Errorf("dev server: %w", err)
	}
	return nil
}