mangaBookmark/backend/internal/userscript/userscript.go

package userscript

import (
	"bytes"
	"log"
	"net/http"
	"os"
	"regexp"
	"time"

	"bookmarkmanager/backend/internal/httpmw"
	"bookmarkmanager/backend/internal/store"
)

// tokenPlaceholder is what the bindmounted userscript carries where the
// Reader's credential goes: in the API_TOKEN constant and in the @downloadURL
// and @updateURL metadata lines. The handler substitutes the requesting
// Reader's credential for it at serve time, so no credential literal is ever
// committed or deployed, and each Reader's copy carries exactly their own.
var tokenPlaceholder = []byte("__API_TOKEN__")

// versionLine matches the userscript metadata block's @version directive.
var versionLine = regexp.MustCompile(`(?m)^// @version[ \t]+.*$`)

// stampVersion replaces the served @version with one derived from the file's
// mtime, discarding whatever the file body says.
//
// Violentmonkey only updates when the served version sorts higher than the
// installed one. Deriving it from the body means one accidental downgrade or
// typo freezes updates forever; an mtime-derived version is monotonic by
// construction, so any later write always outranks any earlier one.
//
// A file with no @version line is returned untouched: such a script never
// auto-updates anyway, and inventing a metadata block is not this handler's job.
func stampVersion(src []byte, mod time.Time) []byte {
	return versionLine.ReplaceAll(src, []byte("// @version      "+mod.UTC().Format("2006.01.02.1504")))
}

// substituteToken replaces every tokenPlaceholder with the Reader's
// credential. A file without the placeholder is returned unchanged so Render
// can warn about it rather than silently serving a credential-less script.
func substituteToken(src []byte, credential string) []byte {
	return bytes.ReplaceAll(src, tokenPlaceholder, []byte(credential))
}

// Render writes one userscript file with the credential substituted and the
// mtime-derived version stamped. Shared by the download path (Handler) and
// the web UI's install endpoints, so both serve byte-identical scripts.
//
// The file is read per request — that is what lets a bindmounted copy be
// edited on the host without a restart. It is ~50 KB and polled about once a
// day.
func Render(w http.ResponseWriter, r *http.Request, path, credential string) {
	info, err := os.Stat(path)
	if err != nil {
		log.Printf("userscript: stat %s: %v", path, err)
		http.NotFound(w, r)
		return
	}
	src, err := os.ReadFile(path)
	if err != nil {
		log.Printf("userscript: read %s: %v", path, err)
		http.NotFound(w, r)
		return
	}
	rendered := substituteToken(src, credential)
	if bytes.Equal(rendered, src) {
		// The bindmounted file was not built for per-Reader rendering. Serving
		// it as written is the operator's freedom, but a credential-less copy
		// is a deployment bug worth one log line — the symptom (silent 401s on
		// every device) is otherwise indistinguishable from a network fault.
		log.Printf("userscript: %s has no %s placeholder; serving as written", path, tokenPlaceholder)
	}
	w.Header().Set("Content-Type", "text/javascript; charset=utf-8")
	w.Header().Set("Cache-Control", "no-cache")
	w.Write(stampVersion(rendered, info.ModTime()))
}

// Handler serves the userscript to Violentmonkey's updater, rendered for the
// Reader whose credential is in the path.
//
// The credential lives in the path because the update poll sends no
// Authorization header, and the rendered file embeds the credential in
// plaintext, so an open path would hand it to anyone who guessed the URL. A
// mismatch answers 404 rather than 401: a prober learns nothing about whether
// the route exists. The same credential authenticates the API bearer header,
// so the two are one secret with one blast radius.
//
// The path segment is the credential itself, so once it resolves it is also
// exactly what the served copy must carry — no re-derivation needed.
func Handler(s *store.Store, path string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		cred := r.PathValue("token")
		if _, ok := httpmw.ResolveReader(s, cred); !ok {
			http.NotFound(w, r)
			return
		}
		Render(w, r, path, cred)
	}
}