backend/main.go

@@ -21,6 +21,9 @@ type Config struct {
 	Port           string
 	// WebPassword gates the browser UI. Empty disables the web routes entirely.
 	WebPassword string
+	// UserscriptPath is the file served at /u/{token}/manga-bookmark.user.js.
+	// Supplied by a bindmount so the script can be edited without a rebuild.
+	UserscriptPath string
 	// LatestPoll configures the background latest-chapter fetcher.
 	LatestPoll LatestPoll
 }
@@ -128,11 +131,12 @@ func loadLatestPoll() LatestPoll {
 
 func loadConfig() Config {
 	c := Config{
-		Token:       os.Getenv("API_TOKEN"),
+		Token:          os.Getenv("API_TOKEN"),
-		DBPath:      envOr("DB_PATH", "/data/bookmarks.db"),
+		DBPath:         envOr("DB_PATH", "/data/bookmarks.db"),
-		Port:        envOr("PORT", "8080"),
+		Port:           envOr("PORT", "8080"),
-		WebPassword: os.Getenv("WEB_PASSWORD"),
+		WebPassword:    os.Getenv("WEB_PASSWORD"),
-		LatestPoll:  loadLatestPoll(),
+		UserscriptPath: envOr("USERSCRIPT_PATH", "/userscript/manga-bookmark.user.js"),
+		LatestPoll:     loadLatestPoll(),
 	}
 	for _, o := range strings.Split(os.Getenv("ALLOWED_ORIGINS"), ",") {
 		if o = strings.TrimSpace(o); o != "" {
@@ -149,6 +153,11 @@ func newRouter(store *Store, cfg Config) http.Handler {
 	mux := http.NewServeMux()
 	mux.HandleFunc("GET /healthz", healthz)
 
+	// Outside withAuth (the updater sends no Authorization header) and outside
+	// the WEB_PASSWORD gate (the script must be installable either way). The
+	// path segment carries the token instead.
+	mux.HandleFunc("GET /u/{token}/manga-bookmark.user.js", userscriptHandler(cfg.Token, cfg.UserscriptPath))
+
 	h := &bookmarkHandler{store: store}
 	protected := http.NewServeMux()
 	protected.HandleFunc("GET /bookmarks", h.list)
@@ -170,7 +179,22 @@ func newRouter(store *Store, cfg Config) http.Handler {
 		web.register(mux)
 	}
 
-	return withCORS(cfg.AllowedOrigins, mux)
+	return withCORS(cfg.AllowedOrigins, guardEmptyUserscriptToken(mux))
+}
+
+// guardEmptyUserscriptToken heads off ServeMux's own path-cleaning redirect:
+// an empty {token} segment makes the request path "/u//manga-bookmark.user.js",
+// and ServeMux 307s that to "/u/manga-bookmark.user.js" before pattern
+// matching ever runs. The endpoint's contract is 404 for any wrong token,
+// including this one, so catch it ahead of the mux.
+func guardEmptyUserscriptToken(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if strings.HasPrefix(r.URL.Path, "/u//") {
+			http.NotFound(w, r)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
 }
 
 func main() {

backend/userscript.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"crypto/subtle"
+	"log"
+	"net/http"
+	"os"
+	"regexp"
+	"time"
+)
+
+// 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")))
+}
+
+// userscriptHandler serves the userscript to Violentmonkey's updater.
+//
+// The token lives in the path because the update poll sends no Authorization
+// header, and the file embeds API_TOKEN in plain text, so an open path would
+// hand that token to anyone who guessed the URL. A mismatch answers 404 rather
+// than 401: a prober learns nothing about whether the route exists.
+//
+// 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 userscriptHandler(token, path string) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if subtle.ConstantTimeCompare([]byte(r.PathValue("token")), []byte(token)) != 1 {
+			http.NotFound(w, r)
+			return
+		}
+		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
+		}
+		w.Header().Set("Content-Type", "text/javascript; charset=utf-8")
+		w.Header().Set("Cache-Control", "no-cache")
+		w.Write(stampVersion(src, info.ModTime()))
+	}
+}

backend/userscript_test.go

@@ -0,0 +1,132 @@
+package main
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+// sampleScript is a stand-in for the real userscript: a metadata block with a
+// @version line, plus a body that must survive the rewrite untouched.
+const sampleScript = `// ==UserScript==
+// @name         Manga Bookmark Sync
+// @version      1.5.0
+// @match        https://asurascans.com/*
+// ==/UserScript==
+(function () { "use strict"; })();
+`
+
+// writeScript drops a userscript in a temp dir with a known mtime and returns
+// its path plus the version string the handler is expected to stamp.
+func writeScript(t *testing.T, body string) (path, wantVersion string) {
+	t.Helper()
+	path = filepath.Join(t.TempDir(), "manga-bookmark.user.js")
+	if err := os.WriteFile(path, []byte(body), 0o644); err != nil {
+		t.Fatalf("write script: %v", err)
+	}
+	mod := time.Date(2026, 7, 28, 16, 42, 0, 0, time.UTC)
+	if err := os.Chtimes(path, mod, mod); err != nil {
+		t.Fatalf("chtimes: %v", err)
+	}
+	return path, "2026.07.28.1642"
+}
+
+func newUserscriptServer(t *testing.T, path string) http.Handler {
+	t.Helper()
+	store, err := OpenStore(filepath.Join(t.TempDir(), "test.db"))
+	if err != nil {
+		t.Fatalf("OpenStore: %v", err)
+	}
+	t.Cleanup(func() { store.Close() })
+	cfg := testConfig()
+	cfg.UserscriptPath = path
+	return newRouter(store, cfg)
+}
+
+func getScript(t *testing.T, srv http.Handler, token string) *httptest.ResponseRecorder {
+	t.Helper()
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, httptest.NewRequest(http.MethodGet, "/u/"+token+"/manga-bookmark.user.js", nil))
+	return rr
+}
+
+func TestUserscriptServedWithStampedVersion(t *testing.T) {
+	path, wantVersion := writeScript(t, sampleScript)
+	rr := getScript(t, newUserscriptServer(t, path), testToken)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if ct := rr.Header().Get("Content-Type"); !strings.HasPrefix(ct, "text/javascript") {
+		t.Errorf("Content-Type = %q, want text/javascript", ct)
+	}
+	if cc := rr.Header().Get("Cache-Control"); cc != "no-cache" {
+		t.Errorf("Cache-Control = %q, want no-cache", cc)
+	}
+	body := rr.Body.String()
+	if !strings.Contains(body, "// @version      "+wantVersion) {
+		t.Errorf("body has no stamped version %q:\n%s", wantVersion, body)
+	}
+	if strings.Contains(body, "1.5.0") {
+		t.Errorf("body still carries the file's own version:\n%s", body)
+	}
+	// Everything outside the @version line is served verbatim.
+	if !strings.Contains(body, `(function () { "use strict"; })();`) {
+		t.Errorf("body was altered beyond the version line:\n%s", body)
+	}
+	if !strings.Contains(body, "// @name         Manga Bookmark Sync") {
+		t.Errorf("metadata block was altered:\n%s", body)
+	}
+}
+
+func TestUserscriptWrongTokenIs404(t *testing.T) {
+	path, _ := writeScript(t, sampleScript)
+	srv := newUserscriptServer(t, path)
+	for _, tok := range []string{"wrong", "", testToken + "x", testToken[:3]} {
+		if got := getScript(t, srv, tok).Code; got != http.StatusNotFound {
+			t.Errorf("token %q: status = %d, want 404", tok, got)
+		}
+	}
+}
+
+func TestUserscriptMissingFileIs404(t *testing.T) {
+	srv := newUserscriptServer(t, filepath.Join(t.TempDir(), "absent.user.js"))
+	if got := getScript(t, srv, testToken).Code; got != http.StatusNotFound {
+		t.Fatalf("status = %d, want 404", got)
+	}
+}
+
+func TestUserscriptWithoutVersionLineServedUnmodified(t *testing.T) {
+	const noVersion = "// ==UserScript==\n// @name x\n// ==/UserScript==\nconsole.log(1);\n"
+	path, _ := writeScript(t, noVersion)
+	rr := getScript(t, newUserscriptServer(t, path), testToken)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rr.Code)
+	}
+	if rr.Body.String() != noVersion {
+		t.Fatalf("body = %q, want it unmodified", rr.Body.String())
+	}
+}
+
+// The endpoint must work on a deployment that never set WEB_PASSWORD, since
+// the web routes are not registered at all in that case.
+func TestUserscriptServedWithWebUIDisabled(t *testing.T) {
+	path, _ := writeScript(t, sampleScript)
+	store, err := OpenStore(filepath.Join(t.TempDir(), "nopass.db"))
+	if err != nil {
+		t.Fatalf("OpenStore: %v", err)
+	}
+	t.Cleanup(func() { store.Close() })
+	cfg := testConfig()
+	cfg.WebPassword = ""
+	cfg.UserscriptPath = path
+
+	if got := getScript(t, newRouter(store, cfg), testToken).Code; got != http.StatusOK {
+		t.Fatalf("status = %d, want 200", got)
+	}
+}