Fix #226: a refused Web Locks request no longer leaves the web app loading

apps/lightcraft-web/src/safety.rs

@@ -93,25 +93,41 @@ pub fn install_panic_hook() {
 const TAB_LOCK: &str = "lightcraft-library";
 
 /// Hold the library lock for as long as this page lives (Web Locks API). `Some(true)`: ours;
-/// `Some(false)`: another tab of this site has it; `None`: the browser can't tell (no Web Locks).
+/// `Some(false)`: another tab of this site has it; `None`: the browser can't tell (no Web Locks, or
+/// it refused the request).
 pub async fn acquire_tab_lock() -> Option<bool> {
     let nav = Reflect::get(&js_sys::global(), &"navigator".into()).ok()?;
     let locks = Reflect::get(&nav, &"locks".into()).ok().filter(|l| !l.is_undefined() && !l.is_null())?;
     let request: js_sys::Function = Reflect::get(&locks, &"request".into()).ok()?.dyn_into().ok()?;
     let opts = js_sys::Object::new();
     Reflect::set(&opts, &"ifAvailable".into(), &true.into()).ok()?;
-    let granted = js_sys::Promise::new(&mut |resolve, _reject| {
+    let granted = js_sys::Promise::new(&mut |resolve, reject| {
         let cb = Closure::once_into_js(move |lock: JsValue| -> JsValue {
             let ours = !lock.is_null() && !lock.is_undefined();
             let _ = resolve.call1(&JsValue::NULL, &ours.into());
             // keep the lock until the page goes away: a promise that never settles
             if ours { js_sys::Promise::new(&mut |_, _| {}).into() } else { JsValue::NULL }
         });
-        if let Err(e) = request.call3(&locks, &TAB_LOCK.into(), &opts, &cb) {
-            log::warn!("Web Locks request failed: {e:?}");
+        // a refused request never calls `cb`: settle with its error instead of waiting forever
+        match request.call3(&locks, &TAB_LOCK.into(), &opts, &cb) {
+            Ok(pending) => {
+                let catch = Reflect::get(&pending, &"catch".into()).ok().and_then(|c| c.dyn_into::<js_sys::Function>().ok());
+                if let Some(catch) = catch {
+                    let _ = catch.call1(&pending, &reject);
+                }
+            }
+            Err(e) => {
+                let _ = reject.call1(&JsValue::NULL, &e);
+            }
         }
     });
-    JsFuture::from(granted).await.ok().map(|v| v.is_truthy())
+    match JsFuture::from(granted).await {
+        Ok(v) => Some(v.is_truthy()),
+        Err(e) => {
+            log::warn!("Web Locks request failed, starting without the one-tab guard: {e:?}");
+            None
+        }
+    }
 }
 
 /// Offer a blob as a download named `name`.

docs/web.md

@@ -122,8 +122,8 @@ are the same in every version, so serve them with `Cache-Control: no-cache` (not
     a reload); a notice says why.
   - One tab at a time: the page holds a Web Lock (`navigator.locks`, `lightcraft-library`); a
     second tab or window shows "LightCraft is already open in another tab" instead of loading
-    its own copy (two copies would overwrite each other's saves). Browsers without Web Locks
-    aren't protected.
+    its own copy (two copies would overwrite each other's saves). Browsers without Web Locks, or
+    that refuse the lock request, aren't protected: the app starts without the guard.
   - If the stored library can't be opened, a notice says the session is temporary and nothing
     is saved; the stored library is left as it was.
   - A panic (wasm is built with `panic = "abort"`) replaces the page with a message saying the