feat(web): keep the saved session on a demo start, bound the config request

An explicit demo startup no longer saves the session, so the demo cannot replace the last visit's project. The config request is aborted after 5 s, stops reading once the body passes the size limit, and is skipped for ?empty and ?demo. The packaged hosting notes link to the published docs.

apps/effectcraft-web/Cargo.toml

@@ -37,6 +37,8 @@ js-sys = "0.3"
 web-sys = { version = "0.3", features = [
     "Blob",
     "BlobPropertyBag",
+    "AbortController",
+    "AbortSignal",
     "CssStyleDeclaration",
     "Document",
     "Element",
@@ -51,6 +53,8 @@ web-sys = { version = "0.3", features = [
     "HtmlInputElement",
     "Location",
     "Navigator",
+    "ReadableStream",
+    "ReadableStreamDefaultReader",
     "Request",
     "RequestCache",
     "RequestInit",

apps/effectcraft-web/src/app.rs

@@ -6,6 +6,7 @@ use std::sync::atomic::{AtomicBool, Ordering};
 
 use effectcraft_engine::Session;
 
+use crate::startup::Startup;
 use crate::{api, audio, browse, config_file, diskcache, files, frames, persist, storage, worker};
 use effectcraft_ui_egui::EffectcraftApp;
 use serde_json::json;
@@ -81,7 +82,7 @@ pub(crate) fn open_or_import(app: &mut EffectcraftApp, paths: Vec<String>) {
 /// The eframe app: the shared EffectCraft UI plus the browser host's per-frame duties.
 pub struct WebApp {
     pub app: EffectcraftApp,
-    /// Keep the session snapshot (off with `?empty`).
+    /// Keep the session snapshot (off with `?empty` and an explicit `"demo"` startup).
     pub persist: bool,
 }
 
@@ -221,7 +222,10 @@ pub async fn start(canvas_id: String) -> Result<(), JsValue> {
     api::set_info("storage", json!({"backend": persist::backend(), "entries": stored, "loadMs": t0.elapsed().as_secs_f64() * 1000.0}));
     audio::install_unlock();
     // `./effectcraft-config.json`: what to open on load.
-    let startup = config_file::startup().await;
+    // `?empty` and `?demo` decide by themselves, so they skip the request.
+    let startup = if empty || query_flag("demo") { Startup::default() } else { config_file::startup().await };
+    // An explicit `"demo"` start leaves the saved session alone: the demo must not overwrite it.
+    let keep_session = !empty && startup.keeps_session();
     browse::restore();
     diskcache::init();
     storage::refresh();
@@ -247,7 +251,7 @@ pub async fn start(canvas_id: String) -> Result<(), JsValue> {
                     // The canvas can't present any more (#225): the page says so over it and
                     // offers a reload, which brings the session back when browser storage keeps it.
                     let lost = gpu_failures.clone();
-                    let kept = !empty && persist::backend() != "memory";
+                    let kept = keep_session && persist::backend() != "memory";
                     rs.device.set_device_lost_callback(move |reason, message| {
                         let message = format!("GPU device lost ({reason:?}): {message}");
                         lost.report(&message, true);
@@ -287,7 +291,7 @@ pub async fn start(canvas_id: String) -> Result<(), JsValue> {
                 app = app.with_control(rx);
                 let backend = cc.wgpu_render_state.as_ref().map(|rs| format!("{:?}", rs.adapter.get_info().backend));
                 api::set_info("backend", json!(backend));
-                Ok(Box::new(WebApp { app, persist: !empty }))
+                Ok(Box::new(WebApp { app, persist: keep_session }))
             }),
         )
         .await?;

apps/effectcraft-web/src/config_file.rs

@@ -2,28 +2,57 @@
 
 use std::cell::RefCell;
 
-use wasm_bindgen::JsCast;
+use wasm_bindgen::{JsCast, closure::Closure};
 use wasm_bindgen_futures::JsFuture;
 
 use crate::startup::{self, Startup};
 
+/// Longest the request and the body read may take (ms); a stalled server must not hold up start-up.
+const TIMEOUT_MS: i32 = 5_000;
+
 thread_local! {
     /// `None`: not fetched yet; `Some(None)`: no usable file.
     static FILE: RefCell<Option<Option<Vec<u8>>>> = const { RefCell::new(None) };
 }
 
+/// The config file's bytes, or `None` when it is missing, too large, unreadable or the request
+/// takes longer than [`TIMEOUT_MS`] (the timer aborts the request and covers the body read).
 async fn fetch() -> Option<Vec<u8>> {
+    let win = web_sys::window()?;
+    let ctl = web_sys::AbortController::new().ok()?;
     let init = web_sys::RequestInit::new();
     // Always ask the server: the file is the operator's switch, never a cached copy.
     init.set_cache(web_sys::RequestCache::NoCache);
-    let req = web_sys::Request::new_with_str_and_init("effectcraft-config.json", &init).ok()?;
-    let resp: web_sys::Response = JsFuture::from(web_sys::window()?.fetch_with_request(&req)).await.ok()?.dyn_into().ok()?;
+    init.set_signal(Some(&ctl.signal()));
+    let on_timeout = Closure::once(move || ctl.abort());
+    let timer = win.set_timeout_with_callback_and_timeout_and_arguments_0(on_timeout.as_ref().unchecked_ref(), TIMEOUT_MS).ok()?;
+    let out = read(&win, &init).await;
+    win.clear_timeout_with_handle(timer);
+    out
+}
+
+/// Fetch the file and read its body chunk by chunk, giving up as soon as it passes `CONFIG_MAX`.
+async fn read(win: &web_sys::Window, init: &web_sys::RequestInit) -> Option<Vec<u8>> {
+    let req = web_sys::Request::new_with_str_and_init("effectcraft-config.json", init).ok()?;
+    let resp: web_sys::Response = JsFuture::from(win.fetch_with_request(&req)).await.ok()?.dyn_into().ok()?;
     if resp.status() != 200 {
         return None;
     }
-    let buf = JsFuture::from(resp.array_buffer().ok()?).await.ok()?;
-    let a = js_sys::Uint8Array::new(&buf);
-    (a.length() as usize <= startup::CONFIG_MAX).then(|| a.to_vec())
+    let reader: web_sys::ReadableStreamDefaultReader = resp.body()?.get_reader().unchecked_into();
+    let mut out = Vec::new();
+    loop {
+        let chunk = JsFuture::from(reader.read()).await.ok()?;
+        if js_sys::Reflect::get(&chunk, &"done".into()).ok()?.as_bool() == Some(true) {
+            return Some(out);
+        }
+        let bytes: js_sys::Uint8Array = js_sys::Reflect::get(&chunk, &"value".into()).ok()?.dyn_into().ok()?;
+        if out.len().saturating_add(bytes.length() as usize) > startup::CONFIG_MAX {
+            // Stop the download: the limit is enforced while reading, not after the whole body arrived.
+            let _ = reader.cancel();
+            return None;
+        }
+        out.extend_from_slice(&bytes.to_vec());
+    }
 }
 
 /// The config file's bytes (`None` when there is none), fetched on the first call.

apps/effectcraft-web/src/startup.rs

@@ -42,6 +42,12 @@ impl Startup {
         self != Startup::Demo
     }
 
+    /// Whether the session snapshot keeps being saved (off for the demo, so it never replaces the
+    /// project the last visit left).
+    pub fn keeps_session(self) -> bool {
+        self != Startup::Demo
+    }
+
     /// Whether the demo project opens when nothing was restored (off for a blank start).
     pub fn demo_when_nothing_restored(self) -> bool {
         self != Startup::Empty

apps/effectcraft-web/tests/it/startup_tests.rs

@@ -2,6 +2,7 @@
 
 use effectcraft_web::startup::{CONFIG_MAX, Startup, parse};
 
+/// The three accepted values parse.
 #[test]
 fn the_three_values() {
     assert_eq!(parse(br#"{"startup":"empty"}"#), Startup::Empty);
@@ -9,6 +10,7 @@ fn the_three_values() {
     assert_eq!(parse(br#"{"startup":"last"}"#), Startup::Last);
 }
 
+/// A missing key, a wrong type, an unknown value or a non-object file gives `Last`.
 #[test]
 fn absent_or_invalid_is_the_default() {
     for bytes in
@@ -18,12 +20,14 @@ fn absent_or_invalid_is_the_default() {
     }
 }
 
+/// Other keys do not hide `startup`.
 #[test]
 fn a_config_with_other_keys_still_has_its_startup() {
     assert_eq!(parse(br#"{"remote":{"mounts":[]},"startup":"empty"}"#), Startup::Empty);
     assert_eq!(parse(br#"{"startup":"demo","unknown":{"a":1}}"#), Startup::Demo);
 }
 
+/// A file over `CONFIG_MAX` is ignored.
 #[test]
 fn an_oversized_file_is_the_default() {
     let mut big = br#"{"startup":"empty","pad":""#.to_vec();
@@ -32,9 +36,17 @@ fn an_oversized_file_is_the_default() {
     assert_eq!(parse(&big), Startup::Last);
 }
 
+/// Which project each value restores or opens.
 #[test]
 fn what_each_value_does_on_load() {
     assert!(Startup::Last.restores() && Startup::Last.demo_when_nothing_restored());
     assert!(Startup::Empty.restores() && !Startup::Empty.demo_when_nothing_restored());
     assert!(!Startup::Demo.restores() && Startup::Demo.demo_when_nothing_restored());
 }
+
+/// Only the explicit demo start stops saving the session.
+#[test]
+fn only_the_demo_start_leaves_the_saved_session_alone() {
+    assert!(Startup::Last.keeps_session() && Startup::Empty.keeps_session());
+    assert!(!Startup::Demo.keeps_session());
+}

docs/web.md

@@ -255,7 +255,7 @@ By default the page opens the last visit's project, and the demo project on a fi
 |---|---|
 | `"last"` (or no key) | the last visit's project; the demo project on a first visit |
 | `"empty"` | the last visit's project; a blank project on a first visit, never the demo |
-| `"demo"` | the demo project, whatever the last visit left |
+| `"demo"` | the demo project, whatever the last visit left; the session is not saved, so the last visit's project stays as it was |
 
 The `?empty` and `?demo` URL flags still win. A missing, invalid or non-JSON file, or an unknown
 value, is the default. The file is fetched once per page load; serve it with

packaging/web/README.md

@@ -67,9 +67,9 @@ To choose what the page opens on load, add an `effectcraft-config.json` next to
 ```
 
 `"empty"` restores the last visit's project and otherwise starts blank (never the demo); `"demo"`
-always opens the demo project; `"last"` (the default) restores the last visit's project and shows
+always opens the demo project without saving the session (the last visit's project stays); `"last"` (the default) restores the last visit's project and shows
 the demo on a first visit. Serve the file with `Cache-Control: no-cache`. Details:
-[docs/web.md](../../docs/web.md#startup-project).
+[docs/web.md](https://github.com/storytold/effectcraft/blob/main/docs/web.md#startup-project).
 
 ## Embedding in a page (iframe)