web: harden drop-open and autosave after adversarial review

Review of the drop-open and autosave commits surfaced one blocker and
five majors; all are fixed here, and two claims in the autosave commit
message are corrected by this one: tab close does NOT clear a snapshot
(matches native .sv$ semantics — saving, restoring, discarding, exit,
and the capped boot scan do), and serialization runs on the main
thread between awaits, not off the paint path.

- The restore prompt can no longer appear with no entries (empty
  "Recover Drawing" modal on every boot once the autosave dir exists).
- Restore no longer consumes the snapshot before the open succeeds:
  the recovered drawing goes through the guarded drop queue, and the
  snapshot is deleted only after the tab opens; cancel or parse
  failure keeps the only copy.
- Saving during an autosave round defers its snapshot cleanup until
  the round finishes, so a stale round can no longer resurrect a
  snapshot newer than the save.
- The boot scan no longer deletes snapshots by mtime comparison
  (re-opening a drawing refreshes the recent copy's mtime, which
  silently destroyed wanted snapshots); GC is now a 16-entry cap with
  oldest-first eviction.
- OPFS write failures abort the writable stream (a leaked lock could
  stall the same-key writer and disable autosave for the session), and
  DOMException errors now carry their name/message.
- Empty (crash-window) snapshots are never offered; an unreadable one
  warns and stays discardable instead of looping forever.
- Drop-open polish: reads start inside the handler and complete in
  drop order (legacy WebKit safety, deterministic multi-file order),
  dragenter is canceled too, the install guard only latches on
  success, non-drawing drops don't steal propagation, a lock-poison
  panic can't cross into JS, and re-dropping an open drawing no
  longer risks clobbering an in-flight open (it re-queues).
- The model-space sheet floor is per-axis (a thin 8x400 mm profile
  keeps its length) and centers content on the enlarged page.

src/io/web_autosave.rs

@@ -8,11 +8,18 @@
 //! offered for restore at boot when newer than the drawing's last explicit
 //! save (the OPFS recent copy of the same name).
 //!
+//! Losing that comparison never deletes a snapshot: the recent copy is
+//! refreshed by every OPEN as well as every save, so a snapshot that just
+//! went stale may still be the only survivor of a crash right after a
+//! re-open. Accumulation is bounded instead — the boot scan keeps the
+//! newest [`SNAPSHOT_CAP`] snapshots and evicts only the oldest ones beyond
+//! that cap.
+//!
 //! OPFS entry names carry only the FNV hash of the drawing name (like
 //! `web_recent`), so a small JSON index maps hashes back to names. The index
 //! is written before its snapshot: a crash between the two writes leaves a
-//! name without bytes, which `list` filters out, rather than an orphan
-//! snapshot nobody can name.
+//! name without bytes — a missing file or a zero-byte one — which `list`
+//! filters out, rather than an orphan snapshot nobody can name.
 
 #[cfg(target_arch = "wasm32")]
 use wasm_bindgen::JsCast;
@@ -38,6 +45,9 @@ pub struct WebAutosaveEntry {
 /// A snapshot is worth offering when it is strictly newer than the last
 /// explicit save, or when no saved copy exists at all (the drawing was never
 /// saved — or its recent copy was evicted — so the snapshot is all there is).
+/// The comparison decides *offering* only, never survival: the recent copy
+/// is refreshed by every open, so "older than the recent copy" is not proof
+/// that a snapshot recovers nothing.
 #[cfg_attr(not(target_arch = "wasm32"), allow(dead_code))] // unit-tested on native
 pub(crate) fn newer_than_saved(autosave_ms: f64, saved_ms: Option<f64>) -> bool {
     match saved_ms {
@@ -46,6 +56,33 @@ pub(crate) fn newer_than_saved(autosave_ms: f64, saved_ms: Option<f64>) -> bool
     }
 }
 
+/// Upper bound on stored snapshots; the boot scan evicts the oldest ones
+/// beyond it. Without a bound, snapshots superseded by a save (or by a
+/// mere re-open) would pile up forever, since age no longer deletes them.
+#[cfg_attr(not(target_arch = "wasm32"), allow(dead_code))] // unit-tested on native
+pub(crate) const SNAPSHOT_CAP: usize = 16;
+
+/// Split `entries` into `(kept, evicted)` so at most `cap` snapshots remain,
+/// both sides ordered by `modified_ms` (newest first within each side; the
+/// evicted names overall run oldest-first, so an interrupted scan deletes
+/// the least valuable snapshots first). This is the only deletion criterion
+/// the boot scan applies — being older than the recent copy is not one.
+#[cfg_attr(not(target_arch = "wasm32"), allow(dead_code))] // unit-tested on native
+pub(crate) fn cap_evictions(
+    mut entries: Vec<WebAutosaveEntry>,
+    cap: usize,
+) -> (Vec<WebAutosaveEntry>, Vec<String>) {
+    entries.sort_by(|a, b| b.modified_ms.total_cmp(&a.modified_ms));
+    let overflow = entries.len().saturating_sub(cap);
+    let mut evicted: Vec<String> = entries
+        .split_off(entries.len() - overflow)
+        .into_iter()
+        .map(|entry| entry.name)
+        .collect();
+    evicted.reverse();
+    (entries, evicted)
+}
+
 /// The name a restored drawing carries as a tab and download: the drawing's
 /// stem plus " (recovered)" and its own extension (DWG when the stored name
 /// has none, matching how autosave serializes untitled tabs). The extension
@@ -89,7 +126,10 @@ pub async fn snapshot(name: &str, bytes: &[u8], thumbnail: Option<&[u8]>) -> Res
 }
 
 /// All live snapshots. Index rows whose file is gone (crash between writes,
-/// partial eviction) are skipped rather than reported.
+/// partial eviction) and zero-byte snapshots (the crash window after the
+/// index write but before the snapshot write) are skipped rather than
+/// reported — an empty file recovers nothing and would only route the user
+/// into the corrupt-file recovery flow.
 #[cfg(target_arch = "wasm32")]
 pub async fn list() -> Result<Vec<WebAutosaveEntry>, String> {
     let directory = autosave_directory(false).await?;
@@ -98,6 +138,9 @@ pub async fn list() -> Result<Vec<WebAutosaveEntry>, String> {
         let Ok(file) = super::web_recent::get_file(&directory, &snapshot_key(&name)).await else {
             continue;
         };
+        if file.size() == 0.0 {
+            continue;
+        }
         entries.push(WebAutosaveEntry {
             name,
             size: file.size() as u64,
@@ -130,22 +173,26 @@ pub async fn remove(name: &str) -> Result<(), String> {
     drawing.map(|_| ())
 }
 
-/// Boot scan: keep the snapshots worth offering (newer than the drawing's
-/// last explicit save, newest first) and silently delete the stale ones — a
-/// snapshot older than the saved copy recovers nothing and would otherwise
-/// pile up across sessions.
+/// Boot scan: offer the snapshots worth recovering — strictly newer than the
+/// drawing's last explicit copy, newest first. Age never deletes: the recent
+/// copy is refreshed by every OPEN as well as every save, so a snapshot that
+/// lost the comparison may still be the only survivor of a crash right after
+/// a re-open. Accumulation is bounded instead — beyond [`SNAPSHOT_CAP`]
+/// snapshots the oldest are evicted, which is the only deletion here.
 #[cfg(target_arch = "wasm32")]
 pub async fn scan() -> Result<Vec<WebAutosaveEntry>, String> {
+    let (kept, evicted) = cap_evictions(list().await?, SNAPSHOT_CAP);
+    for name in evicted {
+        let _ = remove(&name).await;
+    }
     let mut recoverable = Vec::new();
-    for entry in list().await? {
+    for entry in kept {
         let saved_ms = super::web_recent::modified_ms(&entry.name).await;
         if newer_than_saved(entry.modified_ms, saved_ms) {
             recoverable.push(entry);
-        } else {
-            let _ = remove(&entry.name).await;
         }
     }
-    recoverable.sort_by(|a, b| b.modified_ms.total_cmp(&a.modified_ms));
+    // `cap_evictions` already ordered the survivors newest-first.
     Ok(recoverable)
 }
 
@@ -219,4 +266,44 @@ mod tests {
         assert_eq!(recovered_name("a.b.dwg"), "a.b (recovered).dwg");
         assert_eq!(recovered_name(""), " (recovered).dwg");
     }
+
+    #[test]
+    fn under_the_cap_nothing_is_evicted_and_order_is_newest_first() {
+        let (kept, evicted) = cap_evictions(
+            vec![
+                entry("old.dwg", 100.0),
+                entry("new.dwg", 300.0),
+                entry("mid.dwg", 200.0),
+            ],
+            SNAPSHOT_CAP,
+        );
+        assert!(evicted.is_empty());
+        assert_eq!(
+            kept.iter().map(|e| e.name.as_str()).collect::<Vec<_>>(),
+            ["new.dwg", "mid.dwg", "old.dwg"]
+        );
+    }
+
+    #[test]
+    fn over_the_cap_exactly_the_oldest_overflow_is_evicted() {
+        let entries = (0..SNAPSHOT_CAP + 4)
+            .map(|i| entry(&format!("d{i}.dwg"), i as f64))
+            .collect();
+        let (kept, evicted) = cap_evictions(entries, SNAPSHOT_CAP);
+        assert_eq!(kept.len(), SNAPSHOT_CAP);
+        // Newest kept first, oldest evicted first.
+        assert_eq!(kept.first().map(|e| e.name.as_str()), Some("d19.dwg"));
+        assert_eq!(
+            evicted.iter().map(|name| name.as_str()).collect::<Vec<_>>(),
+            ["d0.dwg", "d1.dwg", "d2.dwg", "d3.dwg"]
+        );
+    }
+
+    fn entry(name: &str, modified_ms: f64) -> WebAutosaveEntry {
+        WebAutosaveEntry {
+            name: name.to_string(),
+            size: 16,
+            modified_ms,
+        }
+    }
 }

src/io/web_drop.rs

@@ -5,7 +5,8 @@
 //! queues them for the 50 ms control poll to open through the web-picker flow.
 
 use std::collections::VecDeque;
-use std::sync::{Mutex, OnceLock};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Mutex;
 use wasm_bindgen::JsCast;
 use wasm_bindgen_futures::JsFuture;
 
@@ -14,14 +15,25 @@ use wasm_bindgen_futures::JsFuture;
 const QUEUE_CAP: usize = 8;
 
 static DROPPED: Mutex<VecDeque<(String, Vec<u8>)>> = Mutex::new(VecDeque::new());
-static INSTALLED: OnceLock<()> = OnceLock::new();
+// `OnceLock::take` needs `&mut self`, which an immutable static cannot
+// provide — an AtomicBool swap gives the same set-once marker and can still
+// be reverted when no document exists.
+static INSTALLED: AtomicBool = AtomicBool::new(false);
 
 fn warn(line: &str) {
     web_sys::console::warn_1(&wasm_bindgen::JsValue::from_str(line));
 }
 
+fn lock_queue() -> std::sync::MutexGuard<'static, VecDeque<(String, Vec<u8>)>> {
+    // The critical sections are push/pop only, so poisoning is unreachable;
+    // recover anyway rather than panic across the JS boundary.
+    DROPPED
+        .lock()
+        .unwrap_or_else(|poisoned| poisoned.into_inner())
+}
+
 fn enqueue(name: String, bytes: Vec<u8>) {
-    let mut queue = DROPPED.lock().unwrap();
+    let mut queue = lock_queue();
     if queue.len() >= QUEUE_CAP {
         drop(queue);
         warn(&format!(
@@ -32,6 +44,12 @@ fn enqueue(name: String, bytes: Vec<u8>) {
     queue.push_back((name, bytes));
 }
 
+/// Queue already-read drawing bytes (drag-and-drop, autosave restore) for the
+/// 50 ms poll to open through the guarded web-picker flow.
+pub fn enqueue_open(name: String, bytes: Vec<u8>) {
+    enqueue(name, bytes);
+}
+
 /// JS-callable entry (same shape as `ocs_control_submit`) for hosts that
 /// forward drops themselves.
 #[wasm_bindgen::prelude::wasm_bindgen]
@@ -41,7 +59,7 @@ pub fn ocs_web_drop_file(name: String, bytes: js_sys::Uint8Array) {
 
 /// Pop the oldest dropped file that the poll has not started opening yet.
 pub fn take_dropped() -> Option<(String, Vec<u8>)> {
-    DROPPED.lock().unwrap().pop_front()
+    lock_queue().pop_front()
 }
 
 /// Feed already-read bytes through the same loader tail the web picker uses
@@ -71,35 +89,44 @@ pub async fn open_dropped(
     }
 }
 
-/// Register the document-level `dragover`/`drop` listeners once. `dragover`
-/// MUST preventDefault or the browser navigates to the dropped file instead
-/// of delivering it to the page.
+/// Register the document-level `dragenter`/`dragover`/`drop` listeners once.
+/// `dragover` MUST preventDefault or the browser navigates to the dropped file
+/// instead of delivering it to the page.
 pub fn install() {
-    if INSTALLED.set(()).is_err() {
+    if INSTALLED.swap(true, Ordering::SeqCst) {
         return;
     }
     let Some(document) = web_sys::window().and_then(|window| window.document()) else {
+        // Only mark installed once the listeners really are: an early
+        // no-document context must not permanently disable drops.
+        INSTALLED.store(false, Ordering::SeqCst);
         return;
     };
-    let on_dragover = wasm_bindgen::closure::Closure::<dyn FnMut(web_sys::Event)>::new(
+    let cancel = wasm_bindgen::closure::Closure::<dyn FnMut(web_sys::Event)>::new(
         move |event: web_sys::Event| {
             event.prevent_default();
         },
     );
-    let _ =
-        document.add_event_listener_with_callback("dragover", on_dragover.as_ref().unchecked_ref());
-    on_dragover.forget();
+    // Canceling `dragenter` too is belt-and-braces some engines still want
+    // before treating the page as a drop target.
+    let _ = document.add_event_listener_with_callback("dragenter", cancel.as_ref().unchecked_ref());
+    let _ = document.add_event_listener_with_callback("dragover", cancel.as_ref().unchecked_ref());
+    cancel.forget();
 
     let on_drop = wasm_bindgen::closure::Closure::<dyn FnMut(web_sys::Event)>::new(
         move |event: web_sys::Event| {
             event.prevent_default();
-            event.stop_propagation();
             let Some(drag) = event.dyn_ref::<web_sys::DragEvent>() else {
                 return;
             };
             let Some(files) = drag.data_transfer().and_then(|transfer| transfer.files()) else {
                 return;
             };
+            // Start every read here, inside the handler, and finish them in
+            // one task in drop order: legacy WebKit dislikes drag-originated
+            // reads that begin after the event returns, and parallel reads
+            // would enqueue (and open) in completion order, not drop order.
+            let mut reads: Vec<(String, js_sys::Promise)> = Vec::new();
             for index in 0..files.length() {
                 let Some(file) = files.get(index) else {
                     continue;
@@ -113,15 +140,27 @@ pub fn install() {
                     ));
                     continue;
                 }
-                wasm_bindgen_futures::spawn_local(async move {
-                    match read_file(&file).await {
-                        Ok(bytes) => enqueue(name, bytes),
-                        Err(error) => web_sys::console::error_1(&wasm_bindgen::JsValue::from_str(
-                            &format!("ocs: failed to read dropped file \"{name}\": {error}"),
-                        )),
-                    }
-                });
+                reads.push((name, file.array_buffer()));
             }
+            if reads.is_empty() {
+                return;
+            }
+            // The page is the only intended drop target; hide drawing drops
+            // from host-page handlers only once there is something to open.
+            event.stop_propagation();
+            wasm_bindgen_futures::spawn_local(async move {
+                for (name, read) in reads {
+                    match JsFuture::from(read).await {
+                        Ok(buffer) => enqueue(name, js_sys::Uint8Array::new(&buffer).to_vec()),
+                        Err(error) => {
+                            web_sys::console::error_1(&wasm_bindgen::JsValue::from_str(&format!(
+                                "ocs: failed to read dropped file \"{name}\": {}",
+                                error.as_string().unwrap_or_else(|| format!("{error:?}"))
+                            )))
+                        }
+                    }
+                }
+            });
         },
     );
     let _ = document.add_event_listener_with_callback("drop", on_drop.as_ref().unchecked_ref());
@@ -133,11 +172,3 @@ fn is_drawing_name(name: &str) -> bool {
         matches!(extension.to_ascii_lowercase().as_str(), "dwg" | "dxf")
     })
 }
-
-/// Same Blob → bytes read as the OPFS recent copies (`web_recent`).
-async fn read_file(file: &web_sys::File) -> Result<Vec<u8>, String> {
-    let buffer = JsFuture::from(file.array_buffer())
-        .await
-        .map_err(|error| error.as_string().unwrap_or_else(|| format!("{error:?}")))?;
-    Ok(js_sys::Uint8Array::new(&buffer).to_vec())
-}

src/io/web_recent.rs

@@ -90,10 +90,7 @@ pub async fn store_open(
                 return None;
             };
             if state.latest_id != pending_id {
-                return Some((
-                    state.latest_id,
-                    std::sync::Arc::clone(&state.latest_bytes),
-                ));
+                return Some((state.latest_id, std::sync::Arc::clone(&state.latest_bytes)));
             }
             state.active_writers = state.active_writers.saturating_sub(1);
             if state.active_writers == 0 {
@@ -130,6 +127,24 @@ pub(crate) async fn write_entry(
         .map_err(js_error)?
         .dyn_into::<web_sys::FileSystemWritableFileStream>()
         .map_err(js_error)?;
+    if let Err(error) = write_and_close(&writable, bytes).await {
+        // A failed write must still release the stream: the
+        // FileSystemWritableFileStream holds an exclusive lock on the entry,
+        // and leaving it undrained makes every later write to the same file
+        // fail the same way — one quota error would freeze autosave (its
+        // round would never finish) for the rest of the session. `abort`
+        // discards the pending bytes; ignore its own failure, there is
+        // nothing better to report than `error`.
+        let _ = JsFuture::from(writable.abort()).await;
+        return Err(error);
+    }
+    Ok(())
+}
+
+async fn write_and_close(
+    writable: &web_sys::FileSystemWritableFileStream,
+    bytes: &[u8],
+) -> Result<(), String> {
     let write = writable.write_with_u8_array(bytes).map_err(js_error)?;
     JsFuture::from(write).await.map_err(js_error)?;
     JsFuture::from(writable.close()).await.map_err(js_error)?;
@@ -262,9 +277,22 @@ fn decode_thumbnail(bytes: Vec<u8>) -> Option<Thumbnail> {
 
 /// Shared with the autosave snapshot store (`web_autosave`).
 pub(crate) fn js_error(value: wasm_bindgen::JsValue) -> String {
-    value
-        .as_string()
-        .unwrap_or_else(|| format!("browser storage error: {value:?}"))
+    if let Some(text) = value.as_string() {
+        return text;
+    }
+    // OPFS failures surface as DOMExceptions, which stringify as
+    // "[object DOMException]" — useless in a command-line warning. Their
+    // `name`/`message` properties carry the diagnosis ("QuotaExceededError",
+    // "NotFoundError", …), so render those when present.
+    let property = |key: &str| {
+        js_sys::Reflect::get(&value, &wasm_bindgen::JsValue::from_str(key))
+            .ok()
+            .and_then(|found| found.as_string())
+    };
+    match (property("name"), property("message")) {
+        (Some(name), Some(message)) => format!("{name}: {message}"),
+        _ => format!("browser storage error: {value:?}"),
+    }
 }
 
 /// Last-modified time (ms since the Unix epoch) of a drawing's cached copy,