fix(fuse): list again when a listing changed while it was published (B159)

A lookup in the root publishes the "Shared with me" entry only if the root
listing did not change while its plan was read, and answered EAGAIN when it
had. Right after a restart the root is listed again as the first lookups come
in, so a write could fail with "Resource temporarily unavailable".
ensure_children now takes the listing again, up to four times.

crates/pdfs-fuse/src/lib.rs

@@ -2437,7 +2437,24 @@ impl Core {
     /// Enumerate `ino`'s children from the remote and cache them. No-op if the
     /// directory has already been listed. Network I/O happens without the lock
     /// held so concurrent metadata reads aren't blocked behind a fetch.
+    ///
+    /// A listing that changed while it was being published answers EAGAIN: the
+    /// root listed again after a restart, an event, a newer refresh of a shared
+    /// folder. The listing is still owed, so it is taken again rather than
+    /// handed to the lookup or open that asked (B159).
     fn ensure_children(&self, ino: u64) -> Result<(), Errno> {
+        for _ in 1..LISTING_ATTEMPTS {
+            match self.list_children(ino) {
+                Err(e) if e.code() == Errno::EAGAIN.code() => {
+                    debug!(ino, "listing changed while it was published; listing again");
+                }
+                done => return done,
+            }
+        }
+        self.list_children(ino)
+    }
+
+    fn list_children(&self, ino: u64) -> Result<(), Errno> {
         let (folder_uid, cached) = {
             let st = self.state();
             match st.entries.get(&ino) {
@@ -5777,6 +5794,10 @@ fn api_code(e: &(dyn std::error::Error + 'static)) -> Option<ResponseCode> {
     }
 }
 
+/// How often [`Core::ensure_children`] takes a listing that changed while it
+/// was being published.
+const LISTING_ATTEMPTS: usize = 4;
+
 /// How often a rename or move that Drive calls out of date is retried, and how
 /// long apart.
 const OUT_OF_DATE_RETRIES: u32 = 20;
@@ -8776,11 +8797,28 @@ mod tests {
         assert!(changed.contains("self.note_self_change(uid)"));
     }
 
+    #[test]
+    fn a_listing_changed_while_published_is_taken_again() {
+        // The root listed again after a restart while a lookup was publishing
+        // the "Shared with me" entry, and the lookup answered EAGAIN (B159).
+        let ensure = function_source(include_str!("lib.rs"), "fn ensure_children(");
+        assert_before(
+            ensure,
+            "for _ in 1..LISTING_ATTEMPTS",
+            "Errno::EAGAIN.code()",
+        );
+        assert_eq!(ensure.matches("self.list_children(ino)").count(), 2);
+        assert!(
+            !ensure.contains("?"),
+            "an attempt's error must not skip the retry"
+        );
+    }
+
     #[test]
     fn a_folder_lists_from_the_db_while_the_link_is_down() {
         // A listing dropped as stale mid-run, then a drop: every create in that
         // folder looks the name up first, and answered EIO for want of it.
-        let ensure = function_source(include_str!("lib.rs"), "fn ensure_children(");
+        let ensure = function_source(include_str!("lib.rs"), "fn list_children(");
         assert_before(
             ensure,
             "if !self.is_online()",