Explain why it fails

core/storage/sqlite3_ondisk.rs

@@ -495,6 +495,7 @@ fn finish_read_page(
 pub fn begin_write_btree_page(pager: &Pager, page: &Rc<RefCell<Page>>) -> Result<()> {
     let page_source = &pager.page_io;
     let page_finish = page.clone();
+    // page.borrow() returns RefCell<Page> which remains active for the entire begin_write_btree_page function
     let page = page.borrow();
 
     let contents = page.contents.read().unwrap();
@@ -505,6 +506,7 @@ pub fn begin_write_btree_page(pager: &Pager, page: &Rc<RefCell<Page>>) -> Result
         Box::new(move |bytes_written: i32| {
             let buf_copy = buf_copy.clone();
             let buf_len = buf_copy.borrow().len();
+            // requires a mutable borrow of the page_finish's RefCell<Page>
             page_finish.borrow_mut().clear_dirty();
             if bytes_written < buf_len as i32 {
                 log::error!("wrote({bytes_written}) less than expected({buf_len})");