Resolve borrow_mut error

Author: tiif

src/shims/unix/linux/epoll.rs

@@ -448,22 +448,25 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         // is not close after the thread unblocks.
         let weak_epfd = epfd.downgrade();
 
-        let epoll_file_description = epfd
-            .downcast::<Epoll>()
-            .ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_wait`"))?;
-
-        let binding = epoll_file_description.get_ready_list();
-        let ready_list = binding.borrow_mut();
+        // We just need to know if the ready list is empty. The whole logic is wrapped inside a block
+        // so we don't need to manually drop epfd later.
+        let ready_list_empty;
+        {
+            let epoll_file_description = epfd
+                .downcast::<Epoll>()
+                .ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_wait`"))?;
+            let binding = epoll_file_description.get_ready_list();
+            ready_list_empty = binding.borrow_mut().is_empty();
+        }
 
-        if ready_list.is_empty() {
+        if ready_list_empty {
             if timeout == 0 {
                 // Non-blocking with no notification returned.
                 this.write_scalar(Scalar::from_i32(0), &dest)?;
                 return Ok(());
             } else {
                 // Blocking
                 let duration = Duration::from_millis(timeout.into());
-                drop(ready_list);
                 this.block_thread(
                     BlockReason::Epoll,
                     Some((TimeoutClock::Monotonic, TimeoutAnchor::Relative, duration)),
@@ -488,9 +491,6 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             }
         } else {
             // Immediately return epoll notification without blocking.
-
-            // Attempt to prevent epfd.borrow_mut in blocking_epoll_callback causing ICE.
-            drop(ready_list);
             this.blocking_epoll_callback(epfd_value, weak_epfd, &dest, &event)?;
             return Ok(());
         }