From 2bc2fd9cfd722b354a905df737cb92643aabfd13 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Mon, 18 Mar 2024 16:12:01 +0100
Subject: [PATCH] fixup(#7160 / tokio_epoll_uring_ext): double-panic caused by
 info! in thread-local's drop() (#7164)

Manual testing of the changes in #7160 revealed that, if the
thread-local destructor ever runs (it apparently doesn't in our test
suite runs, otherwise #7160 would not have auto-merged), we can
encounter an `abort()` due to a double-panic in the tracing code.

This github comment here contains the stack trace:
https://github.com/neondatabase/neon/pull/7160#issuecomment-2003778176

This PR reverts #7160 and uses a atomic counter to identify the
thread-local in log messages, instead of the memory address of the
thread local, which may be re-used.
---
 .../io_engine/tokio_epoll_uring_ext.rs        | 29 +++++--------------
 1 file changed, 8 insertions(+), 21 deletions(-)

diff --git a/pageserver/src/virtual_file/io_engine/tokio_epoll_uring_ext.rs b/pageserver/src/virtual_file/io_engine/tokio_epoll_uring_ext.rs
index 9b2efef5d4..6ea19d6b2d 100644
--- a/pageserver/src/virtual_file/io_engine/tokio_epoll_uring_ext.rs
+++ b/pageserver/src/virtual_file/io_engine/tokio_epoll_uring_ext.rs
@@ -5,8 +5,8 @@
 //! on older kernels, such as some (but not all) older kernels in the Linux 5.10 series.
 //! See <https://github.com/neondatabase/neon/issues/6373#issuecomment-1905814391> for more details.
 
-use std::sync::atomic::AtomicU32;
-use std::sync::{Arc, Weak};
+use std::sync::atomic::{AtomicU32, AtomicU64, Ordering};
+use std::sync::Arc;
 
 use tokio_util::sync::CancellationToken;
 use tracing::{error, info, info_span, warn, Instrument};
@@ -24,38 +24,25 @@ struct ThreadLocalState(Arc<ThreadLocalStateInner>);
 struct ThreadLocalStateInner {
     cell: tokio::sync::OnceCell<SystemHandle>,
     launch_attempts: AtomicU32,
-    weak_self: Weak<ThreadLocalStateInner>,
+    /// populated through fetch_add from [`THREAD_LOCAL_STATE_ID`]
+    thread_local_state_id: u64,
 }
 
 impl ThreadLocalState {
     pub fn new() -> Self {
-        Self(Arc::new_cyclic(|weak| ThreadLocalStateInner {
+        Self(Arc::new(ThreadLocalStateInner {
             cell: tokio::sync::OnceCell::default(),
             launch_attempts: AtomicU32::new(0),
-            weak_self: Weak::clone(weak),
+            thread_local_state_id: THREAD_LOCAL_STATE_ID.fetch_add(1, Ordering::Relaxed),
         }))
     }
-}
 
-impl ThreadLocalStateInner {
     pub fn make_id_string(&self) -> String {
-        format!("0x{:p}", self.weak_self.as_ptr())
+        format!("{}", self.0.thread_local_state_id)
     }
 }
 
-impl Drop for ThreadLocalStateInner {
-    fn drop(&mut self) {
-        info!(parent: None, id=%self.make_id_string(), "tokio_epoll_uring_ext: thread-local state is being dropped and id might be re-used in the future");
-    }
-}
-
-impl std::ops::Deref for ThreadLocalState {
-    type Target = ThreadLocalStateInner;
-
-    fn deref(&self) -> &Self::Target {
-        &self.0
-    }
-}
+static THREAD_LOCAL_STATE_ID: AtomicU64 = AtomicU64::new(0);
 
 thread_local! {
     static THREAD_LOCAL: ThreadLocalState = ThreadLocalState::new();