From b44070d0c78552c75dcd119e5599ad54a08c8468 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Tue, 14 Jan 2025 13:30:48 +0100
Subject: [PATCH] experiment: what if we make Handle !Send so it can't be held
 across await points

Result: the whole point of having a Handle at hand is to be holding a GateGuard
while performing a Timeline operation. Dont' do it.
---
 pageserver/src/tenant/timeline/handle.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/pageserver/src/tenant/timeline/handle.rs b/pageserver/src/tenant/timeline/handle.rs
index 7ef237734f..278bbdb863 100644
--- a/pageserver/src/tenant/timeline/handle.rs
+++ b/pageserver/src/tenant/timeline/handle.rs
@@ -173,6 +173,7 @@ pub(crate) struct ShardTimelineId {
 pub(crate) struct Handle<T: Types> {
     timeline: Arc<T::Timeline>,
     inner: tokio::sync::OwnedMutexGuard<HandleInner>,
+    _dont_hold_across_await_points: std::marker::PhantomData<*const u8>,
 }
 pub(crate) struct WeakHandle<T: Types> {
     timeline: Arc<T::Timeline>,
@@ -400,6 +401,7 @@ impl<T: Types> Cache<T> {
                 Ok(Handle {
                     timeline,
                     inner: handle_locked,
+                    _dont_hold_across_await_points: Default::default(),
                 })
             }
             Err(e) => Err(GetError::TenantManager(e)),
@@ -421,6 +423,7 @@ impl<T: Types> WeakHandle<T> {
             HandleInner::KeepingTimelineGateOpen { .. } => Ok(Handle {
                 timeline: Arc::clone(&self.timeline),
                 inner: lock_guard,
+                _dont_hold_across_await_points: Default::default(),
             }),
             HandleInner::ShutDown => Err(HandleUpgradeError::ShutDown),
         }
@@ -439,6 +442,9 @@ impl<T: Types> Deref for Handle<T> {
 }
 
 impl<T: Types> Handle<T> {
+    pub(crate) fn timeline(&self) -> &T::Timeline {
+        &self.timeline
+    }
     pub(crate) fn downgrade(&self) -> WeakHandle<T> {
         WeakHandle {
             timeline: Arc::clone(&self.timeline),