From 1121a8eb23f6f0edd9f41cdc08331d40e18105ee Mon Sep 17 00:00:00 2001 From: Eliza Weisman Date: Fri, 3 Apr 2020 15:45:29 -0700 Subject: sync: ensure Mutex, RwLock, and Semaphore futures are Send + Sync (#2375) Previously, the `Mutex::lock`, `RwLock::{read, write}`, and `Semaphore::acquire` futures in `tokio::sync` implemented `Send + Sync` automatically. This was by virtue of being implemented using a `poll_fn` that only closed over `Send + Sync` types. However, this broke in PR #2325, which rewrote those types using the new `batch_semaphore`. Now, they await an `Acquire` future, which contains a `Waiter`, which internally contains an `UnsafeCell`, and thus does not implement `Sync`. Since removing previously implemented traits breaks existing code, this inadvertantly caused a breaking change. There were tests ensuring that the `Mutex`, `RwLock`, and `Semaphore` types themselves were `Send + Sync`, but no tests that the _futures they return_ implemented those traits. I've fixed this by adding an explicit impl of `Sync` for the `batch_semaphore::Acquire` future. Since the `Waiter` type held by this struct is only accessed when borrowed mutably, it is safe for it to implement `Sync`. Additionally, I've added to the bounds checks for the effected `tokio::sync` types to ensure that returned futures continue to implement `Send + Sync` in the future. --- tokio/src/sync/semaphore.rs | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'tokio/src/sync/semaphore.rs') diff --git a/tokio/src/sync/semaphore.rs b/tokio/src/sync/semaphore.rs index e34e49cc..4cce7e8f 100644 --- a/tokio/src/sync/semaphore.rs +++ b/tokio/src/sync/semaphore.rs @@ -39,8 +39,15 @@ pub struct TryAcquireError(()); #[cfg(not(loom))] fn bounds() { fn check_unpin() {} + // This has to take a value, since the async fn's return type is unnameable. + fn check_send_sync_val(_t: T) {} + fn check_send_sync() {} check_unpin::(); check_unpin::>(); + check_send_sync::(); + + let semaphore = Semaphore::new(0); + check_send_sync_val(semaphore.acquire()); } impl Semaphore { -- cgit v1.2.3