Disallow taking two instances of the same mutex at the same time

[rust-lightning] / lightning / src / sync / mod.rs

diff --git a/lightning/src/sync/mod.rs b/lightning/src/sync/mod.rs
index 50ef40e295f50d0a0d9095a0f3dfe4c73916a340..1b2b9a739b8c5d3078204917f55b0fef86e87f13 100644 (file)
--- a/lightning/src/sync/mod.rs
+++ b/lightning/src/sync/mod.rs
@@ -7,8 +7,17 @@ pub(crate) enum LockHeldState {
        Unsupported,
 }
 
-pub(crate) trait LockTestExt {
+pub(crate) trait LockTestExt<'a> {
        fn held_by_thread(&self) -> LockHeldState;
+       type ExclLock;
+       /// If two instances of the same mutex are being taken at the same time, it's very easy to have
+       /// a lockorder inversion and risk deadlock. Thus, we default to disabling such locks.
+       ///
+       /// However, sometimes they cannot be avoided. In such cases, this method exists to take a
+       /// mutex while avoiding a test failure. It is deliberately verbose and includes the term
+       /// "unsafe" to indicate that special care needs to be taken to ensure no deadlocks are
+       /// possible.
+       fn unsafe_well_ordered_double_lock_self(&'a self) -> Self::ExclLock;
 }
 
 #[cfg(all(feature = "std", not(feature = "_bench_unstable"), test))]
@@ -27,13 +36,19 @@ pub use {std::sync::{Arc, Mutex, Condvar, MutexGuard, RwLock, RwLockReadGuard, R
 #[cfg(all(feature = "std", any(feature = "_bench_unstable", not(test))))]
 mod ext_impl {
        use super::*;
-       impl<T> LockTestExt for Mutex<T> {
+       impl<'a, T: 'a> LockTestExt<'a> for Mutex<T> {
                #[inline]
                fn held_by_thread(&self) -> LockHeldState { LockHeldState::Unsupported }
+               type ExclLock = MutexGuard<'a, T>;
+               #[inline]
+               fn unsafe_well_ordered_double_lock_self(&'a self) -> MutexGuard<T> { self.lock().unwrap() }
        }
-       impl<T> LockTestExt for RwLock<T> {
+       impl<'a, T: 'a> LockTestExt<'a> for RwLock<T> {
                #[inline]
                fn held_by_thread(&self) -> LockHeldState { LockHeldState::Unsupported }
+               type ExclLock = RwLockWriteGuard<'a, T>;
+               #[inline]
+               fn unsafe_well_ordered_double_lock_self(&'a self) -> RwLockWriteGuard<T> { self.write().unwrap() }
        }
 }