Merge 7a943ebba6 into 40f835afd2

The example originally showed a certain deadlock, which was not as clear
as it could be. The new version shows intentionally racy code that may
result in a successful execution but may also deadlock.

.github/workflows/ci.yml

@@ -15,30 +15,43 @@ jobs:
     strategy:
       matrix:
         rust:
-          - "1.70" # minimum stable rust version
           - stable
           - beta
           - nightly
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: dtolnay/rust-toolchain@v1
         with:
           toolchain: ${{ matrix.rust }}
           components: rustfmt, clippy
 
+      # Make sure we test with recent deps
+      - run: cargo update
       - run: cargo build --all-features --all-targets
       - run: cargo test --all-features
       - run: cargo fmt --all -- --check
       - run: cargo clippy --all-features --all-targets -- -D warnings
 
+  msrv:
+    name: MSRV
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: dtolnay/rust-toolchain@v1
+        with:
+          toolchain: "1.70"
+
+      - run: cargo test --all-features
+
   docs:
     name: Documentation build
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: dtolnay/rust-toolchain@v1
         with:

.gitignore

@@ -1,2 +1 @@
 /target
-Cargo.lock

CHANGELOG.md

@@ -6,6 +6,10 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Changed
+
+- Reworked CI to better test continued support for the minimum supported Rust version
+
 ## [0.3.0] - 2023-09-09
 
 ### Added

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Cargo.toml

@@ -36,3 +36,6 @@ backtraces = []
 # Feature names do not match crate names pending namespaced features.
 lockapi = ["lock_api"]
 parkinglot = ["parking_lot", "lockapi"]
+
+[build-dependencies]
+autocfg = "1.4.0"

README.md

@@ -56,10 +56,10 @@ introduce a cyclic dependency between your locks, the operation panics instead.
 immediately notice the cyclic dependency rather than be eventually surprised by it in production.
 
 Mutex tracing is efficient, but it is not completely overhead-free. If you cannot spare the
-performance penalty in your production environment, this library also offers debug-only tracing.
+performance penalty in your production environment, this library also offers debug-only tracing. The
-`DebugMutex`, also found in the `stdsync` module, is a type alias that evaluates to `TracingMutex`
+type aliases in `tracing_mutex::stdsync` correspond to tracing primitives from
-when debug assertions are enabled, and to `Mutex` when they are not. Similar helper types are
+`tracing_mutex::stdsync::tracing` when debug assertions are enabled, and to primitives from
-available for other synchronization primitives.
+`std::sync::Mutex` when they are not. A similar structure exists for other 
 
 The minimum supported Rust version is 1.70. Increasing this is not considered a breaking change, but
 will be avoided within semver-compatible releases if possible.
@@ -68,6 +68,7 @@ will be avoided within semver-compatible releases if possible.
 
 - Dependency-tracking wrappers for all locking primitives
 - Optional opt-out for release mode code
+- Optional backtrace capture to aid with reproducing cyclic mutex chains
 - Support for primitives from:
   - `std::sync`
   - `parking_lot`
@@ -76,7 +77,6 @@ will be avoided within semver-compatible releases if possible.
 ## Future improvements
 
 - Improve performance in lock tracing
-- Optional logging to make debugging easier
 - Better and configurable error handling when detecting cyclic dependencies
 - Support for other locking libraries
 - Support for async locking libraries

build.rs

@@ -0,0 +1,10 @@
+use autocfg::AutoCfg;
+
+fn main() {
+    // To avoid bumping MSRV unnecessarily, we can sniff certain features. Reevaluate this on major
+    // releases.
+    let ac = AutoCfg::new().unwrap();
+    ac.emit_has_path("std::sync::LazyLock");
+
+    autocfg::rerun_path("build.rs");
+}

examples/mutex_cycle.rs

@@ -1,26 +1,62 @@
 //! Show what a crash looks like
 //!
-//! This shows what a traceback of a cycle detection looks like. It is expected to crash.
+//! This shows what a traceback of a cycle detection looks like. It is expected to crash when run in
+//! debug mode, because it might deadlock. In release mode, no tracing is used and the program may
+//! do any of the following:
+//!
+//! - Return a random valuation of `a`, `b`, and `c`. The implementation has a race-condition by
+//!   design. I have observed (4, 3, 6), but also (6, 3, 5).
+//! - Deadlock forever.
+//!
+//! One can increase the SLEEP_TIME constant to increase the likelihood of a deadlock to occur. On
+//! my machine, 1ns of sleep time gives about a 50/50 chance of the program deadlocking.
+use std::thread;
+use std::time::Duration;
+
 use tracing_mutex::stdsync::Mutex;
 
 fn main() {
-    let a = Mutex::new(());
+    let a = Mutex::new(1);
-    let b = Mutex::new(());
+    let b = Mutex::new(2);
-    let c = Mutex::new(());
+    let c = Mutex::new(3);
 
-    // Create an edge from a to b
+    // Increase this time to increase the likelihood of a deadlock.
-    {
+    const SLEEP_TIME: Duration = Duration::from_nanos(1);
-        let _a = a.lock();
-        let _b = b.lock();
-    }
 
-    // Create an edge from b to c
+    // Depending on random CPU performance, this section may deadlock, or may return a result. With
-    {
+    // tracing enabled, the potential deadlock is always detected and a backtrace should be
-        let _b = b.lock();
+    // produced.
-        let _c = c.lock();
+    thread::scope(|s| {
-    }
+        // Create an edge from a to b
+        s.spawn(|| {
+            let a = a.lock().unwrap();
+            thread::sleep(SLEEP_TIME);
+            *b.lock().unwrap() += *a;
+        });
 
-    // Now crash by trying to add an edge from c to a
+        // Create an edge from b to c
-    let _c = c.lock();
+        s.spawn(|| {
-    let _a = a.lock(); // This line will crash
+            let b = b.lock().unwrap();
+            thread::sleep(SLEEP_TIME);
+            *c.lock().unwrap() += *b;
+        });
+
+        // Create an edge from c to a
+        //
+        // N.B. the program can crash on any of the three edges, as there is no guarantee which
+        // thread will execute first. Nevertheless, any one of them is guaranteed to panic with
+        // tracing enabled.
+        s.spawn(|| {
+            let c = c.lock().unwrap();
+            thread::sleep(SLEEP_TIME);
+            *a.lock().unwrap() += *c;
+        });
+    });
+
+    println!(
+        "{}, {}, {}",
+        a.into_inner().unwrap(),
+        b.into_inner().unwrap(),
+        c.into_inner().unwrap()
+    );
 }

src/lib.rs

@@ -105,7 +105,7 @@ thread_local! {
     ///
     /// Assuming that locks are roughly released in the reverse order in which they were acquired,
     /// a stack should be more efficient to keep track of the current state than a set would be.
-    static HELD_LOCKS: RefCell<Vec<usize>> = RefCell::new(Vec::new());
+    static HELD_LOCKS: RefCell<Vec<usize>> = const { RefCell::new(Vec::new()) };
 }
 
 /// Dedicated ID type for Mutexes
@@ -193,6 +193,14 @@ impl MutexId {
             unreachable!("Tried to drop lock for mutex {:?} but it wasn't held", self)
         });
     }
+
+    /// Execute the given closure while the guard is held.
+    pub fn with_held<T>(&self, f: impl FnOnce() -> T) -> T {
+        // Note: we MUST construct the RAII guard, we cannot simply mark held + mark released, as
+        // f() may panic and corrupt our state.
+        let _guard = self.get_borrowed();
+        f()
+    }
 }
 
 impl Default for MutexId {
@@ -275,7 +283,7 @@ struct BorrowedMutex<'a> {
 ///
 /// This function panics if the lock did not appear to be handled by this thread. If that happens,
 /// that is an indication of a serious design flaw in this library.
-impl<'a> Drop for BorrowedMutex<'a> {
+impl Drop for BorrowedMutex<'_> {
     fn drop(&mut self) {
         // Safety: the only way to get a BorrowedMutex is by locking the mutex.
         unsafe { self.id.mark_released() };

src/lockapi.rs

@@ -90,6 +90,8 @@ unsafe impl<T> RawMutex for TracingWrapper<T>
 where
     T: RawMutex,
 {
+    // Known issue with legacy initialisers, allow
+    #[allow(clippy::declare_interior_mutable_const)]
     const INIT: Self = Self {
         inner: T::INIT,
         id: LazyMutexId::new(),
@@ -154,6 +156,8 @@ unsafe impl<T> RawRwLock for TracingWrapper<T>
 where
     T: RawRwLock,
 {
+    // Known issue with legacy initialisers, allow
+    #[allow(clippy::declare_interior_mutable_const)]
     const INIT: Self = Self {
         inner: T::INIT,
         id: LazyMutexId::new(),

src/parkinglot.rs

@@ -138,16 +138,14 @@ pub mod tracing {
         /// This method will panic if `f` panics, poisoning this `Once`. In addition, this function
         /// panics when the lock acquisition order is determined to be inconsistent.
         pub fn call_once(&self, f: impl FnOnce()) {
-            let _borrow = self.id.get_borrowed();
+            self.id.with_held(|| self.inner.call_once(f));
-            self.inner.call_once(f);
         }
 
         /// Performs the given initialization routine once and only once.
         ///
         /// This method is identical to [`Once::call_once`] except it ignores poisoning.
         pub fn call_once_force(&self, f: impl FnOnce(OnceState)) {
-            let _borrow = self.id.get_borrowed();
+            self.id.with_held(|| self.inner.call_once_force(f));
-            self.inner.call_once_force(f);
         }
     }
 }

src/stdsync.rs

@@ -30,6 +30,12 @@ pub use tracing::{
     Condvar, Mutex, MutexGuard, Once, OnceLock, RwLock, RwLockReadGuard, RwLockWriteGuard,
 };
 
+#[cfg(all(has_std__sync__LazyLock, debug_assertions))]
+pub use tracing::LazyLock;
+
+#[cfg(all(has_std__sync__LazyLock, not(debug_assertions)))]
+pub use std::sync::LazyLock;
+
 /// Dependency tracing versions of [`std::sync`].
 pub mod tracing {
     use std::fmt;
@@ -47,6 +53,12 @@ pub mod tracing {
     use crate::BorrowedMutex;
     use crate::LazyMutexId;
 
+    #[cfg(has_std__sync__LazyLock)]
+    pub use lazy_lock::LazyLock;
+
+    #[cfg(has_std__sync__LazyLock)]
+    mod lazy_lock;
+
     /// Wrapper for [`std::sync::Mutex`].
     ///
     /// Refer to the [crate-level][`crate`] documentation for the differences between this struct and
@@ -161,7 +173,7 @@ pub mod tracing {
         }
     }
 
-    impl<'a, T> Deref for MutexGuard<'a, T> {
+    impl<T> Deref for MutexGuard<'_, T> {
         type Target = T;
 
         fn deref(&self) -> &Self::Target {
@@ -169,13 +181,13 @@ pub mod tracing {
         }
     }
 
-    impl<'a, T> DerefMut for MutexGuard<'a, T> {
+    impl<T> DerefMut for MutexGuard<'_, T> {
         fn deref_mut(&mut self) -> &mut Self::Target {
             &mut self.inner
         }
     }
 
-    impl<'a, T: fmt::Display> fmt::Display for MutexGuard<'a, T> {
+    impl<T: fmt::Display> fmt::Display for MutexGuard<'_, T> {
         fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
             self.inner.fmt(f)
         }
@@ -409,7 +421,7 @@ pub mod tracing {
         }
     }
 
-    impl<'a, L, T> Deref for TracingRwLockGuard<'a, L>
+    impl<L, T> Deref for TracingRwLockGuard<'_, L>
     where
         L: Deref<Target = T>,
     {
@@ -420,7 +432,7 @@ pub mod tracing {
         }
     }
 
-    impl<'a, T, L> DerefMut for TracingRwLockGuard<'a, L>
+    impl<T, L> DerefMut for TracingRwLockGuard<'_, L>
     where
         L: Deref<Target = T> + DerefMut,
     {
@@ -439,6 +451,8 @@ pub mod tracing {
         mutex_id: LazyMutexId,
     }
 
+    // New without default is intentional, `std::sync::Once` doesn't implement it either
+    #[allow(clippy::new_without_default)]
     impl Once {
         /// Create a new `Once` value.
         pub const fn new() -> Self {
@@ -458,8 +472,7 @@ pub mod tracing {
         where
             F: FnOnce(),
         {
-            let _guard = self.mutex_id.get_borrowed();
+            self.mutex_id.with_held(|| self.inner.call_once(f))
-            self.inner.call_once(f);
         }
 
         /// Performs the same operation as [`call_once`][Once::call_once] except it ignores
@@ -473,8 +486,7 @@ pub mod tracing {
         where
             F: FnOnce(&OnceState),
         {
-            let _guard = self.mutex_id.get_borrowed();
+            self.mutex_id.with_held(|| self.inner.call_once_force(f))
-            self.inner.call_once_force(f);
         }
 
         /// Returns true if some `call_once` has completed successfully.
@@ -548,9 +560,7 @@ pub mod tracing {
         /// As this method may block until initialization is complete, it participates in cycle
         /// detection.
         pub fn set(&self, value: T) -> Result<(), T> {
-            let _guard = self.id.get_borrowed();
+            self.id.with_held(|| self.inner.set(value))
-
-            self.inner.set(value)
         }
 
         /// Gets the contents of the cell, initializing it with `f` if the cell was empty.
@@ -560,8 +570,7 @@ pub mod tracing {
         where
             F: FnOnce() -> T,
         {
-            let _guard = self.id.get_borrowed();
+            self.id.with_held(|| self.inner.get_or_init(f))
-            self.inner.get_or_init(f)
         }
 
         /// Takes the value out of this `OnceLock`, moving it back to an uninitialized state.

src/stdsync/tracing/lazy_lock.rs

@@ -0,0 +1,117 @@
+//! Wrapper implementation for LazyLock
+//!
+//! This lives in a separate module as LazyLock would otherwise raise our MSRV to 1.80. Reevaluate
+//! this in the future.
+use std::fmt;
+use std::fmt::Debug;
+use std::ops::Deref;
+
+use crate::LazyMutexId;
+
+/// Wrapper for [`std::sync::LazyLock`]
+///
+/// This wrapper participates in cycle detection like all other primitives in this crate. It should
+/// only be possible to encounter cycles when acquiring mutexes in the initialisation function.
+///
+/// # Examples
+///
+/// ```
+/// use tracing_mutex::stdsync::tracing::LazyLock;
+///
+/// static LOCK: LazyLock<i32> = LazyLock::new(|| {
+///     println!("Hello, world!");
+///     42
+/// });
+///
+/// // This should print "Hello, world!"
+/// println!("{}", *LOCK);
+/// // This should not.
+/// println!("{}", *LOCK);
+/// ```
+pub struct LazyLock<T, F = fn() -> T> {
+    // MSRV violation is fine, this is gated behind a cfg! check
+    #[allow(clippy::incompatible_msrv)]
+    inner: std::sync::LazyLock<T, F>,
+    id: LazyMutexId,
+}
+
+impl<T, F: FnOnce() -> T> LazyLock<T, F> {
+    /// Creates a new lazy value with the given initializing function.
+    pub const fn new(f: F) -> LazyLock<T, F> {
+        Self {
+            id: LazyMutexId::new(),
+            // MSRV violation is fine, this is gated behind a cfg! check
+            #[allow(clippy::incompatible_msrv)]
+            inner: std::sync::LazyLock::new(f),
+        }
+    }
+
+    /// Force this lazy lock to be evaluated.
+    ///
+    /// This is equivalent to dereferencing, but is more explicit.
+    pub fn force(this: &LazyLock<T, F>) -> &T {
+        this
+    }
+}
+
+impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.id.with_held(|| &*self.inner)
+    }
+}
+
+impl<T: Default> Default for LazyLock<T> {
+    /// Return a `LazyLock` that is initialized through [`Default`].
+    fn default() -> Self {
+        Self::new(Default::default)
+    }
+}
+
+impl<T: Debug, F> Debug for LazyLock<T, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Cannot implement this ourselves because the get() used is nightly, so delegate.
+        self.inner.fmt(f)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::stdsync::Mutex;
+
+    use super::*;
+
+    #[test]
+    fn test_only_init_once() {
+        let mut init_counter = 0;
+
+        let lock = LazyLock::new(|| {
+            init_counter += 1;
+            42
+        });
+
+        assert_eq!(*lock, 42);
+        LazyLock::force(&lock);
+
+        // Ensure we can access the init counter
+        drop(lock);
+
+        assert_eq!(init_counter, 1);
+    }
+
+    #[test]
+    #[should_panic(expected = "Found cycle")]
+    fn test_panic_with_cycle() {
+        let mutex = Mutex::new(());
+
+        let lock = LazyLock::new(|| *mutex.lock().unwrap());
+
+        // Establish the relation from lock to mutex
+        LazyLock::force(&lock);
+
+        // Now do it the other way around, which should crash
+        let _guard = mutex.lock().unwrap();
+        LazyLock::force(&lock);
+    }
+}