Merge #33

33: Prepare for release 0.3.0 r=bertptrs a=bertptrs

Double check

- [x] documentation
- [x] changelog
- [x] tests


Co-authored-by: Bert Peters <bert@bertptrs.nl>

CHANGELOG.md

@@ -6,16 +6,26 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+## [0.3.0] - 2023-09-09
+
 ### Added
 
 - The minimum supported Rust version is now defined as 1.70. Previously it was undefined.
 - Wrappers for `std::sync` primitives can now be `const` constructed.
 - Add support for `std::sync::OnceLock`
+- Added backtraces of mutex allocations to the cycle report. Capturing backtraces does incur some
+  overhead, this can be mitigated by disabling the `backtraces` feature which is enabled by default.
 
 ### Breaking
 
 - Update [`parking_lot`][parking_lot] dependency to `0.12`.
-- Restructured the crate to reduce typename verbosity. For details, see: #25.
+- Restructured the crate to reduce typename verbosity. Wrapper names now match the name of the
+  primitive they wrap. Specific always/debug tracing versions have now moved to separate modules.
+  For example, `tracing_mutex::stdsync::TracingMutex` is now
+  `tracing_mutex::stdsync::tracing::Mutex`, and `tracing_mutex::stdsync::DebugMutex` is now called
+  `tracing_mutex::stdsync::Mutex`. This hopefully reduces the visual noise while reading code that
+  uses this in practice. Unwrapped primitives are reexported under `tracing_mutex::stdsync::raw` for
+  convenience.
 
 ### Fixed
 
@@ -83,7 +93,8 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 Initial release.
 
-[Unreleased]: https://github.com/bertptrs/tracing-mutex/compare/v0.2.1...HEAD
+[Unreleased]: https://github.com/bertptrs/tracing-mutex/compare/v0.3.0...HEAD
+[0.3.0]: https://github.com/bertptrs/tracing-mutex/compare/v0.2.1...v0.3.0
 [0.2.1]: https://github.com/bertptrs/tracing-mutex/compare/v0.2.0...v0.2.1
 [0.2.0]: https://github.com/bertptrs/tracing-mutex/compare/v0.1.2...v0.2.0
 [0.1.2]: https://github.com/bertptrs/tracing-mutex/compare/v0.1.1...v0.1.2

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "tracing-mutex"
-version = "0.2.1"
+version = "0.3.0"
 authors = ["Bert Peters <bert@bertptrs.nl>"]
 edition = "2021"
 license = "MIT OR Apache-2.0"
@@ -31,6 +31,8 @@ name = "mutex"
 harness = false
 
 [features]
+default = ["backtraces"]
+backtraces = []
 # Feature names do not match crate names pending namespaced features.
 lockapi = ["lock_api"]
 parkinglot = ["parking_lot", "lockapi"]

examples/mutex_cycle.rs

@@ -0,0 +1,26 @@
+//! Show what a crash looks like
+//!
+//! This shows what a traceback of a cycle detection looks like. It is expected to crash.
+use tracing_mutex::stdsync::Mutex;
+
+fn main() {
+    let a = Mutex::new(());
+    let b = Mutex::new(());
+    let c = Mutex::new(());
+
+    // Create an edge from a to b
+    {
+        let _a = a.lock();
+        let _b = b.lock();
+    }
+
+    // Create an edge from b to c
+    {
+        let _b = b.lock();
+        let _c = c.lock();
+    }
+
+    // Now crash by trying to add an edge from c to a
+    let _c = c.lock();
+    let _a = a.lock(); // This line will crash
+}

src/graph.rs

@@ -1,4 +1,5 @@
 use std::cell::Cell;
+use std::collections::hash_map::Entry;
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::hash::Hash;
@@ -19,23 +20,24 @@ type Order = usize;
 /// visibly changed.
 ///
 /// [paper]: https://whileydave.com/publications/pk07_jea/
-#[derive(Default, Debug)]
-pub struct DiGraph<V>
+#[derive(Debug)]
+pub struct DiGraph<V, E>
 where
     V: Eq + Hash + Copy,
 {
-    nodes: HashMap<V, Node<V>>,
-    /// Next topological sort order
-    next_ord: Order,
+    nodes: HashMap<V, Node<V, E>>,
+    // Instead of reordering the orders in the graph whenever a node is deleted, we maintain a list
+    // of unused ids that can be handed out later again.
+    unused_order: Vec<Order>,
 }
 
 #[derive(Debug)]
-struct Node<V>
+struct Node<V, E>
 where
     V: Eq + Hash + Clone,
 {
     in_edges: HashSet<V>,
-    out_edges: HashSet<V>,
+    out_edges: HashMap<V, E>,
     // The "Ord" field is a Cell to ensure we can update it in an immutable context.
     // `std::collections::HashMap` doesn't let you have multiple mutable references to elements, but
     // this way we can use immutable references and still update `ord`. This saves quite a few
@@ -43,7 +45,7 @@ where
     ord: Cell<Order>,
 }
 
-impl<V> DiGraph<V>
+impl<V, E> DiGraph<V, E>
 where
     V: Eq + Hash + Copy,
 {
@@ -54,12 +56,18 @@ where
     /// the node in the topological order.
     ///
     /// New nodes are appended to the end of the topological order when added.
-    fn add_node(&mut self, n: V) -> (&mut HashSet<V>, &mut HashSet<V>, Order) {
-        let next_ord = &mut self.next_ord;
+    fn add_node(&mut self, n: V) -> (&mut HashSet<V>, &mut HashMap<V, E>, Order) {
+        // need to compute next id before the call to entry() to avoid duplicate borrow of nodes
+        let fallback_id = self.nodes.len();
 
         let node = self.nodes.entry(n).or_insert_with(|| {
-            let order = *next_ord;
-            *next_ord = next_ord.checked_add(1).expect("Topological order overflow");
+            let order = if let Some(id) = self.unused_order.pop() {
+                // Reuse discarded ordering entry
+                id
+            } else {
+                // Allocate new order id
+                fallback_id
+            };
 
             Node {
                 ord: Cell::new(order),
@@ -77,9 +85,12 @@ where
             Some(Node {
                 out_edges,
                 in_edges,
-                ..
+                ord,
             }) => {
-                out_edges.into_iter().for_each(|m| {
+                // Return ordering to the pool of unused ones
+                self.unused_order.push(ord.get());
+
+                out_edges.into_keys().for_each(|m| {
                     self.nodes.get_mut(&m).unwrap().in_edges.remove(&n);
                 });
 
@@ -96,18 +107,29 @@ where
     ///
     /// Nodes, both from and to, are created as needed when creating new edges. If the new edge
     /// would introduce a cycle, the edge is rejected and `false` is returned.
-    pub(crate) fn add_edge(&mut self, x: V, y: V) -> bool {
+    ///
+    /// # Errors
+    ///
+    /// If the edge would introduce the cycle, the underlying graph is not modified and a list of
+    /// all the edge data in the would-be cycle is returned instead.
+    pub(crate) fn add_edge(&mut self, x: V, y: V, e: impl FnOnce() -> E) -> Result<(), Vec<E>>
+    where
+        E: Clone,
+    {
         if x == y {
             // self-edges are always considered cycles
-            return false;
+            return Err(Vec::new());
         }
 
         let (_, out_edges, ub) = self.add_node(x);
 
-        if !out_edges.insert(y) {
-            // Edge already exists, nothing to be done
-            return true;
-        }
+        match out_edges.entry(y) {
+            Entry::Occupied(_) => {
+                // Edge already exists, nothing to be done
+                return Ok(());
+            }
+            Entry::Vacant(entry) => entry.insert(e()),
+        };
 
         let (in_edges, _, lb) = self.add_node(y);
 
@@ -119,7 +141,7 @@ where
             let mut delta_f = Vec::new();
             let mut delta_b = Vec::new();
 
-            if !self.dfs_f(&self.nodes[&y], ub, &mut visited, &mut delta_f) {
+            if let Err(cycle) = self.dfs_f(&self.nodes[&y], ub, &mut visited, &mut delta_f) {
                 // This edge introduces a cycle, so we want to reject it and remove it from the
                 // graph again to keep the "does not contain cycles" invariant.
 
@@ -129,7 +151,7 @@ where
                 self.nodes.get_mut(&x).map(|node| node.out_edges.remove(&y));
 
                 // No edge was added
-                return false;
+                return Err(cycle);
             }
 
             // No need to check as we should've found the cycle on the forward pass
@@ -141,44 +163,49 @@ where
             self.reorder(delta_f, delta_b);
         }
 
-        true
+        Ok(())
     }
 
     /// Forwards depth-first-search
     fn dfs_f<'a>(
         &'a self,
-        n: &'a Node<V>,
+        n: &'a Node<V, E>,
         ub: Order,
         visited: &mut HashSet<V>,
-        delta_f: &mut Vec<&'a Node<V>>,
-    ) -> bool {
+        delta_f: &mut Vec<&'a Node<V, E>>,
+    ) -> Result<(), Vec<E>>
+    where
+        E: Clone,
+    {
         delta_f.push(n);
 
-        n.out_edges.iter().all(|w| {
+        for (w, e) in &n.out_edges {
             let node = &self.nodes[w];
             let ord = node.ord.get();
 
             if ord == ub {
                 // Found a cycle
-                false
+                return Err(vec![e.clone()]);
             } else if !visited.contains(w) && ord < ub {
                 // Need to check recursively
                 visited.insert(*w);
-                self.dfs_f(node, ub, visited, delta_f)
-            } else {
-                // Already seen this one or not interesting
-                true
+                if let Err(mut chain) = self.dfs_f(node, ub, visited, delta_f) {
+                    chain.push(e.clone());
+                    return Err(chain);
+                }
             }
-        })
+        }
+
+        Ok(())
     }
 
     /// Backwards depth-first-search
     fn dfs_b<'a>(
         &'a self,
-        n: &'a Node<V>,
+        n: &'a Node<V, E>,
         lb: Order,
         visited: &mut HashSet<V>,
-        delta_b: &mut Vec<&'a Node<V>>,
+        delta_b: &mut Vec<&'a Node<V, E>>,
     ) {
         delta_b.push(n);
 
@@ -192,7 +219,7 @@ where
         }
     }
 
-    fn reorder(&self, mut delta_f: Vec<&Node<V>>, mut delta_b: Vec<&Node<V>>) {
+    fn reorder(&self, mut delta_f: Vec<&Node<V, E>>, mut delta_b: Vec<&Node<V, E>>) {
         self.sort(&mut delta_f);
         self.sort(&mut delta_b);
 
@@ -213,12 +240,25 @@ where
         }
     }
 
-    fn sort(&self, ids: &mut [&Node<V>]) {
+    fn sort(&self, ids: &mut [&Node<V, E>]) {
         // Can use unstable sort because mutex ids should not be equal
         ids.sort_unstable_by_key(|v| &v.ord);
     }
 }
 
+// Manual `Default` impl as derive causes unnecessarily strong bounds.
+impl<V, E> Default for DiGraph<V, E>
+where
+    V: Eq + Hash + Copy,
+{
+    fn default() -> Self {
+        Self {
+            nodes: Default::default(),
+            unused_order: Default::default(),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use rand::seq::SliceRandom;
@@ -226,12 +266,14 @@ mod tests {
 
     use super::*;
 
+    fn nop() {}
+
     #[test]
     fn test_no_self_cycle() {
         // Regression test for https://github.com/bertptrs/tracing-mutex/issues/7
         let mut graph = DiGraph::default();
 
-        assert!(!graph.add_edge(1, 1));
+        assert!(graph.add_edge(1, 1, nop).is_err());
     }
 
     #[test]
@@ -239,16 +281,16 @@ mod tests {
         let mut graph = DiGraph::default();
 
         // Add some safe edges
-        assert!(graph.add_edge(0, 1));
-        assert!(graph.add_edge(1, 2));
-        assert!(graph.add_edge(2, 3));
-        assert!(graph.add_edge(4, 2));
+        assert!(graph.add_edge(0, 1, nop).is_ok());
+        assert!(graph.add_edge(1, 2, nop).is_ok());
+        assert!(graph.add_edge(2, 3, nop).is_ok());
+        assert!(graph.add_edge(4, 2, nop).is_ok());
 
         // Try to add an edge that introduces a cycle
-        assert!(!graph.add_edge(3, 1));
+        assert!(graph.add_edge(3, 1, nop).is_err());
 
         // Add an edge that should reorder 0 to be after 4
-        assert!(graph.add_edge(4, 0));
+        assert!(graph.add_edge(4, 0, nop).is_ok());
     }
 
     /// Fuzz the DiGraph implementation by adding a bunch of valid edges.
@@ -256,7 +298,7 @@ mod tests {
     /// This test generates all possible forward edges in a 100-node graph consisting of natural
     /// numbers, shuffles them, then adds them to the graph. This will always be a valid directed,
     /// acyclic graph because there is a trivial order (the natural numbers) but because the edges
-    /// are added in a random order the DiGraph will still occassionally need to reorder nodes.
+    /// are added in a random order the DiGraph will still occasionally need to reorder nodes.
     #[test]
     fn fuzz_digraph() {
         // Note: this fuzzer is quadratic in the number of nodes, so this cannot be too large or it
@@ -277,7 +319,7 @@ mod tests {
         let mut graph = DiGraph::default();
 
         for (x, y) in edges {
-            assert!(graph.add_edge(x, y));
+            assert!(graph.add_edge(x, y, nop).is_ok());
         }
     }
 }

src/lib.rs

@@ -18,8 +18,23 @@
 //! # Structure
 //!
 //! Each module in this crate exposes wrappers for a specific base-mutex with dependency trakcing
-//! added. For now, that is limited to [`stdsync`] which provides wrappers for the base locks in the
-//! standard library. More back-ends may be added as features in the future.
+//! added. This includes [`stdsync`] which provides wrappers for the base locks in the standard
+//! library, and more depending on enabled compile-time features. More back-ends may be added as
+//! features in the future.
+//!
+//! # Feature flags
+//!
+//! `tracing-mutex` uses feature flags to reduce the impact of this crate on both your compile time
+//! and runtime overhead. Below are the available flags. Modules are annotated with the features
+//! they require.
+//!
+//! - `backtraces`: Enables capturing backtraces of mutex dependencies, to make it easier to
+//!   determine what sequence of events would trigger a deadlock. This is enabled by default, but if
+//!   the performance overhead is unaccceptable, it can be disabled by disabling default features.
+//!
+//! - `lockapi`: Enables the wrapper lock for [`lock_api`][lock_api] locks
+//!
+//! - `parkinglot`: Enables wrapper types for [`parking_lot`][parking_lot] mutexes
 //!
 //! # Performance considerations
 //!
@@ -44,7 +59,13 @@
 //! (such as [`stdsync::Mutex`]) which evaluate to a tracing mutex when debug assertions are
 //! enabled, and to the underlying mutex when they're not.
 //!
+//! For ease of debugging, this crate will, by default, capture a backtrace when establishing a new
+//! dependency between two mutexes. This has an additional overhead of over 60%. If this additional
+//! debugging aid is not required, it can be disabled by disabling default features.
+//!
 //! [paper]: https://whileydave.com/publications/pk07_jea/
+//! [lock_api]: https://docs.rs/lock_api/0.4/lock_api/index.html
+//! [parking_lot]: https://docs.rs/parking_lot/0.12.1/parking_lot/
 #![cfg_attr(docsrs, feature(doc_cfg))]
 use std::cell::RefCell;
 use std::fmt;
@@ -64,6 +85,8 @@ pub use lock_api;
 #[cfg(feature = "parkinglot")]
 #[cfg_attr(docsrs, doc(cfg(feature = "parkinglot")))]
 pub use parking_lot;
+use reporting::Dep;
+use reporting::Reportable;
 
 use crate::graph::DiGraph;
 
@@ -74,6 +97,7 @@ pub mod lockapi;
 #[cfg(feature = "parkinglot")]
 #[cfg_attr(docsrs, doc(cfg(feature = "parkinglot")))]
 pub mod parkinglot;
+mod reporting;
 pub mod stdsync;
 
 thread_local! {
@@ -137,19 +161,18 @@ impl MutexId {
     ///
     /// This method panics if the new dependency would introduce a cycle.
     pub fn mark_held(&self) {
-        let creates_cycle = HELD_LOCKS.with(|locks| {
+        let opt_cycle = HELD_LOCKS.with(|locks| {
             if let Some(&previous) = locks.borrow().last() {
                 let mut graph = get_dependency_graph();
 
-                !graph.add_edge(previous, self.value())
+                graph.add_edge(previous, self.value(), Dep::capture).err()
             } else {
-                false
+                None
             }
         });
 
-        if creates_cycle {
-            // Panic without holding the lock to avoid needlessly poisoning it
-            panic!("Mutex order graph should not have cycles");
+        if let Some(cycle) = opt_cycle {
+            panic!("{}", Dep::panic_message(&cycle))
         }
 
         HELD_LOCKS.with(|locks| locks.borrow_mut().push(self.value()));
@@ -260,8 +283,8 @@ impl<'a> Drop for BorrowedMutex<'a> {
 }
 
 /// Get a reference to the current dependency graph
-fn get_dependency_graph() -> impl DerefMut<Target = DiGraph<usize>> {
-    static DEPENDENCY_GRAPH: OnceLock<Mutex<DiGraph<usize>>> = OnceLock::new();
+fn get_dependency_graph() -> impl DerefMut<Target = DiGraph<usize, Dep>> {
+    static DEPENDENCY_GRAPH: OnceLock<Mutex<DiGraph<usize, Dep>>> = OnceLock::new();
 
     DEPENDENCY_GRAPH
         .get_or_init(Default::default)
@@ -292,11 +315,11 @@ mod tests {
         let c = LazyMutexId::new();
 
         let mut graph = get_dependency_graph();
-        assert!(graph.add_edge(a.value(), b.value()));
-        assert!(graph.add_edge(b.value(), c.value()));
+        assert!(graph.add_edge(a.value(), b.value(), Dep::capture).is_ok());
+        assert!(graph.add_edge(b.value(), c.value(), Dep::capture).is_ok());
 
         // Creating an edge c → a should fail as it introduces a cycle.
-        assert!(!graph.add_edge(c.value(), a.value()));
+        assert!(graph.add_edge(c.value(), a.value(), Dep::capture).is_err());
 
         // Drop graph handle so we can drop vertices without deadlocking
         drop(graph);
@@ -304,7 +327,9 @@ mod tests {
         drop(b);
 
         // If b's destructor correctly ran correctly we can now add an edge from c to a.
-        assert!(get_dependency_graph().add_edge(c.value(), a.value()));
+        assert!(get_dependency_graph()
+            .add_edge(c.value(), a.value(), Dep::capture)
+            .is_ok());
     }
 
     /// Test creating a cycle, then panicking.

src/reporting.rs

@@ -0,0 +1,64 @@
+//! Cycle reporting primitives
+//!
+//! This module exposes [`Dep`], which resolves to either something that tracks dependencies or to
+//! something that doesn't.  It should only be assumed to implement the [`Reportable`] trait.
+use std::backtrace::Backtrace;
+use std::borrow::Cow;
+use std::fmt::Write;
+use std::sync::Arc;
+
+#[cfg(feature = "backtraces")]
+pub type Dep = MutexDep<Arc<Backtrace>>;
+#[cfg(not(feature = "backtraces"))]
+pub type Dep = MutexDep<()>;
+
+// Base message to be reported when cycle is detected
+const BASE_MESSAGE: &str = "Found cycle in mutex dependency graph:";
+
+pub trait Reportable: Clone {
+    /// Capture the current state
+    fn capture() -> Self;
+
+    /// Format a trace of state for human readable consumption.
+    fn panic_message(trace: &[Self]) -> Cow<'static, str>;
+}
+
+#[derive(Clone)]
+pub struct MutexDep<T>(T);
+
+/// Use a unit as tracing data: no tracing.
+///
+/// This should have no runtime overhead for capturing traces and should therefore be cheap enough
+/// for most purposes.
+impl Reportable for MutexDep<()> {
+    fn capture() -> Self {
+        Self(())
+    }
+
+    fn panic_message(_trace: &[Self]) -> Cow<'static, str> {
+        Cow::Borrowed(BASE_MESSAGE)
+    }
+}
+
+/// Use a full backtrace as tracing data
+///
+/// Capture the entire backtrace which may be expensive. This implementation does not force capture
+/// in the event that backtraces are disabled at runtime, so the exact overhead can still be
+/// controlled a little.
+///
+/// N.B. the [`Backtrace`] needs to be wrapped in an Arc as backtraces are not [`Clone`].
+impl Reportable for MutexDep<Arc<Backtrace>> {
+    fn capture() -> Self {
+        Self(Arc::new(Backtrace::capture()))
+    }
+
+    fn panic_message(trace: &[Self]) -> Cow<'static, str> {
+        let mut message = format!("{BASE_MESSAGE}\n");
+
+        for entry in trace {
+            let _ = writeln!(message, "{}", entry.0);
+        }
+
+        message.into()
+    }
+}

src/stdsync.rs

@@ -696,7 +696,7 @@ pub mod tracing {
         }
 
         #[test]
-        #[should_panic(expected = "Mutex order graph should not have cycles")]
+        #[should_panic(expected = "Found cycle in mutex dependency graph")]
         fn test_detect_cycle() {
             let a = Mutex::new(());
             let b = Mutex::new(());