Refactor MutexID to be self tracking

This avoids the need to implement Drop on every wrapped mutex, and
removes the need for unsafe code in this crate.

src/graph.rs

@@ -1,7 +1,6 @@
 use std::collections::HashMap;
 use std::collections::HashSet;
-
-use crate::MutexId;
+use std::hash::Hash;
 
 type Order = usize;
 
@@ -20,19 +19,25 @@ type Order = usize;
 ///
 /// [paper]: https://whileydave.com/publications/pk07_jea/
 #[derive(Clone, Default, Debug)]
-pub struct DiGraph {
-    in_edges: HashMap<MutexId, HashSet<MutexId>>,
-    out_edges: HashMap<MutexId, HashSet<MutexId>>,
+pub struct DiGraph<V>
+where
+    V: Eq + Hash + Copy,
+{
+    in_edges: HashMap<V, HashSet<V>>,
+    out_edges: HashMap<V, HashSet<V>>,
     /// Next topological sort order
     next_ord: Order,
     /// Poison flag, set if a cycle is detected when adding a new edge and
     /// unset when removing a node successfully removed the cycle.
     contains_cycle: bool,
     /// Topological sort order. Order is not guaranteed to be contiguous
-    ord: HashMap<MutexId, Order>,
+    ord: HashMap<V, Order>,
 }
 
-impl DiGraph {
+impl<V> DiGraph<V>
+where
+    V: Eq + Hash + Copy,
+{
     /// Add a new node to the graph.
     ///
     /// If the node already existed, this function does not add it and uses the existing node data.
@@ -40,7 +45,7 @@ impl DiGraph {
     /// 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: MutexId) -> (&mut HashSet<MutexId>, &mut HashSet<MutexId>, Order) {
+    fn add_node(&mut self, n: V) -> (&mut HashSet<V>, &mut HashSet<V>, Order) {
         let next_ord = &mut self.next_ord;
         let in_edges = self.in_edges.entry(n).or_default();
         let out_edges = self.out_edges.entry(n).or_default();
@@ -54,7 +59,7 @@ impl DiGraph {
         (in_edges, out_edges, order)
     }
 
-    pub(crate) fn remove_node(&mut self, n: MutexId) -> bool {
+    pub(crate) fn remove_node(&mut self, n: V) -> bool {
         match self.out_edges.remove(&n) {
             None => false,
             Some(out_edges) => {
@@ -79,7 +84,7 @@ impl DiGraph {
     /// Add an edge to the graph
     ///
     /// Nodes, both from and to, are created as needed when creating new edges.
-    pub(crate) fn add_edge(&mut self, x: MutexId, y: MutexId) -> bool {
+    pub(crate) fn add_edge(&mut self, x: V, y: V) -> bool {
         if x == y {
             // self-edges are not considered cycles
             return false;
@@ -120,13 +125,7 @@ impl DiGraph {
     }
 
     /// Forwards depth-first-search
-    fn dfs_f(
-        &self,
-        n: MutexId,
-        ub: Order,
-        visited: &mut HashSet<MutexId>,
-        delta_f: &mut Vec<MutexId>,
-    ) -> bool {
+    fn dfs_f(&self, n: V, ub: Order, visited: &mut HashSet<V>, delta_f: &mut Vec<V>) -> bool {
         visited.insert(n);
         delta_f.push(n);
 
@@ -147,13 +146,7 @@ impl DiGraph {
     }
 
     /// Backwards depth-first-search
-    fn dfs_b(
-        &self,
-        n: MutexId,
-        lb: Order,
-        visited: &mut HashSet<MutexId>,
-        delta_b: &mut Vec<MutexId>,
-    ) {
+    fn dfs_b(&self, n: V, lb: Order, visited: &mut HashSet<V>, delta_b: &mut Vec<V>) {
         visited.insert(n);
         delta_b.push(n);
 
@@ -164,7 +157,7 @@ impl DiGraph {
         }
     }
 
-    fn reorder(&mut self, mut delta_f: Vec<MutexId>, mut delta_b: Vec<MutexId>) {
+    fn reorder(&mut self, mut delta_f: Vec<V>, mut delta_b: Vec<V>) {
         self.sort(&mut delta_f);
         self.sort(&mut delta_b);
 
@@ -190,7 +183,7 @@ impl DiGraph {
         }
     }
 
-    fn sort(&self, ids: &mut [MutexId]) {
+    fn sort(&self, ids: &mut [V]) {
         // Can use unstable sort because mutex ids should not be equal
         ids.sort_unstable_by_key(|v| self.ord[v]);
     }
@@ -239,10 +232,10 @@ impl DiGraph {
     /// Helper function for `Self::recompute_topological_order`.
     fn visit(
         &self,
-        v: MutexId,
-        permanent_marks: &mut HashSet<MutexId>,
-        temporary_marks: &mut HashSet<MutexId>,
-        rev_order: &mut Vec<MutexId>,
+        v: V,
+        permanent_marks: &mut HashSet<V>,
+        temporary_marks: &mut HashSet<V>,
+        rev_order: &mut Vec<V>,
     ) -> bool {
         if permanent_marks.contains(&v) {
             return true;
@@ -271,28 +264,26 @@ impl DiGraph {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::MutexId;
 
     #[test]
     fn test_digraph() {
-        let id: Vec<MutexId> = (0..5).map(|_| MutexId::new()).collect();
         let mut graph = DiGraph::default();
 
         // Add some safe edges
-        graph.add_edge(id[0], id[1]);
-        graph.add_edge(id[1], id[2]);
-        graph.add_edge(id[2], id[3]);
-        graph.add_edge(id[4], id[2]);
+        graph.add_edge(0, 1);
+        graph.add_edge(1, 2);
+        graph.add_edge(2, 3);
+        graph.add_edge(4, 2);
 
         // Should not have a cycle yet
         assert!(!graph.has_cycles());
 
         // Introduce cycle 3 → 1 → 2 → 3
-        graph.add_edge(id[3], id[1]);
+        graph.add_edge(3, 1);
         assert!(graph.has_cycles());
 
         // Removing 3 should remove that cycle
-        assert!(graph.remove_node(id[3]));
+        assert!(graph.remove_node(3));
         assert!(!graph.has_cycles())
     }
 }

src/lib.rs

@@ -70,11 +70,11 @@ 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<MutexId>> = RefCell::new(Vec::new());
+    static HELD_LOCKS: RefCell<Vec<usize>> = RefCell::new(Vec::new());
 }
 
 lazy_static! {
-    static ref DEPENDENCY_GRAPH: Mutex<DiGraph> = Default::default();
+    static ref DEPENDENCY_GRAPH: Mutex<DiGraph<usize>> = Default::default();
 }
 
 /// Dedicated ID type for Mutexes
@@ -86,7 +86,6 @@ lazy_static! {
 ///
 /// One possible alteration is to make this type not `Copy` but `Drop`, and handle deregistering
 /// the lock from there.
-#[derive(Copy, Clone, PartialEq, Eq, Hash)]
 struct MutexId(usize);
 
 impl MutexId {
@@ -105,6 +104,10 @@ impl MutexId {
             .expect("Mutex ID wraparound happened, results unreliable")
     }
 
+    pub fn value(&self) -> usize {
+        self.0
+    }
+
     /// Get a borrowed guard for this lock.
     ///
     /// This method adds checks adds this Mutex ID to the dependency graph as needed, and adds the
@@ -113,12 +116,12 @@ impl MutexId {
     /// # Panics
     ///
     /// This method panics if the new dependency would introduce a cycle.
-    pub fn get_borrowed(self) -> BorrowedMutex {
+    pub fn get_borrowed(&self) -> BorrowedMutex {
         let creates_cycle = HELD_LOCKS.with(|locks| {
             if let Some(&previous) = locks.borrow().last() {
                 let mut graph = get_depedency_graph();
 
-                graph.add_edge(previous, self) && graph.has_cycles()
+                graph.add_edge(previous, self.value()) && graph.has_cycles()
             } else {
                 false
             }
@@ -129,7 +132,7 @@ impl MutexId {
             panic!("Mutex order graph should not have cycles");
         }
 
-        HELD_LOCKS.with(|locks| locks.borrow_mut().push(self));
+        HELD_LOCKS.with(|locks| locks.borrow_mut().push(self.value()));
         BorrowedMutex(self)
     }
 }
@@ -140,8 +143,14 @@ impl fmt::Debug for MutexId {
     }
 }
 
+impl Drop for MutexId {
+    fn drop(&mut self) {
+        get_depedency_graph().remove_node(self.value());
+    }
+}
+
 #[derive(Debug)]
-struct BorrowedMutex(MutexId);
+struct BorrowedMutex<'a>(&'a MutexId);
 
 /// Drop a lock held by the current thread.
 ///
@@ -149,7 +158,7 @@ struct BorrowedMutex(MutexId);
 ///
 /// 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 Drop for BorrowedMutex {
+impl<'a> Drop for BorrowedMutex<'a> {
     fn drop(&mut self) {
         let id = self.0;
 
@@ -157,7 +166,7 @@ impl Drop for BorrowedMutex {
             let mut locks = locks.borrow_mut();
 
             for (i, &lock) in locks.iter().enumerate().rev() {
-                if lock == id {
+                if lock == id.value() {
                     locks.remove(i);
                     return;
                 }
@@ -170,7 +179,7 @@ impl Drop for BorrowedMutex {
 }
 
 /// Get a reference to the current dependency graph
-fn get_depedency_graph() -> impl DerefMut<Target = DiGraph> {
+fn get_depedency_graph() -> impl DerefMut<Target = DiGraph<usize>> {
     DEPENDENCY_GRAPH
         .lock()
         .unwrap_or_else(PoisonError::into_inner)

src/stdsync.rs

@@ -14,10 +14,8 @@
 //! rwlock.read().unwrap();
 //! ```
 use std::fmt;
-use std::mem;
 use std::ops::Deref;
 use std::ops::DerefMut;
-use std::ptr;
 use std::sync::LockResult;
 use std::sync::Mutex;
 use std::sync::MutexGuard;
@@ -28,7 +26,6 @@ use std::sync::RwLockWriteGuard;
 use std::sync::TryLockError;
 use std::sync::TryLockResult;
 
-use crate::get_depedency_graph;
 use crate::BorrowedMutex;
 use crate::MutexId;
 
@@ -69,7 +66,7 @@ pub struct TracingMutex<T> {
 #[derive(Debug)]
 pub struct TracingMutexGuard<'a, T> {
     inner: MutexGuard<'a, T>,
-    mutex: BorrowedMutex,
+    mutex: BorrowedMutex<'a>,
 }
 
 fn map_lockresult<T, I, F>(result: LockResult<I>, mapper: F) -> LockResult<T>
@@ -138,17 +135,7 @@ impl<T> TracingMutex<T> {
     }
 
     pub fn into_inner(self) -> LockResult<T> {
-        self.deregister();
-
-        // Safety: we forget the original immediately after
-        let inner = unsafe { ptr::read(&self.inner) };
-        mem::forget(self);
-
-        inner.into_inner()
-    }
-
-    fn deregister(&self) {
-        get_depedency_graph().remove_node(self.id);
+        self.inner.into_inner()
     }
 }
 
@@ -164,12 +151,6 @@ impl<T> From<T> for TracingMutex<T> {
     }
 }
 
-impl<T> Drop for TracingMutex<T> {
-    fn drop(&mut self) {
-        self.deregister();
-    }
-}
-
 impl<'a, T> Deref for TracingMutexGuard<'a, T> {
     type Target = MutexGuard<'a, T>;
 
@@ -201,15 +182,15 @@ pub struct TracingRwLock<T> {
 ///
 /// Please refer to [`TracingReadGuard`] and [`TracingWriteGuard`] for usable types.
 #[derive(Debug)]
-pub struct TracingRwLockGuard<L> {
+pub struct TracingRwLockGuard<'a, L> {
     inner: L,
-    mutex: BorrowedMutex,
+    mutex: BorrowedMutex<'a>,
 }
 
 /// Wrapper around [`std::sync::RwLockReadGuard`].
-pub type TracingReadGuard<'a, T> = TracingRwLockGuard<RwLockReadGuard<'a, T>>;
+pub type TracingReadGuard<'a, T> = TracingRwLockGuard<'a, RwLockReadGuard<'a, T>>;
 /// Wrapper around [`std::sync::RwLockWriteGuard`].
-pub type TracingWriteGuard<'a, T> = TracingRwLockGuard<RwLockWriteGuard<'a, T>>;
+pub type TracingWriteGuard<'a, T> = TracingRwLockGuard<'a, RwLockWriteGuard<'a, T>>;
 
 impl<T> TracingRwLock<T> {
     pub fn new(t: T) -> Self {
@@ -256,24 +237,7 @@ impl<T> TracingRwLock<T> {
     }
 
     pub fn into_inner(self) -> LockResult<T> {
-        self.deregister();
-
-        // Grab our contents and then forget ourselves
-        // Safety: we immediately forget the mutex after copying
-        let inner = unsafe { ptr::read(&self.inner) };
-        mem::forget(self);
-
-        inner.into_inner()
-    }
-
-    fn deregister(&self) {
-        get_depedency_graph().remove_node(self.id);
-    }
-}
-
-impl<T> Drop for TracingRwLock<T> {
-    fn drop(&mut self) {
-        self.deregister();
+        self.inner.into_inner()
     }
 }
 
@@ -286,7 +250,7 @@ where
     }
 }
 
-impl<L, T> Deref for TracingRwLockGuard<L>
+impl<'a, L, T> Deref for TracingRwLockGuard<'a, L>
 where
     L: Deref<Target = T>,
 {
@@ -297,7 +261,7 @@ where
     }
 }
 
-impl<T, L> DerefMut for TracingRwLockGuard<L>
+impl<'a, T, L> DerefMut for TracingRwLockGuard<'a, L>
 where
     L: Deref<Target = T> + DerefMut,
 {