Use dedicated type for Mutex IDs

This should prevent tiny mistakes in handling the value.

src/graph.rs

@@ -1,21 +1,23 @@
 use std::collections::HashMap;
 use std::collections::HashSet;
 
+use crate::MutexID;
+
 #[derive(Clone, Default, Debug)]
 pub struct DiGraph {
-    in_edges: HashMap<usize, Vec<usize>>,
-    out_edges: HashMap<usize, Vec<usize>>,
+    in_edges: HashMap<MutexID, Vec<MutexID>>,
+    out_edges: HashMap<MutexID, Vec<MutexID>>,
 }
 
 impl DiGraph {
-    fn add_node(&mut self, node: usize) -> (&mut Vec<usize>, &mut Vec<usize>) {
+    fn add_node(&mut self, node: MutexID) -> (&mut Vec<MutexID>, &mut Vec<MutexID>) {
         let in_edges = self.in_edges.entry(node).or_default();
         let out_edges = self.out_edges.entry(node).or_default();
 
         (in_edges, out_edges)
     }
 
-    pub fn remove_node(&mut self, node: usize) -> bool {
+    pub(crate) fn remove_node(&mut self, node: MutexID) -> bool {
         match self.out_edges.remove(&node) {
             None => false,
             Some(out_edges) => {
@@ -38,7 +40,10 @@ impl DiGraph {
         }
     }
 
-    pub fn add_edge(&mut self, from: usize, to: usize) -> bool {
+    /// 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, from: MutexID, to: MutexID) -> bool {
         if from == to {
             return false;
         }
@@ -65,7 +70,12 @@ impl DiGraph {
             .any(|node| !self.visit(node, &mut marks, &mut temp))
     }
 
-    fn visit(&self, node: usize, marks: &mut HashSet<usize>, temp: &mut HashSet<usize>) -> bool {
+    fn visit(
+        &self,
+        node: MutexID,
+        marks: &mut HashSet<MutexID>,
+        temp: &mut HashSet<MutexID>,
+    ) -> bool {
         if marks.contains(&node) {
             return true;
         }
@@ -92,25 +102,29 @@ impl DiGraph {
 
 #[cfg(test)]
 mod tests {
-    use super::DiGraph;
+    use super::*;
+    use crate::MutexID;
 
     #[test]
     fn test_digraph() {
+        let id: Vec<MutexID> = (0..5).map(|_| MutexID::new()).collect();
         let mut graph = DiGraph::default();
 
-        graph.add_edge(1, 2);
-        graph.add_edge(2, 3);
-        graph.add_edge(3, 4);
-        graph.add_edge(5, 2);
+        // 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]);
 
+        // Should not have a cycle yet
         assert!(!graph.has_cycles());
 
-        graph.add_edge(4, 2);
-
+        // Introduce cycle 3 → 1 → 2 → 3
+        graph.add_edge(id[3], id[1]);
         assert!(graph.has_cycles());
 
-        assert!(graph.remove_node(4));
-
+        // Removing 3 should remove that cycle
+        assert!(graph.remove_node(id[3]));
         assert!(!graph.has_cycles())
     }
 }

src/lib.rs

@@ -13,6 +13,8 @@ mod graph;
 pub mod stdsync;
 
 /// Counter for Mutex IDs. Atomic avoids the need for locking.
+///
+/// Should be part of the `MutexID` impl but static items are not yet a thing.
 static ID_SEQUENCE: AtomicUsize = AtomicUsize::new(0);
 
 thread_local! {
@@ -20,17 +22,40 @@ 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<MutexID>> = RefCell::new(Vec::new());
 }
 
 lazy_static! {
     static ref DEPENDENCY_GRAPH: Mutex<DiGraph> = Default::default();
 }
 
-fn next_mutex_id() -> usize {
-    ID_SEQUENCE
-        .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |id| id.checked_add(1))
-        .expect("Mutex ID wraparound happened, results unreliable")
+/// Dedicated ID type for Mutexes
+///
+/// # Unstable
+///
+/// This type is currently private to prevent usage while the exact implementation is figured out,
+/// but it will likely be public in the future.
+///
+/// One possible alteration is to make this type not `Copy` but `Drop`, and handle deregistering
+/// the lock from there.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+struct MutexID(usize);
+
+impl MutexID {
+    /// Get a new, unique, mutex ID.
+    ///
+    /// This ID is guaranteed to be unique within the runtime of the program.
+    ///
+    /// # Panics
+    ///
+    /// This function may panic when there are no more mutex IDs available. The number of mutex ids
+    /// is `usize::MAX - 1` which should be plenty for most practical applications.
+    pub fn new() -> Self {
+        ID_SEQUENCE
+            .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |id| id.checked_add(1))
+            .map(|id| Self(id))
+            .expect("Mutex ID wraparound happened, results unreliable")
+    }
 }
 
 /// Get a reference to the current dependency graph
@@ -41,11 +66,17 @@ fn get_depedency_graph() -> impl DerefMut<Target = DiGraph> {
 }
 
 /// Register that a lock is currently held
-fn register_lock(lock: usize) {
+fn register_lock(lock: MutexID) {
     HELD_LOCKS.with(|locks| locks.borrow_mut().push(lock))
 }
 
-fn drop_lock(id: usize) {
+/// Drop a lock held by the current thread.
+///
+/// # Panics
+///
+/// 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.
+fn drop_lock(id: MutexID) {
     HELD_LOCKS.with(|locks| {
         let mut locks = locks.borrow_mut();
 
@@ -56,7 +87,7 @@ fn drop_lock(id: usize) {
             }
         }
 
-        panic!("Tried to drop lock for mutex {} but it wasn't held", id)
+        panic!("Tried to drop lock for mutex {:?} but it wasn't held", id)
     });
 }
 
@@ -64,16 +95,23 @@ fn drop_lock(id: usize) {
 ///
 /// If the dependency is new, check for cycles in the dependency graph. If not, there shouldn't be
 /// any cycles so we don't need to check.
-fn register_dependency(lock: usize) {
-    HELD_LOCKS.with(|locks| {
+///
+/// # Panics
+///
+/// This function panics if the new dependency would introduce a cycle.
+fn register_dependency(lock: MutexID) {
+    if HELD_LOCKS.with(|locks| {
         if let Some(&previous) = locks.borrow().last() {
             let mut graph = get_depedency_graph();
 
-            if graph.add_edge(previous, lock) && graph.has_cycles() {
-                panic!("Mutex order graph should not have cycles");
-            }
+            graph.add_edge(previous, lock) && graph.has_cycles()
+        } else {
+            false
         }
-    })
+    }) {
+        // Panic without holding the lock to avoid needlessly poisoning it
+        panic!("Mutex order graph should not have cycles");
+    }
 }
 
 #[cfg(test)]
@@ -87,10 +125,10 @@ mod tests {
 
     #[test]
     fn test_next_mutex_id() {
-        let initial = next_mutex_id();
-        let next = next_mutex_id();
+        let initial = MutexID::new();
+        let next = MutexID::new();
 
         // Can't assert N + 1 because multiple threads running tests
-        assert!(initial < next);
+        assert!(initial.0 < next.0);
     }
 }

src/stdsync.rs

@@ -30,15 +30,15 @@ use std::sync::TryLockResult;
 
 use crate::drop_lock;
 use crate::get_depedency_graph;
-use crate::next_mutex_id;
 use crate::register_dependency;
 use crate::register_lock;
+use crate::MutexID;
 
 /// Wrapper for `std::sync::Mutex`
 #[derive(Debug)]
 pub struct TracingMutex<T> {
     inner: Mutex<T>,
-    id: usize,
+    id: MutexID,
 }
 
 /// Wrapper for `std::sync::MutexGuard`
@@ -75,7 +75,7 @@ impl<T> TracingMutex<T> {
     pub fn new(t: T) -> Self {
         Self {
             inner: Mutex::new(t),
-            id: next_mutex_id(),
+            id: MutexID::new(),
         }
     }
 
@@ -107,10 +107,6 @@ impl<T> TracingMutex<T> {
         map_trylockresult(result, mapper)
     }
 
-    pub fn get_id(&self) -> usize {
-        self.id
-    }
-
     pub fn is_poisoned(&self) -> bool {
         self.inner.is_poisoned()
     }
@@ -182,7 +178,7 @@ impl<'a, T> Drop for TracingMutexGuard<'a, T> {
 #[derive(Debug)]
 pub struct TracingRwLock<T> {
     inner: RwLock<T>,
-    id: usize,
+    id: MutexID,
 }
 
 /// Hybrid wrapper for both `std::sync::RwLockReadGuard` and `std::sync::RwLockWriteGuard`.
@@ -203,14 +199,10 @@ impl<T> TracingRwLock<T> {
     pub fn new(t: T) -> Self {
         Self {
             inner: RwLock::new(t),
-            id: next_mutex_id(),
+            id: MutexID::new(),
         }
     }
 
-    pub fn get_id(&self) -> usize {
-        self.id
-    }
-
     #[track_caller]
     pub fn read(&self) -> LockResult<TracingReadGuard<T>> {
         register_dependency(self.id);