fix race condition around dropping arc handle

Sigh, I always make this mistake.

src/handle.rs

@@ -9,7 +9,10 @@ use crate::{storage::HasStorage, Event, EventKind};
 /// When you attempt to modify the database, you call `get_mut`, which will set the cancellation flag,
 /// causing other handles to get panics. Once all other handles are dropped, you can proceed.
 pub struct Handle<Db: HasStorage> {
-    db: Arc<Db>,
+    /// Reference to the database. This is always `Some` except during destruction.
+    db: Option<Arc<Db>>,
+
+    /// Coordination data.
     coordinate: Arc<Coordinate>,
 }
 
@@ -23,7 +26,7 @@ struct Coordinate {
 impl<Db: HasStorage> Handle<Db> {
     pub fn new(db: Db) -> Self {
         Self {
-            db: Arc::new(db),
+            db: Some(Arc::new(db)),
             coordinate: Arc::new(Coordinate {
                 clones: Mutex::new(1),
                 cvar: Default::default(),
@@ -31,12 +34,29 @@ impl<Db: HasStorage> Handle<Db> {
         }
     }
 
+    fn db(&self) -> &Arc<Db> {
+        self.db.as_ref().unwrap()
+    }
+
+    fn db_mut(&mut self) -> &mut Arc<Db> {
+        self.db.as_mut().unwrap()
+    }
+
     /// Returns a mutable reference to the inner database.
     /// If other handles are active, this method sets the cancellation flag
     /// and blocks until they are dropped.
     pub fn get_mut(&mut self) -> &mut Db {
         self.cancel_others();
-        Arc::get_mut(&mut self.db).expect("no other handles")
+
+        // Once cancellation above completes, the other handles are being dropped.
+        // However, because the signal is sent before the destructor completes, it's
+        // possible that they have not *yet* dropped.
+        //
+        // Therefore, we may have to do a (short) bit of
+        // spinning before we observe the thread-count reducing to 0.
+        //
+        // An alternative would be to
+        Arc::get_mut(self.db_mut()).expect("other threads remain active despite cancellation")
     }
 
     // ANCHOR: cancel_other_workers
@@ -46,10 +66,10 @@ impl<Db: HasStorage> Handle<Db> {
     /// This could deadlock if there is a single worker with two handles to the
     /// same database!
     fn cancel_others(&mut self) {
-        let storage = self.db.storage();
+        let storage = self.db().storage();
         storage.runtime().set_cancellation_flag();
 
-        self.db.salsa_event(Event {
+        self.db().salsa_event(Event {
             thread_id: std::thread::current().id(),
 
             kind: EventKind::DidSetCancellationFlag,
@@ -65,6 +85,10 @@ impl<Db: HasStorage> Handle<Db> {
 
 impl<Db: HasStorage> Drop for Handle<Db> {
     fn drop(&mut self) {
+        // Drop the database handle *first*
+        self.db.take();
+
+        // *Now* decrement the number of clones and notify once we have completed
         *self.coordinate.clones.lock() -= 1;
         self.coordinate.cvar.notify_all();
     }
@@ -74,7 +98,7 @@ impl<Db: HasStorage> std::ops::Deref for Handle<Db> {
     type Target = Db;
 
     fn deref(&self) -> &Self::Target {
-        &self.db
+        self.db()
     }
 }
 
@@ -83,7 +107,7 @@ impl<Db: HasStorage> Clone for Handle<Db> {
         *self.coordinate.clones.lock() += 1;
 
         Self {
-            db: Arc::clone(&self.db),
+            db: Some(Arc::clone(self.db())),
             coordinate: Arc::clone(&self.coordinate),
         }
     }