add panic hooks

To implement cancellation via unwinding, one needs to throw `Canceled`
value in `if_current_revision_is_canceled` and `on_propagated_panic`.

src/derived.rs

@@ -480,7 +480,7 @@ where
                             },
                         });
 
-                        let value = rx.recv().unwrap();
+                        let value = rx.recv().unwrap_or_else(|_| db.on_propagated_panic());
                         ProbeState::UpToDate(Ok(value))
                     }
 
@@ -731,7 +731,7 @@ where
                         // can complete.
                         std::mem::drop(map);
 
-                        let value = rx.recv().unwrap();
+                        let value = rx.recv().unwrap_or_else(|_| db.on_propagated_panic());
                         return value.changed_at.changed_since(revision);
                     }
 

src/lib.rs

@@ -104,6 +104,12 @@ pub trait Database: plumbing::DatabaseStorageTypes + plumbing::DatabaseOps {
     fn salsa_event(&self, event_fn: impl Fn() -> Event<Self>) {
         #![allow(unused_variables)]
     }
+
+    /// This function is invoked when a depndent query is being computed by the
+    /// other thread, and that thread panics.
+    fn on_propagated_panic(&self) -> ! {
+        panic!("concurrent salsa query paniced")
+    }
 }
 
 /// The `Event` struct identifies various notable things that can

src/runtime.rs

@@ -223,6 +223,16 @@ where
         }
     }
 
+    /// Calls the callback if the current revision is cancelled.
+    ///
+    /// Observing cancellation may lead to inconsistencies in database storage,
+    /// so the callback must panic.
+    pub fn if_current_revision_is_canceled(&self, cb: fn() -> !) {
+        if self.pending_revision() > self.current_revision() {
+            cb()
+        }
+    }
+
     /// Acquires the **global query write lock** (ensuring that no
     /// queries are executing) and then increments the current
     /// revision counter; invokes `op` with the global query write