From add15d83eaa865571cd33e60b2eb611a366ca3ff Mon Sep 17 00:00:00 2001
From: Aleksey Kladov <aleksey.kladov@gmail.com>
Date: Thu, 10 Jan 2019 11:57:24 +0300
Subject: [PATCH] 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 |  4 ++--
 src/lib.rs     |  6 ++++++
 src/runtime.rs | 10 ++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/derived.rs b/src/derived.rs
index e5f7b81e..c30ddea5 100644
--- a/src/derived.rs
+++ b/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);
                     }
 
diff --git a/src/lib.rs b/src/lib.rs
index 12c74e8d..b42581a5 100644
--- a/src/lib.rs
+++ b/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
diff --git a/src/runtime.rs b/src/runtime.rs
index 4f0e6d08..5eeb701a 100644
--- a/src/runtime.rs
+++ b/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