Use Event API and add docs

src/derived.rs

@@ -628,7 +628,6 @@ where
             // panicked before it could be removed. At this point, we
             // therefore "own" unique access to our slot, so we can just
             // remove the `InProgress` marker.
-            let waiting = {
             let mut map = self.map.write();
             match map.remove(self.key) {
                 Some(QueryState::InProgress { id, waiting }) => {
@@ -636,11 +635,11 @@ where
 
                     let waiting = waiting.into_inner();
 
-                        if waiting.is_empty() {
+                    if !waiting.is_empty() {
-                            // if nobody is waiting, we are done here
+                        // We want to propagate our panic to those waiting on
-                            return;
+                        // us. By dropping `waiting`, others will panic in
-                        } else {
+                        // `.recv`.
-                            waiting
+                        std::mem::drop(waiting)
                     }
                 }
 
@@ -655,11 +654,6 @@ Unexpected panic during query evaluation, aborting the process.
 Please report this bug to https://github.com/salsa-rs/salsa/issues."
                 ),
             }
-            };
-
-            // We want to propagate our panic to those waiting on us. By dropping
-            // `waiting`, others will panic in `.recv`.
-            std::mem::drop(waiting)
         } else {
             // If no panic occurred, then panic guard ought to be
             // "forgotten" and so this Drop code should never run.

src/plumbing.rs

@@ -112,7 +112,7 @@ where
     /// Check if `key` is (currently) believed to be a constant.
     fn is_constant(&self, db: &DB, key: &Q::Key) -> bool;
 
-    /// Check if `key` is (currently) believed to be a constant.
+    /// Get the (current) set of the keys in the query storage
     fn keys<C>(&self, db: &DB) -> C
     where
         C: std::iter::FromIterator<Q::Key>;

tests/parallel/true_parallel.rs

@@ -84,12 +84,17 @@ fn true_parallel_same_keys() {
     assert_eq!(thread2.join().unwrap(), 111);
 }
 
+/// Add a test that tries to trigger a conflict, where we fetch `sum("a")`
+/// from two threads simultaneously. After `thread2` begins blocking,
+/// we force `thread1` to panic and should see that propagate to `thread2`.
 #[test]
 fn true_parallel_propagate_panic() {
     let db = ParDatabaseImpl::default();
 
     db.query(Input).set('a', 1);
 
+    // `thread1` will wait_for a barrier in the start of `sum`. Once it can
+    // continue, it will panic.
     let thread1 = std::thread::spawn({
         let db = db.fork();
         move || {
@@ -102,11 +107,13 @@ fn true_parallel_propagate_panic() {
         }
     });
 
+    // `thread2` will wait until `thread1` has entered sum and then -- once it
+    // has set itself to block -- signal `thread1` to continue.
     let thread2 = std::thread::spawn({
         let db = db.fork();
         move || {
             db.knobs().signal.wait_for(1);
-            db.knobs().signal.signal(2);
+            db.knobs().signal_on_will_block.set(2);
             db.sum("a")
         }
     });