From 49a1184bcfabf45e44b85cf1ee8a5c434f46b595 Mon Sep 17 00:00:00 2001
From: Jonas Schievink <jonasschievink@gmail.com>
Date: Tue, 18 May 2021 14:41:45 +0200
Subject: [PATCH] Improve `Canceled` API

---
 src/lib.rs               | 37 +++++++++++++++++++------------------
 tests/parallel/race.rs   |  4 ++--
 tests/parallel/stress.rs |  4 ++--
 3 files changed, 23 insertions(+), 22 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index da4da81c..39ab7246 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -647,13 +647,28 @@ where
 
 /// A panic payload indicating that a salsa revision was canceled.
 #[derive(Debug)]
-pub struct Canceled {
-    _private: (),
-}
+#[non_exhaustive]
+pub struct Canceled;
 
 impl Canceled {
     fn throw() -> ! {
-        std::panic::resume_unwind(Box::new(Self { _private: () }));
+        // We use resume and not panic here to avoid running the panic
+        // hook (that is, to avoid collecting and printing backtrace).
+        std::panic::resume_unwind(Box::new(Self));
+    }
+
+    /// Runs `f`, and catches any salsa cancellation.
+    pub fn catch<F, T>(f: F) -> Result<T, Canceled>
+    where
+        F: FnOnce() -> T + UnwindSafe,
+    {
+        match panic::catch_unwind(f) {
+            Ok(t) => Ok(t),
+            Err(payload) => match payload.downcast() {
+                Ok(canceled) => Err(*canceled),
+                Err(payload) => panic::resume_unwind(payload),
+            },
+        }
     }
 }
 
@@ -665,20 +680,6 @@ impl std::fmt::Display for Canceled {
 
 impl std::error::Error for Canceled {}
 
-/// Runs `f`, and catches any salsa cancelation.
-pub fn catch_cancellation<F, T>(f: F) -> Result<T, Canceled>
-where
-    F: FnOnce() -> T + UnwindSafe,
-{
-    match panic::catch_unwind(f) {
-        Ok(t) => Ok(t),
-        Err(payload) => match payload.downcast() {
-            Ok(canceled) => Err(*canceled),
-            Err(payload) => panic::resume_unwind(payload),
-        },
-    }
-}
-
 // Re-export the procedural macros.
 #[allow(unused_imports)]
 #[macro_use]
diff --git a/tests/parallel/race.rs b/tests/parallel/race.rs
index 4d44e521..6247a9bb 100644
--- a/tests/parallel/race.rs
+++ b/tests/parallel/race.rs
@@ -1,7 +1,7 @@
 use std::panic::AssertUnwindSafe;
 
 use crate::setup::{ParDatabase, ParDatabaseImpl};
-use salsa::ParallelDatabase;
+use salsa::{Canceled, ParallelDatabase};
 
 /// Test where a read and a set are racing with one another.
 /// Should be atomic.
@@ -16,7 +16,7 @@ fn in_par_get_set_race() {
     let thread1 = std::thread::spawn({
         let db = db.snapshot();
         move || {
-            salsa::catch_cancellation(AssertUnwindSafe(|| {
+            Canceled::catch(AssertUnwindSafe(|| {
                 let v = db.sum("abc");
                 v
             }))
diff --git a/tests/parallel/stress.rs b/tests/parallel/stress.rs
index 07dd9c95..bafb0db8 100644
--- a/tests/parallel/stress.rs
+++ b/tests/parallel/stress.rs
@@ -1,10 +1,10 @@
 use rand::seq::SliceRandom;
 use rand::Rng;
 
-use salsa::Database;
 use salsa::ParallelDatabase;
 use salsa::Snapshot;
 use salsa::SweepStrategy;
+use salsa::{Canceled, Database};
 
 // Number of operations a reader performs
 const N_MUTATOR_OPS: usize = 100;
@@ -191,7 +191,7 @@ fn stress_test() {
                 check_cancellation,
             } => all_threads.push(std::thread::spawn({
                 let db = db.snapshot();
-                move || salsa::catch_cancellation(|| db_reader_thread(&db, ops, check_cancellation))
+                move || Canceled::catch(|| db_reader_thread(&db, ops, check_cancellation))
             })),
         }
     }