Allow to temporarily set the current registry even if it is not assoc…

…iated with a worker thread

rayon-core/src/registry.rs

@@ -217,6 +217,36 @@ fn default_global_registry() -> Result<Arc<Registry>, ThreadPoolBuildError> {
     result
 }
 
+// This is used to temporarily overwrite the current registry.
+//
+// This either null, a pointer to the global registry if it was
+// ever used to access the global registry or a pointer to a
+// registry which is temporarily made current because the current
+// thread is not a worker thread but is running a scope associated
+// to a specific thread pool.
+thread_local! {
+    static CURRENT_REGISTRY: Cell<*const Arc<Registry>> = const { Cell::new(ptr::null()) };
+}
+
+#[cold]
+fn set_current_registry_to_global_registry() -> *const Arc<Registry> {
+    let global = global_registry();
+
+    CURRENT_REGISTRY.with(|current_registry| current_registry.set(global));
+
+    global
+}
+
+fn current_registry() -> *const Arc<Registry> {
+    let mut current = CURRENT_REGISTRY.with(Cell::get);
+
+    if current.is_null() {
+        current = set_current_registry_to_global_registry();
+    }
+
+    current
+}
+
 struct Terminator<'a>(&'a Arc<Registry>);
 
 impl<'a> Drop for Terminator<'a> {
@@ -315,14 +345,47 @@ impl Registry {
         unsafe {
             let worker_thread = WorkerThread::current();
             let registry = if worker_thread.is_null() {
-                global_registry()
+                &*current_registry()
             } else {
                 &(*worker_thread).registry
             };
             Arc::clone(registry)
         }
     }
 
+    /// Optionally install a specific registry as the current one.
+    ///
+    /// This is used when a thread which is not a worker executes
+    /// a scope which should use the specific thread pool instead of
+    /// the global one.
+    pub(super) fn with_current<F, R>(registry: Option<&Arc<Registry>>, f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        struct Guard {
+            current: *const Arc<Registry>,
+        }
+
+        impl Guard {
+            fn new(registry: &Arc<Registry>) -> Self {
+                let current =
+                    CURRENT_REGISTRY.with(|current_registry| current_registry.replace(registry));
+
+                Self { current }
+            }
+        }
+
+        impl Drop for Guard {
+            fn drop(&mut self) {
+                CURRENT_REGISTRY.with(|current_registry| current_registry.set(self.current));
+            }
+        }
+
+        let _guard = registry.map(Guard::new);
+
+        f()
+    }
+
     /// Returns the number of threads in the current registry.  This
     /// is better than `Registry::current().num_threads()` because it
     /// avoids incrementing the `Arc`.

rayon-core/src/scope/mod.rs

@@ -416,9 +416,11 @@ pub(crate) fn do_in_place_scope<'scope, OP, R>(registry: Option<&Arc<Registry>>,
 where
     OP: FnOnce(&Scope<'scope>) -> R,
 {
-    let thread = unsafe { WorkerThread::current().as_ref() };
-    let scope = Scope::<'scope>::new(thread, registry);
-    scope.base.complete(thread, || op(&scope))
+    Registry::with_current(registry, || {
+        let thread = unsafe { WorkerThread::current().as_ref() };
+        let scope = Scope::<'scope>::new(thread, registry);
+        scope.base.complete(thread, || op(&scope))
+    })
 }
 
 /// Creates a "fork-join" scope `s` with FIFO order, and invokes the