3 files changed, 77 insertions, 3 deletions

diff --git a/core/src/async_util/executor.rs b/core/src/async_util/executor.rs
new file mode 100644
index 0000000..3e7aa06
--- /dev/null
+++ b/core/src/async_util/executor.rs
@@ -0,0 +1,30 @@
+use std::{panic::catch_unwind, sync::Arc, thread};
+
+use async_lock::OnceCell;
+use smol::Executor as SmolEx;
+
+static GLOBAL_EXECUTOR: OnceCell<Arc<smol::Executor<'_>>> = OnceCell::new();
+
+/// A pointer to an Executor
+pub type Executor<'a> = Arc<SmolEx<'a>>;
+
+/// Returns a single-threaded global executor
+pub(crate) fn global_executor() -> Executor<'static> {
+    fn init_executor() -> Executor<'static> {
+        let ex = smol::Executor::new();
+        thread::Builder::new()
+            .spawn(|| loop {
+                catch_unwind(|| {
+                    smol::block_on(global_executor().run(smol::future::pending::<()>()))
+                })
+                .ok();
+            })
+            .expect("cannot spawn executor thread");
+        // Prevent spawning another thread by running the process driver on this
+        // thread. see https://github.com/smol-rs/smol/blob/master/src/spawn.rs
+        ex.spawn(async_process::driver()).detach();
+        Arc::new(ex)
+    }
+
+    GLOBAL_EXECUTOR.get_or_init_blocking(init_executor).clone()
+}
diff --git a/core/src/async_util/mod.rs b/core/src/async_util/mod.rs
index c871bad..2916118 100644
--- a/core/src/async_util/mod.rs
+++ b/core/src/async_util/mod.rs
@@ -1,6 +1,7 @@
 mod backoff;
 mod condvar;
 mod condwait;
+mod executor;
 mod select;
 mod task_group;
 mod timeout;
@@ -8,6 +9,7 @@ mod timeout;
 pub use backoff::Backoff;
 pub use condvar::CondVar;
 pub use condwait::CondWait;
+pub use executor::Executor;
 pub use select::{select, Either};
 pub use task_group::{TaskGroup, TaskResult};
 pub use timeout::timeout;
diff --git a/core/src/async_util/task_group.rs b/core/src/async_util/task_group.rs
index 7129af2..0fb4855 100644
--- a/core/src/async_util/task_group.rs
+++ b/core/src/async_util/task_group.rs
@@ -2,9 +2,7 @@ use std::{future::Future, sync::Arc, sync::Mutex};
 
 use async_task::FallibleTask;
 
-use crate::Executor;
-
-use super::{select, CondWait, Either};
+use super::{executor::global_executor, select, CondWait, Either, Executor};
 
 /// TaskGroup is a group of spawned tasks.
 ///
@@ -35,6 +33,19 @@ pub struct TaskGroup<'a> {
     executor: Executor<'a>,
 }
 
+impl TaskGroup<'static> {
+    /// Creates a new task group without providing an executor
+    ///
+    /// This will Spawn a task onto a global executor (single-threaded by default).
+    pub fn new_without_executor() -> Self {
+        Self {
+            tasks: Mutex::new(Vec::new()),
+            stop_signal: Arc::new(CondWait::new()),
+            executor: global_executor(),
+        }
+    }
+}
+
 impl<'a> TaskGroup<'a> {
     /// Creates a new task group
     pub fn new(executor: Executor<'a>) -> Self {
@@ -191,4 +202,35 @@ mod tests {
             group.cancel().await;
         }));
     }
+
+    #[test]
+    fn test_task_group_without_executor() {
+        smol::block_on(async {
+            let group = Arc::new(TaskGroup::new_without_executor());
+
+            group.spawn(future::ready(0), |res| async move {
+                assert!(matches!(res, TaskResult::Completed(0)));
+            });
+
+            group.spawn(future::pending::<()>(), |res| async move {
+                assert!(matches!(res, TaskResult::Cancelled));
+            });
+
+            let groupc = group.clone();
+            group.spawn(
+                async move {
+                    groupc.spawn(future::pending::<()>(), |res| async move {
+                        assert!(matches!(res, TaskResult::Cancelled));
+                    });
+                },
+                |res| async move {
+                    assert!(matches!(res, TaskResult::Completed(_)));
+                },
+            );
+
+            // Do something
+            smol::Timer::after(std::time::Duration::from_millis(50)).await;
+            group.cancel().await;
+        });
+    }
 }