1 files changed, 66 insertions, 0 deletions
diff --git a/xpcom/rust/moz_task/src/event_loop.rs b/xpcom/rust/moz_task/src/event_loop.rs
new file mode 100644
index 0000000000..f8d113ed57
--- /dev/null
+++ b/xpcom/rust/moz_task/src/event_loop.rs
@@ -0,0 +1,66 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+extern crate nsstring;
+
+use cstr::cstr;
+use nserror::{nsresult, NS_ERROR_SERVICE_NOT_AVAILABLE, NS_ERROR_UNEXPECTED, NS_OK};
+use nsstring::*;
+use std::cell::RefCell;
+use std::future::Future;
+use xpcom::{interfaces::nsIThreadManager, xpcom, xpcom_method};
+
+#[xpcom(implement(nsINestedEventLoopCondition), nonatomic)]
+struct FutureCompleteCondition<T: 'static> {
+    value: RefCell<Option<T>>,
+}
+
+impl<T: 'static> FutureCompleteCondition<T> {
+    xpcom_method!(is_done => IsDone() -> bool);
+    fn is_done(&self) -> Result<bool, nsresult> {
+        Ok(self.value.borrow().is_some())
+    }
+}
+
+/// Spin the event loop on the current thread until `future` is resolved.
+///
+/// # Safety
+///
+/// Spinning a nested event loop should always be avoided when possible, as it
+/// can cause hangs, break JS run-to-completion guarantees, and break other C++
+/// code currently on the stack relying on heap invariants. While in a pure-rust
+/// codebase this method would only be ill-advised and not technically "unsafe",
+/// it is marked as unsafe due to the potential for triggering unsafety in
+/// unrelated C++ code.
+pub unsafe fn spin_event_loop_until<F>(
+    reason: &'static str,
+    future: F,
+) -> Result<F::Output, nsresult>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
+    let thread_manager =
+        xpcom::get_service::<nsIThreadManager>(cstr!("@mozilla.org/thread-manager;1"))
+            .ok_or(NS_ERROR_SERVICE_NOT_AVAILABLE)?;
+
+    let cond = FutureCompleteCondition::<F::Output>::allocate(InitFutureCompleteCondition {
+        value: RefCell::new(None),
+    });
+
+    // Spawn our future onto the current thread event loop, and record the
+    // completed value as it completes.
+    let cond2 = cond.clone();
+    crate::spawn_local(reason, async move {
+        let rv = future.await;
+        *cond2.value.borrow_mut() = Some(rv);
+    })
+    .detach();
+
+    thread_manager
+        .SpinEventLoopUntil(&*nsCStr::from(reason), cond.coerce())
+        .to_result()?;
+    let rv = cond.value.borrow_mut().take();
+    rv.ok_or(NS_ERROR_UNEXPECTED)
+}