perf: avoid holding async mutex guards across await points

tokio recommends using a sync mutex with a notifier instead of the
async mutex where possible. Rust forbids holding a sync mutex guard
across await points so we can prevent a potential deadlock this way.

This adds a custom channel based on the tokio mpmc example for
handling gvfs events from callbacks to avoid the async mutex
requirement. Messages are held in a `VecDeque` behind a sync mutex
and the receiver will get notified via the notifier when a message
is added to the queue.

Weak references used in gio callbacks in case the sender is dropped
by the application.

src/channel.rs

@@ -0,0 +1,77 @@
+// Copyright 2025 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+use std::{
+    collections::VecDeque,
+    sync::{
+        Arc, Mutex,
+        atomic::{AtomicBool, Ordering},
+    },
+};
+
+/// Create a channel backed by `tokio::sync::Notify` and a sync mutex with a vec deque.
+pub fn channel<Message>() -> (Sender<Message>, Receiver<Message>) {
+    let channel = Arc::new(Channel {
+        queue: Mutex::new(VecDeque::default()),
+        notify: tokio::sync::Notify::const_new(),
+        closed: AtomicBool::new(false),
+    });
+
+    (Sender(channel.clone()), Receiver(channel))
+}
+
+/// A channel backed by `tokio::sync::Notify` and a sync mutex with a vec deque.
+struct Channel<Message> {
+    pub(self) queue: Mutex<VecDeque<Message>>,
+    /// Set when a new message has been stored.
+    pub(self) notify: tokio::sync::Notify,
+    /// Set when the receiver is dropped.
+    pub(self) closed: AtomicBool,
+}
+
+pub struct Sender<Message>(Arc<Channel<Message>>);
+
+impl<Message> Sender<Message> {
+    pub fn send(&self, message: Message) {
+        self.0.queue.lock().unwrap().push_back(message);
+        self.0.notify.notify_one();
+    }
+}
+
+impl<Message> Drop for Sender<Message> {
+    fn drop(&mut self) {
+        self.0.closed.store(true, Ordering::SeqCst);
+        self.0.notify.notify_one();
+    }
+}
+
+pub struct Receiver<Message>(Arc<Channel<Message>>);
+
+impl<Message> Receiver<Message> {
+    /// Returns a value until the sender is dropped.
+    pub async fn recv(&self) -> Option<Message> {
+        loop {
+            {
+                let mut queue = self.0.queue.lock().unwrap();
+                if let Some(value) = queue.pop_front() {
+                    if queue.capacity() - queue.len() > 32 {
+                        let capacity = queue.len().next_power_of_two();
+                        queue.shrink_to(capacity);
+                    }
+                    drop(queue);
+                    return Some(value);
+                }
+            }
+
+            if self.0.closed.load(Ordering::SeqCst) {
+                return None;
+            }
+
+            self.0.notify.notified().await;
+        }
+    }
+
+    pub fn try_recv(&self) -> Option<Message> {
+        self.0.queue.lock().unwrap().pop_front()
+    }
+}