do not deadlock due to all paths being in standby

quiche/src/lib.rs

@@ -3634,6 +3634,12 @@ impl Connection {
         let multipath_enabled = self.paths.multipath();
         let paths = &mut self.paths;
 
+        // Avoid being deadlocked if all available paths are standby.
+        if paths.all_available_paths_standby() {
+            // Force the availability of this path.
+            paths.set_path_status(send_pid, PathStatus::Available)?;
+        }
+
         let mut left = b.cap();
 
         let path = paths.get_mut(send_pid)?;

quiche/src/path.rs

@@ -1209,6 +1209,11 @@ impl PathMap {
         self.path_status_to_advertise.pop_front();
     }
 
+    /// Are all paths in standby state?
+    pub fn all_available_paths_standby(&self) -> bool {
+        !self.paths.iter().all(|p| p.1.active() && !p.1.is_standby())
+    }
+
     /// Handles the reception of PATH_STANDBY/PATH_AVAILABLE.
     pub fn on_path_status_received(
         &mut self, path_id: PathId, seq_num: u64, available: bool,