add session closing callback API

Cargo.toml

@@ -163,6 +163,7 @@ serde = { version = "1.0.210", default-features = false, features = [
 serde_json = "1.0.128"
 serde_with = "3.12.0"
 serde_yaml = "0.9.34"
+slab = "0.4.10"
 static_init = "1.0.3"
 stabby = "36.1.1"
 sha3 = "0.10.8"

zenoh/Cargo.toml

@@ -101,6 +101,7 @@ rand = { workspace = true, features = ["default"] }
 ref-cast = { workspace = true }
 serde = { workspace = true, features = ["default"] }
 serde_json = { workspace = true }
+slab = { workspace = true }
 socket2 = { workspace = true }
 uhlc = { workspace = true, features = ["default"] }
 vec_map = { workspace = true }

zenoh/src/api/session.rs

@@ -156,6 +156,7 @@
     pub(crate) aggregated_subscribers: Vec<OwnedKeyExpr>,
     pub(crate) aggregated_publishers: Vec<OwnedKeyExpr>,
     pub(crate) publisher_qos_tree: KeBoxTree<PublisherQoSConfig>,
+    pub(crate) closing_callbacks: ClosingCallbackList,
 }
 
 impl SessionState {
@@ -190,6 +191,7 @@
             aggregated_subscribers,
             aggregated_publishers,
             publisher_qos_tree,
+            closing_callbacks: Default::default(),
         }
     }
 }
@@ -1249,6 +1251,63 @@
             source_info: SourceInfo::empty(),
         }
     }
+
+    /// Registers a closing callback to the session.
+    ///
+    /// The callback will be called when the session will be closed. It returns an id to unregister
+    /// the callback with [`unregister_closing_callback`](Self::unregister_closing_callback). If
+    /// the session is already closed, the callback is returned in an error.
+    ///
+    /// Execution order of callbacks is unspecified.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///
+    /// let session = zenoh::open(zenoh::Config::default()).await.unwrap();
+    /// if let Err(_) = session.register_closing_callback(|| println!("session closed")) {
+    ///     println!("session already closed");
+    /// }
+    /// # }
+    /// ```
+    #[zenoh_macros::internal]
+    pub fn register_closing_callback<F: FnOnce() + Send + Sync + 'static>(
+        &self,
+        callback: F,
+    ) -> Result<ClosingCallbackId, F> {
+        let mut state = zwrite!(self.0.state);
+        if state.primitives().is_err() {
+            return Err(callback);
+        }
+        Ok(state.closing_callbacks.insert_callback(Box::new(callback)))
+    }
+
+    /// Unregisters a closing callback.
+    ///
+    /// The callback must have been registered with
+    /// [`register_closing_callback`](Self::register_closing_callback).
+    /// It will no longer be called on session closing.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # #[tokio::main]
+    /// # async fn main() {
+    ///
+    /// let session = zenoh::open(zenoh::Config::default()).await.unwrap();
+    /// let Ok(id) = session.register_closing_callback(|| println!("session closed")) else {
+    ///     panic!("session already closed");
+    /// };
+    /// session.unregister_closing_callback(id);
+    /// # }
+    /// ```
+    #[zenoh_macros::internal]
+    pub fn unregister_closing_callback(&self, callback_id: ClosingCallbackId) {
+        let mut state = zwrite!(self.0.state);
+        state.closing_callbacks.remove_callback(callback_id);
+    }
 }
 
 impl Session {
@@ -3213,6 +3272,7 @@
             // will be stabilized.
             let mut state = zwrite!(self.state);
             let _matching_listeners = std::mem::take(&mut state.matching_listeners);
+            let _closing_callbacks = std::mem::take(&mut state.closing_callbacks);
             drop(state);
         }
     }
@@ -3225,3 +3285,49 @@
         self.0.clone()
     }
 }
+
+#[derive(Default)]
+pub(crate) struct ClosingCallbackList {
+    callbacks: slab::Slab<ClosingCallback>,
+    generation: usize,
+}
+
+impl ClosingCallbackList {
+    fn insert_callback(&mut self, callback: Box<dyn FnOnce() + Send + Sync>) -> ClosingCallbackId {
+        let generation = self.generation;
+        let index = self.callbacks.insert(ClosingCallback {
+            callback,
+            generation,
+        });
+        self.generation = self.generation.wrapping_add(1);
+        ClosingCallbackId { index, generation }
+    }
+
+    fn remove_callback(&mut self, id: ClosingCallbackId) {
+        if matches!( self.callbacks.get(id.index), Some(cb) if cb.generation == id.generation) {
+            self.callbacks.remove(id.index);
+        }
+    }
+}
+
+impl Drop for ClosingCallbackList {
+    fn drop(&mut self) {
+        for cb in self.callbacks.drain() {
+            (cb.callback)();
+        }
+    }
+}
+
+struct ClosingCallback {
+    callback: Box<dyn FnOnce() + Send + Sync>,
+    generation: usize,
+}
+
+/// The id of a registered session closing callback.
+///
+/// See [`Session::register_closing_callback`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ClosingCallbackId {
+    index: usize,
+    generation: usize,
+}

zenoh/src/lib.rs

@@ -208,6 +208,8 @@ pub mod session {
 
     #[zenoh_macros::internal]
     pub use crate::api::builders::session::{init, InitBuilder};
+    #[zenoh_macros::internal]
+    pub use crate::api::session::ClosingCallbackId;
     pub use crate::api::{
         builders::{
             close::CloseBuilder,