refactor: allows retrieving a session ref from a weak-session

The previous implementation of `WeakSession` relied on a weak counter
 wrapped in a mutex. It was unnecessarily complex, as an atomic
 strong counter does the job better.

zenoh/src/api/session.rs

@@ -17,10 +17,11 @@ use std::{
     collections::HashMap,
     convert::TryInto,
     fmt,
+    mem::ManuallyDrop,
     ops::Deref,
     sync::{
-        atomic::{AtomicU16, Ordering},
-        Arc, Mutex, RwLock,
+        atomic::{AtomicU16, AtomicUsize, Ordering},
+        Arc, RwLock,
     },
     time::{Duration, SystemTime, UNIX_EPOCH},
 };
@@ -527,7 +528,7 @@ impl<T, S> Undeclarable<S> for T where T: UndeclarableSealed<S> {}
 
 pub(crate) struct SessionInner {
     /// See [`WeakSession`] doc
-    weak_counter: Mutex<usize>,
+    strong_counter: AtomicUsize,
     pub(crate) runtime: Runtime,
     pub(crate) state: RwLock<SessionState>,
     pub(crate) id: u16,
@@ -573,16 +574,14 @@ impl fmt::Debug for Session {
 
 impl Clone for Session {
     fn clone(&self) -> Self {
-        let _weak = self.0.weak_counter.lock().unwrap();
+        self.0.strong_counter.fetch_add(1, Ordering::Relaxed);
         Self(self.0.clone())
     }
 }
 
 impl Drop for Session {
     fn drop(&mut self) {
-        let weak = self.0.weak_counter.lock().unwrap();
-        if Arc::strong_count(&self.0) == *weak + /* the `Arc` currently dropped */ 1 {
-            drop(weak);
+        if self.0.strong_counter.fetch_sub(1, Ordering::Relaxed) == 1 {
             if let Err(error) = self.close().wait() {
                 tracing::error!(error)
             }
@@ -596,32 +595,17 @@ impl Drop for Session {
 /// When all `Session` instance are dropped, [`Session::close`] is be called and cleans
 /// the reference cycles, allowing the underlying `Arc` to be properly reclaimed.
 ///
-/// The pseudo-weak algorithm relies on a counter wrapped in a mutex. It was indeed the simplest
-/// to implement it, because atomic manipulations to achieve this semantic would not have been
-/// trivial at all — what could happen if a pseudo-weak is cloned while the last session instance
-/// is dropped? With a mutex, it's simple, and it works perfectly fine, as we don't care about the
-/// performance penalty when it comes to session entities cloning/dropping.
-///
 /// (Although it was planed to be used initially, `Weak` was in fact causing errors in the session
 /// closing, because the primitive implementation seemed to be used in the closing operation.)
+#[derive(Clone)]
 pub(crate) struct WeakSession(Arc<SessionInner>);
 
 impl WeakSession {
     fn new(session: &Arc<SessionInner>) -> Self {
-        let mut weak = session.weak_counter.lock().unwrap();
-        *weak += 1;
         Self(session.clone())
     }
 }
 
-impl Clone for WeakSession {
-    fn clone(&self) -> Self {
-        let mut weak = self.0.weak_counter.lock().unwrap();
-        *weak += 1;
-        Self(self.0.clone())
-    }
-}
-
 impl fmt::Debug for WeakSession {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         self.0.fmt(f)
@@ -636,13 +620,6 @@ impl Deref for WeakSession {
     }
 }
 
-impl Drop for WeakSession {
-    fn drop(&mut self) {
-        let mut weak = self.0.weak_counter.lock().unwrap();
-        *weak -= 1;
-    }
-}
-
 /// Error indicating the operation cannot proceed because the session is closed.
 ///
 /// It may be returned by operations like [`Session::get`] or [`Publisher::put`](crate::api::publisher::Publisher::put) when
@@ -677,7 +654,7 @@ impl Session {
                 publisher_qos.into(),
             ));
             let session = Session(Arc::new(SessionInner {
-                weak_counter: Mutex::new(0),
+                strong_counter: AtomicUsize::new(1),
                 runtime: runtime.clone(),
                 state,
                 id: SESSION_ID_COUNTER.fetch_add(1, Ordering::SeqCst),