Un-reserve the new session immediately (project-chip#180)

* Un-reserve the new session immediately

* Address review feedback

rs-matter/src/secure_channel/case.rs

@@ -182,6 +182,13 @@ impl Case {
                         Some(&session_keys[32..48]),
                     )?;
 
+                    // Complete the reserved session and thus make the `Session` instance
+                    // immediately available for use by the system.
+                    //
+                    // We need to do this _before_ we send the response to the peer, or else we risk missing
+                    // (dropping) the first messages the peer would send us on the newly-established session,
+                    // as it might start using it right after it receives the response, while it is still marked
+                    // as reserved.
                     session.complete();
 
                     SCStatusCodes::SessionEstablishmentSuccess

rs-matter/src/secure_channel/pake.rs

@@ -224,6 +224,14 @@ impl Pake {
                     .pase_mgr
                     .borrow_mut()
                     .disable_pase_session(mdns)?;
+
+                // Complete the reserved session and thus make the `Session` instance
+                // immediately available for use by the system.
+                //
+                // We need to do this _before_ we send the response to the peer, or else we risk missing
+                // (dropping) the first messages the peer would send us on the newly-established session,
+                // as it might start using it right after it receives the response, while it is still marked
+                // as reserved.
                 session.complete();
 
                 SCStatusCodes::SessionEstablishmentSuccess

rs-matter/src/transport/session.rs

@@ -497,7 +497,7 @@ impl<'a> ReservedSession<'a> {
         Ok(())
     }
 
-    pub fn complete(&mut self) {
+    pub fn complete(mut self) {
         self.complete = true;
     }
 }