Addition of SessionSharedPtr

src/transport/BUILD.gn

@@ -47,6 +47,7 @@ static_library("transport") {
     "SessionManager.h",
     "SessionMessageCounter.h",
     "SessionMessageDelegate.h",
+    "SessionSharedPtr.h",
     "TransportMgr.h",
     "TransportMgrBase.cpp",
     "TransportMgrBase.h",

src/transport/SessionHandle.h

@@ -18,7 +18,9 @@
 #pragma once
 
 #include <access/SubjectDescriptor.h>
+#include <lib/core/Optional.h>
 #include <lib/support/ReferenceCountedHandle.h>
+#include <transport/SessionSharedPtr.h>
 
 namespace chip {
 
@@ -29,27 +31,29 @@ class Session;
 class SessionHolder;
 
 /** @brief
- *    Non-copyable session reference. All SessionHandles are created within SessionManager. SessionHandle is not
- *    reference *counted, hence it is not allowed to store SessionHandle anywhere except for function arguments and
- *    return values. SessionHandle is short-lived as it is only available as stack variable, so it is never dangling. */
-class SessionHandle
+ *    A non-copyable version of SessionSharedPtr that is always guaranteed to point to a valid Session. This is always
+ *    created within SessionManager.
+ *
+ *    Since this is a strong reference, this should be short-lived and only used as a stack variable.
+ */
+class SessionHandle : protected SessionSharedPtr
 {
 public:
-    SessionHandle(Transport::Session & session) : mSession(session) {}
+    SessionHandle(Transport::Session & session) : SessionSharedPtr(session) {}
     ~SessionHandle() {}
 
     SessionHandle(const SessionHandle &) = delete;
     SessionHandle operator=(const SessionHandle &) = delete;
     SessionHandle(SessionHandle &&)                = default;
     SessionHandle & operator=(SessionHandle &&) = delete;
 
-    bool operator==(const SessionHandle & that) const { return &mSession.Get() == &that.mSession.Get(); }
+    bool operator==(const SessionHandle & that) const { return Get() == that.Get(); }
 
-    Transport::Session * operator->() const { return mSession.operator->(); }
+    Transport::Session * operator->() const { return SessionSharedPtr::operator->(); }
 
 private:
+    // TODO: Remove this once SessionHolder pivots to truly being a weak reference (#18399).
     friend class SessionHolder;
-    ReferenceCountedHandle<Transport::Session> mSession;
 };
 
 } // namespace chip

src/transport/SessionHolder.cpp

@@ -75,7 +75,7 @@ SessionHolder & SessionHolder::operator=(SessionHolder && that)
 void SessionHolder::Grab(const SessionHandle & session)
 {
     Release();
-    mSession.Emplace(session.mSession);
+    mSession.Emplace(session.mSession.Value());
     session->AddHolder(*this);
 }
 

src/transport/SessionHolder.h

@@ -44,7 +44,8 @@ class SessionHolder : public SessionDelegate, public IntrusiveListNodeBase
 
     bool Contains(const SessionHandle & session) const
     {
-        return mSession.HasValue() && &mSession.Value().Get() == &session.mSession.Get();
+        auto ourSession = ToOptional();
+        return ourSession.HasValue() && ourSession.Value() == session;
     }
 
     void Grab(const SessionHandle & session);

src/transport/SessionSharedPtr.h

@@ -0,0 +1,105 @@
+/*
+ *
+ *    Copyright (c) 2021 Project CHIP Authors
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#pragma once
+
+#include <access/SubjectDescriptor.h>
+#include <lib/core/Optional.h>
+#include <lib/support/ReferenceCountedHandle.h>
+
+namespace chip {
+
+namespace Transport {
+class Session;
+} // namespace Transport
+
+class SessionHolder;
+
+/** @brief
+ *    A shared_ptr like smart pointer to manage the lifetime of a Session object. Like a shared_ptr,
+ *    this object can start out not tracking any Session and be attached to a Session there-after.
+ *    The underlying Session is guaranteed to remain active and resident until all references to it from SessionSharedPtr
+ *    instances in the system have gone away, at which point it invokes its custom destructor.
+ *
+ *    Just because a Session is refcounted does not mean it actually gets destroyed upon reaching a count of 0.
+ *    UnauthenticatedSession and SecureSession have different logic that gets invoked when the count hits 0.
+ *
+ *    This should really only be used during session setup by the entity setting up the session.
+ *    Once setup, the session should transfer ownership to the SessionManager at which point,
+ *    all clients in the system should only be holding SessionWeakPtrs (SessionWeakPtr doesn't exist yet, but once
+ *    #18399 is complete, SessionHolder will become SessionWeakPtr).
+ *
+ *    This is copy-constructible.
+ */
+class SessionSharedPtr
+{
+public:
+    SessionSharedPtr() {}
+    SessionSharedPtr(Transport::Session & session) { mSession.Emplace(session); }
+
+    SessionSharedPtr(const SessionSharedPtr & session) { Grab(session); }
+
+    /*
+     * Add ourselves as a shared owner on the passed in session (if it points to
+     * a valid session). The passed in
+     * session object can now be free'ed safely without impacting the underlying
+     * Session.
+     */
+    void operator=(const SessionSharedPtr & session) { Grab(session); }
+
+    /*
+     * If we're currently pointing to a valid session, remove ourselves
+     * as a shared owner of that session. If there are no more shared owners
+     * on that session, that session MAY be reclaimed.
+     */
+    void Release() { mSession.ClearValue(); }
+
+    /*
+     * Assume shared ownership of the provided session.
+     */
+    void Grab(Transport::Session & session) { mSession.Emplace(session); }
+
+    Transport::Session * operator->() const { return Get(); }
+
+protected:
+    Transport::Session * Get() const
+    {
+        if (mSession.HasValue())
+        {
+            return &mSession.Value().Get();
+        }
+
+        return nullptr;
+    }
+
+private:
+    void Grab(const SessionSharedPtr & session)
+    {
+        auto * underlyingSession = session.Get();
+        if (underlyingSession)
+        {
+            mSession.Emplace(*underlyingSession);
+        }
+    }
+
+    Optional<ReferenceCountedHandle<Transport::Session>> mSession;
+
+    // TODO: Remove this once SessionHolder pivots to truly being a weak reference (#18399).
+    friend class SessionHolder;
+};
+
+} // namespace chip