[PATCH v21 5/9] win32u: Add a weak reference count to the shared session.

[Rémi Bernon]

From: Rémi Bernon

---
 dlls/win32u/winstation.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/dlls/win32u/winstation.c b/dlls/win32u/winstation.c
index e5fe36ff566..deb4cd0bf02 100644
--- a/dlls/win32u/winstation.c
+++ b/dlls/win32u/winstation.c
@@ -49,27 +49,45 @@ static struct shared_session *shared_session;
 struct shared_session
 {
     LONG ref;
+    LONG weak_ref;
     UINT64 id;
     SIZE_T size;
     UINT object_capacity;
     const session_shm_t *shared;
 };

+/* try acquiring a strong reference from a possibly weak referenced session */
 static struct shared_session *shared_session_acquire( struct shared_session *session )
 {
-    int ref = InterlockedIncrement( &session->ref );
-    TRACE( "session %p incrementing ref to %d\n", session, ref );
-    return session;
+    if (InterlockedOr( &session->ref, 0 ))
+    {
+        int ref = InterlockedIncrement( &session->ref );
+        TRACE( "session %p incrementing ref to %d\n", session, ref );
+        return session;
+    }
+
+    TRACE( "session %p failed to upgrade from weak ref\n", session );
+    return NULL;
+}
+
+/* release a weak reference on a session */
+static void shared_session_release_weak( struct shared_session *session )
+{
+    int ref = InterlockedDecrement( &session->weak_ref );
+    TRACE( "session %p decrementing weak_ref to %d\n", session, ref );
+    if (!ref) free( session );
 }

+/* release a strong reference on a session */
 static void shared_session_release( struct shared_session *session )
 {
     int ref = InterlockedDecrement( &session->ref );
     TRACE( "session %p decrementing ref to %d\n", session, ref );
     if (!ref)
     {
+        /* global shared_session has a strong reference, unmap when strong refcount reaches 0. */
         NtUnmapViewOfSection( GetCurrentProcess(), (void *)session->shared );
-        free( session );
+        shared_session_release_weak( session );
     }
 }

@@ -99,6 +117,7 @@ static NTSTATUS map_shared_session_section( struct shared_session *session, HAND
     return status;
 }

+/* return a strong reference on the last known valid session */
 static struct shared_session *get_shared_session( BOOL force )
 {
     struct shared_session *session;
@@ -123,6 +142,7 @@ static struct shared_session *get_shared_session( BOOL force )
             return NULL;
         }
         session->ref = 1;
+        session->weak_ref = 1;

InitializeObjectAttributes( &attr, &name, 0, NULL, NULL );
         if (!(status = NtOpenSection( &handle, SECTION_MAP_READ | SECTION_QUERY, &attr )))