Do not capture this in on_new_flx_subscription_set callback

CHANGELOG.md

@@ -15,6 +15,7 @@
 * Compare actual users (`SyncUser::operator!=`), not pointers (`shared_ptr<SyncUser>::operator!=`). ([#realm/realm-dart#1055](https://github.com/realm/realm-dart/issues/1055), since v10.2.0)
 * Core should not alter the order of the properties for additive schemas. ([#6134](https://github.com/realm/realm-core/issues/6134))
 * Core should not use the version passed in `Realm` constructor to load the schema at some particular version, but only for setting `m_frozen_version`.
+* Fixed possible segfault in sync client where async callback was not extending lifetime of `this` ([#6053](https://github.com/realm/realm-core/issues/6053), since v11.7.0)
 
 ### Breaking changes
 * Core no longer provides any vcpkg infrastructure (the ports submodule and overlay triplets), because it handles dependant libraries internally now.

src/realm/sync/client.cpp

@@ -1058,20 +1058,20 @@ void SessionWrapper::on_new_flx_subscription_set(int64_t new_version)
         return;
     }
 
-    m_client.post([new_version, this](Status status) {
+    auto self = util::bind_ptr<SessionWrapper>(this);
+    m_client.get_service().post([new_version, self = std::move(self)](Status status) {
         if (status == ErrorCodes::OperationAborted)
             return;
         else if (!status.is_ok())
             throw ExceptionForStatus(status);
-
-        REALM_ASSERT(m_actualized);
-        if (REALM_UNLIKELY(!m_sess)) {
+        REALM_ASSERT(self->m_actualized);
+        if (REALM_UNLIKELY(!self->m_sess)) {
             return; // Already finalized
         }
-
-        m_sess->recognize_sync_version(m_db->get_version_of_latest_snapshot());
-        m_flx_latest_version = new_version;
-        m_sess->on_new_flx_subscription_set(new_version);
+        auto& sess = *self->m_sess;
+        sess.recognize_sync_version(self->m_db->get_version_of_latest_snapshot());
+        self->m_flx_latest_version = new_version;
+        sess.on_new_flx_subscription_set(new_version);
     });
 }