Fix data races in the connection state changes tests

The assignment to token1 on the main thread and the read of it on the sync
worker thread didn't have any intervening synchronization and in theory the
callback could read it before it's actually written. Fixing this requires
adding some locking.

Conversely, listener2 doesn't actually need to be atomic since the second
callback should only ever be invokved synchronously inside log_out(), and if
it's called at some other time that's a bug.

It doesn't matter here, but `listener1_call_cnt = listener1_call_cnt + 1` is a
nonatomic increment that will drop updates if it happens on multiple threads at
once, while `++` will not.

CHANGELOG.md

@@ -17,7 +17,7 @@
 -----------
 
 ### Internals
-* None.
+* Fix a thread sanitizer failure in the "unregister connection change listener during callback" test ([PR #7871](https://github.com/realm/realm-core/pull/7871)).
 
 ----------------------------------------------
 

test/object-store/sync/session/connection_change_notifications.cpp

@@ -69,7 +69,7 @@ TEST_CASE("sync: Connection state changes", "[sync][session][connection change]"
 
         auto token1 = session->register_connection_change_callback(
             [&](SyncSession::ConnectionState, SyncSession::ConnectionState) {
-                listener1_call_cnt = listener1_call_cnt + 1;
+                ++listener1_call_cnt;
             });
         session->unregister_connection_change_callback(token1);
         // One call may have been in progress when unregistered
@@ -87,16 +87,18 @@ TEST_CASE("sync: Connection state changes", "[sync][session][connection change]"
     }
 
     SECTION("unregister connection change listener during callback") {
-        uint64_t token1;
-        std::atomic<int> listener1_call_cnt(0);
-        std::atomic<bool> listener2_called(false);
+        int listener1_call_cnt = 0;
         auto session = sync_session(
             user, "/connection-state-changes-3", [](auto, auto) {}, SyncSessionStopPolicy::AfterChangesUploaded);
-        token1 = session->register_connection_change_callback(
+        std::mutex mutex;
+        std::unique_lock lock(mutex);
+        uint64_t token1 = session->register_connection_change_callback(
             [&](SyncSession::ConnectionState, SyncSession::ConnectionState) {
-                listener1_call_cnt = listener1_call_cnt + 1;
+                std::lock_guard lock(mutex);
+                ++listener1_call_cnt;
                 session->unregister_connection_change_callback(token1);
             });
+        lock.unlock();
 
         EventLoop::main().run_until([&] {
             return sessions_are_active(*session);
@@ -105,6 +107,7 @@ TEST_CASE("sync: Connection state changes", "[sync][session][connection change]"
             return sessions_are_connected(*session);
         });
 
+        bool listener2_called = false;
         session->register_connection_change_callback([&](SyncSession::ConnectionState, SyncSession::ConnectionState) {
             listener2_called = true;
         });