Ensure scheduler is not deleted until callback has been done

This is fixing a problem introduced in 14210d1

The issue was discovered in realm-js, where async open would
hang because the scheduler was deleted before the callback was done on the
main thread. Beforehand the EventLoopDispatcher has a State member that
would retain the scheduler until the invocations were done.

CHANGELOG.md

@@ -10,6 +10,7 @@
 * Wrong error code returned from C-API on MacOS ([#5233](https://github.com/realm/realm-core/issues/5233), since v10.0.0)
 * Mixed::compare() used inconsistent rounding for comparing a Decimal128 to a float, giving different results from comparing those values directly ([#5270](https://github.com/realm/realm-core/pull/5270)).
 * Calling Realm::async_begin_transaction() from within a write transaction while the async state was idle would hit an assertion failure (since v11.10.0).
+* Fix issue with scheduler being deleted on wrong thread. This caused async open to hang in eg. realm-js. ([#5287](https://github.com/realm/realm-core/pull/5287), Since v11.10.0)
 
 ### Breaking changes
 * None.

src/realm/object-store/util/event_loop_dispatcher.hpp

@@ -54,9 +54,12 @@ class EventLoopDispatcher<void(Args...)> {
             (*m_func)(std::forward<Args>(args)...);
             return;
         }
-        m_scheduler->invoke([func = m_func, args = std::make_tuple(std::forward<Args>(args)...)]() mutable {
-            std::apply(*func, std::move(args));
-        });
+        m_scheduler->invoke(
+            [scheduler = m_scheduler, func = m_func, args = std::make_tuple(std::forward<Args>(args)...)]() mutable {
+                std::apply(*func, std::move(args));
+                // Each invocation block will retain the scheduler, so the scheduler
+                // will not be released until all blocks are called
+            });
     }
 };
 

test/object-store/realm.cpp

@@ -33,6 +33,7 @@
 #include <realm/object-store/schema.hpp>
 #include <realm/object-store/thread_safe_reference.hpp>
 #include <realm/object-store/util/scheduler.hpp>
+#include <realm/object-store/util/event_loop_dispatcher.hpp>
 
 #if REALM_ENABLE_SYNC
 #include <realm/object-store/sync/async_open_task.hpp>
@@ -616,16 +617,15 @@ TEST_CASE("Get Realm using Async Open", "[asyncOpen]") {
     std::mutex mutex;
     SECTION("can open synced Realms that don't already exist") {
         ThreadSafeReference realm_ref;
-        std::atomic<bool> called{false};
+        bool called = false;
         auto task = Realm::get_synchronized_realm(config);
-        task->start([&](auto ref, auto error) {
-            std::lock_guard<std::mutex> lock(mutex);
+        task->start(realm::util::EventLoopDispatcher([&](ThreadSafeReference&& ref, std::exception_ptr error) {
             REQUIRE(!error);
             called = true;
             realm_ref = std::move(ref);
-        });
+        }));
         util::EventLoop::main().run_until([&] {
-            return called.load();
+            return called;
         });
         std::lock_guard<std::mutex> lock(mutex);
         REQUIRE(called);