Make it possible to disable the commit notifier background worker

This makes it much easier to write tests which test the work done on the
background thread.

src/impl/realm_coordinator.cpp

@@ -66,7 +66,7 @@ std::shared_ptr<Realm> RealmCoordinator::get_realm(Realm::Config config)
     std::lock_guard<std::mutex> lock(m_realm_mutex);
     if ((!m_config.read_only && !m_notifier) || (m_config.read_only && m_weak_realm_notifiers.empty())) {
         m_config = config;
-        if (!config.read_only && !m_notifier) {
+        if (!config.read_only && !m_notifier && config.automatic_change_notifications) {
             try {
                 m_notifier = std::make_unique<ExternalCommitHelper>(*this);
             }
@@ -202,7 +202,9 @@ void RealmCoordinator::clear_cache()
 void RealmCoordinator::send_commit_notifications()
 {
     REALM_ASSERT(!m_config.read_only);
-    m_notifier->notify_others();
+    if (m_notifier) {
+        m_notifier->notify_others();
+    }
 }
 
 void RealmCoordinator::pin_version(uint_fast64_t version, uint_fast32_t index)

src/shared_realm.cpp

@@ -35,13 +35,14 @@ using namespace realm::_impl;
 
 Realm::Config::Config(const Config& c)
 : path(c.path)
+, encryption_key(c.encryption_key)
+, schema_version(c.schema_version)
+, migration_function(c.migration_function)
 , read_only(c.read_only)
 , in_memory(c.in_memory)
 , cache(c.cache)
 , disable_format_upgrade(c.disable_format_upgrade)
-, encryption_key(c.encryption_key)
-, schema_version(c.schema_version)
-, migration_function(c.migration_function)
+, automatic_change_notifications(c.automatic_change_notifications)
 {
     if (c.schema) {
         schema = std::make_unique<Schema>(*c.schema);

src/shared_realm.hpp

@@ -47,20 +47,40 @@ namespace realm {
       public:
         typedef std::function<void(SharedRealm old_realm, SharedRealm realm)> MigrationFunction;
 
-        struct Config
-        {
+        struct Config {
             std::string path;
-            bool read_only = false;
-            bool in_memory = false;
-            bool cache = true;
-            bool disable_format_upgrade = false;
+            // User-supplied encryption key. Must be either empty or 64 bytes.
             std::vector<char> encryption_key;
 
+            // Optional schema for the file. If nullptr, the existing schema
+            // from the file opened will be used. If present, the file will be
+            // migrated to the schema if needed.
             std::unique_ptr<Schema> schema;
             uint64_t schema_version;
 
             MigrationFunction migration_function;
 
+            bool read_only = false;
+            bool in_memory = false;
+
+            // The following are intended for internal/testing purposes and
+            // should not be publically exposed in binding APIs
+
+            // If false, always return a new Realm instance, and don't return
+            // that Realm instance for other requests for a cached Realm. Useful
+            // for dynamic Realms and for tests that need multiple instances on
+            // one thread
+            bool cache = true;
+            // Throw an exception rather than automatically upgrading the file
+            // format. Used by the browser to warn the user that it'll modify
+            // the file.
+            bool disable_format_upgrade = false;
+            // Disable the background worker thread for producing change
+            // notifications. Useful for tests for those notifications so that
+            // everything can be done deterministically on one thread, and
+            // speeds up tests that don't need notifications.
+            bool automatic_change_notifications = true;
+
             Config();
             Config(Config&&);
             Config(const Config& c);