Check if there are any new versions before refreshing in SubscriptionSet

CHANGELOG.md

@@ -4,12 +4,11 @@
 * <New feature description> (PR [#????](https://github.com/realm/realm-core/pull/????))
 * Allow multiple anonymous sessions. ([PR #5693](https://github.com/realm/realm-core/pull/5693)).
 * Introducing query parser support for constant list expressions such as `fruit IN {'apple', 'orange'}`. This also includes general query support for list vs list matching such as `NONE fruits IN {'apple', 'orange'}`. ([Issue #4266](https://github.com/realm/realm-core/issues/4266))
+* SubscriptionSet::refresh() does less work if no commits have been made since the last call to refresh(). ([PR #5695](https://github.com/realm/realm-core/pull/5695))
 
 ### Fixed
 * Fix error message when validating outgoing links from asymmetric objects to non-embedded objects. ([PR #5702](https://github.com/realm/realm-core/pull/5702))
 
-
-
 ### Breaking changes
 * None.
 

src/realm/sync/subscriptions.cpp

@@ -364,11 +364,9 @@ MutableSubscriptionSet SubscriptionSet::make_mutable_copy() const
 void SubscriptionSet::refresh()
 {
     auto mgr = get_flx_subscription_store(); // Throws
-    auto refreshed_self = mgr->get_by_version(version());
-    m_state = refreshed_self.m_state;
-    m_error_str = refreshed_self.m_error_str;
-    m_cur_version = refreshed_self.m_cur_version;
-    *this = mgr->get_by_version(version());
+    if (mgr->would_refresh(m_cur_version)) {
+        *this = mgr->get_by_version(version());
+    }
 }
 
 util::Future<SubscriptionSet::State> SubscriptionSet::get_state_change_notification(State notify_when) const
@@ -832,4 +830,9 @@ MutableSubscriptionSet SubscriptionStore::make_mutable_copy(const SubscriptionSe
     return new_set_obj;
 }
 
+bool SubscriptionStore::would_refresh(DB::version_type version) const noexcept
+{
+    return version < m_db->get_version_of_latest_snapshot();
+}
+
 } // namespace realm::sync

src/realm/sync/subscriptions.hpp

@@ -328,6 +328,9 @@ class SubscriptionStore : public std::enable_shared_from_this<SubscriptionStore>
     // affect the mutable subscription identified by the parameter.
     void supercede_all_except(MutableSubscriptionSet& mut_sub) const;
 
+    // Returns true if there have been commits to the DB since the given version
+    bool would_refresh(DB::version_type version) const noexcept;
+
     using TableSet = std::set<std::string, std::less<>>;
     TableSet get_tables_for_latest(const Transaction& tr) const;