RCORE-1977 Empty reciprocal changesets lead to crashes or data diverg…

…ence (#7955)

* Fix bugs related to storing and retrieving reciprocal changesets

CHANGELOG.md

@@ -9,6 +9,8 @@
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Fixed an "invalid column key" exception when using a RQL "BETWEEN" query on an int or timestamp property across links. ([#7935](https://github.com/realm/realm-core/issues/7935), since v14.10.1)
+* Fixed conflict resolution bug related to ArrayErase and Clear instructions, which could sometimes cause an "Invalid prior_size" exception to prevent synchronization ([#7893](https://github.com/realm/realm-core/issues/7893), since v14.8.0).
+* Fixed bug which would prevent eventual consistency during conflict resolution. Affected clients would experience data divergence and potentially consistency errors as a result. ([PR #7955](https://github.com/realm/realm-core/pull/7955), since v14.8.0)
 
 ### Breaking changes
 * None.

src/realm/sync/noinst/client_history_impl.cpp

@@ -714,7 +714,7 @@ void ClientHistory::set_reciprocal_transform(version_type version, BinaryData da
     std::size_t index = size_t(version - m_sync_history_base_version) - 1;
     REALM_ASSERT(index < sync_history_size());
 
-    if (data.is_null()) {
+    if (data.size() == 0) {
         m_arrays->reciprocal_transforms.set(index, BinaryData{"", 0}); // Throws
         return;
     }

src/realm/sync/transform.cpp

@@ -2590,8 +2590,9 @@ size_t Transformer::transform_remote_changesets(TransformHistory& history, file_
 Changeset& Transformer::get_reciprocal_transform(TransformHistory& history, file_ident_type local_file_ident,
                                                  version_type version, const HistoryEntry& history_entry)
 {
-    auto& changeset = m_reciprocal_transform_cache[version]; // Throws
-    if (changeset.empty()) {
+    auto [it, success] = m_reciprocal_transform_cache.insert({version, Changeset{}}); // Throws
+    if (success) {
+        Changeset& changeset = it->second;
         bool is_compressed = false;
         ChunkedBinaryData data = history.get_reciprocal_transform(version, is_compressed);
         ChunkedBinaryInputStream in{data};
@@ -2613,7 +2614,7 @@ Changeset& Transformer::get_reciprocal_transform(TransformHistory& history, file
             origin_file_ident = local_file_ident;
         changeset.origin_file_ident = origin_file_ident;
     }
-    return changeset;
+    return it->second;
 }