Fix corruption of freelist during compaction

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Fixed wrong assertion on query error that could result in a crash. ([#6038](https://github.com/realm/realm-core/issues/6038), since v11.7.0)
+* Freelist may be corrupted if compaction was initiated ([#6054](https://github.com/realm/realm-core/pull/6054), since v13.0.0)
 
 ### Breaking changes
 * Updated `logger_factory` in SyncClientConfig to return a `shared_ptr` instead of a `unique_ptr` ([PR #5980](https://github.com/realm/realm-core/pull/5980))

src/realm/group_writer.cpp

@@ -766,6 +766,26 @@ ref_type GroupWriter::write_group()
     top.set(Group::s_free_version_ndx, from_ref(free_versions_ref));            // Throws
     top.set(Group::s_version_ndx, RefOrTagged::make_tagged(m_current_version)); // Throws
 
+    if (m_evacuation_limit == 0 && m_backoff == 0) {
+        size_t used_space = m_logical_size - m_free_space_size;
+        // Compacting files smaller than 1 Mb is not worth the effort. Arbitrary chosen value.
+        static constexpr size_t minimal_compaction_size = 0x100000;
+        // If we make the file too small, there is a big chance it will grow immediately afterwards
+        static constexpr size_t minimal_evac_limit = 0x10000;
+        if (m_logical_size >= minimal_compaction_size && m_free_space_size - m_locked_space_size > 2 * used_space) {
+            // Clean up potential
+            auto limit = util::round_up_to_page_size(used_space + used_space / 2 + m_locked_space_size);
+            m_evacuation_limit = std::max(minimal_evac_limit, limit);
+            // From now on, we will only allocate below this limit
+            // Save the limit in the file
+            while (top.size() <= Group::s_evacuation_point_ndx) {
+                top.add(0);
+            }
+            top.set(Group::s_evacuation_point_ndx, RefOrTagged::make_tagged(m_evacuation_limit));
+            // std::cout << "Evacuation point = " << std::hex << m_evacuation_limit << std::dec << std::endl;
+        }
+    }
+
     // Get final sizes
     size_t top_byte_size = top.get_byte_size();
     ref_type end_ref = top_ref + top_byte_size;
@@ -791,26 +811,6 @@ ref_type GroupWriter::write_group()
     m_free_lengths.set(reserve_ndx, value_9);   // Throws
     m_free_space_size += rest;
 
-    if (m_evacuation_limit == 0 && m_backoff == 0) {
-        size_t used_space = m_logical_size - m_free_space_size;
-        // Compacting files smaller than 1 Mb is not worth the effort. Arbitrary chosen value.
-        static constexpr size_t minimal_compaction_size = 0x100000;
-        // If we make the file too small, there is a big chance it will grow immediately afterwards
-        static constexpr size_t minimal_evac_limit = 0x10000;
-        if (m_logical_size >= minimal_compaction_size && m_free_space_size - m_locked_space_size > 2 * used_space) {
-            // Clean up potential
-            auto limit = util::round_up_to_page_size(used_space + used_space / 2 + m_locked_space_size);
-            m_evacuation_limit = std::max(minimal_evac_limit, limit);
-            // From now on, we will only allocate below this limit
-            // Save the limit in the file
-            while (top.size() <= Group::s_evacuation_point_ndx) {
-                top.add(0);
-            }
-            top.set(Group::s_evacuation_point_ndx, RefOrTagged::make_tagged(m_evacuation_limit));
-            // std::cout << "Evacuation point = " << std::hex << m_evacuation_limit << std::dec << std::endl;
-        }
-    }
-
 #if REALM_ALLOC_DEBUG
     std::cout << "  Final Freelist:" << std::endl;
     for (size_t j = 0; j < m_free_positions.size(); ++j) {

test/test_compaction.cpp

@@ -59,11 +59,11 @@ TEST(Compaction_WhileGrowing)
     for (int i = 0; i < 5000; ++i) {
         w[i] = '0' + (i % 64);
     }
-    int num = (REALM_MAX_BPNODE_SIZE == 1000) ? 1490 : 1400;
+    int num = (REALM_MAX_BPNODE_SIZE == 1000) ? 1400 : 1300;
     tr->promote_to_write();
     CHECK(db->get_evacuation_stage() == DB::EvacStage::idle);
     for (int j = 0; j < num; ++j) {
-        table1->create_object().set(col_bin1, BinaryData(w, 400));
+        table1->create_object().set(col_bin1, BinaryData(w, 450));
         table2->create_object().set(col_bin2, BinaryData(w, 200));
         if (j % 10 == 0) {
             tr->commit_and_continue_as_read();
@@ -113,7 +113,7 @@ TEST(Compaction_WhileGrowing)
     tr->commit_and_continue_as_read();
     // Now there should be room for compaction
 
-    auto n = 20; // Ensure that test will end
+    auto n = 50; // Ensure that test will end
     do {
         tr->promote_to_write();
         tr->commit_and_continue_as_read();