Fix reattaching the allocator after compact

CHANGELOG.md

@@ -6,6 +6,7 @@
 ### Fixed
 * <How to hit and notice issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Fix upgrade bug. Could cause assertions like "Assertion failed: ref != 0" during opning of a realm. ([#6644](https://github.com/realm/realm-cocoa/issues/6644), since V6.0.7)
+* A use-after-free would occur if a Realm was compacted, opened on multiple threads prior to the first write, then written to while reads were happening on other threads. This could result in a variety of crashes, often inside realm::util::EncryptedFileMapping::read_barrier. (Since v6.0.0, [Cocoa #6652](https://github.com/realm/realm-cocoa/issues/6652), [Cocoa #6628](https://github.com/realm/realm-cocoa/issues/6628), [Cocoa #6655](https://github.com/realm/realm-cocoa/issues/6555)).
 
 ### Breaking changes
 * None.

src/realm/db.cpp

@@ -1453,6 +1453,7 @@ bool DB::compact(bool bump_version_number, util::Optional<const char*> output_en
         cfg.encryption_key = write_key;
         ref_type top_ref;
         top_ref = m_alloc.attach_file(m_db_path, cfg);
+        m_alloc.m_youngest_live_version = info->latest_version_number;
         info->number_of_versions = 1;
         // info->latest_version_number is unchanged
         SharedInfo* r_info = m_reader_map.get_addr();

test/test_shared.cpp

@@ -499,6 +499,31 @@ TEST(Shared_CompactingOnTheFly)
 }
 
 
+TEST(Shared_ReadAfterCompact)
+{
+    SHARED_GROUP_TEST_PATH(path);
+    DBRef sg = DB::create(path);
+    {
+        WriteTransaction wt(sg);
+        auto table = wt.add_table("table");
+        table->add_column(type_Int, "col");
+        table->create_object().set_all(1);
+        wt.commit();
+    }
+    sg->compact();
+    auto rt = sg->start_read();
+    auto table = rt->get_table("table");
+    for (int i = 2; i < 4; ++i) {
+        WriteTransaction wt(sg);
+        wt.get_table("table")->create_object().set_all(i);
+        wt.commit();
+    }
+
+    CHECK_EQUAL(table->size(), 1);
+    CHECK_EQUAL(table->get_object(0).get<int64_t>("col"), 1);
+}
+
+
 TEST(Shared_EncryptedRemap)
 {
     // Attempts to trigger code coverage in util::mremap() for the case where the file is encrypted.