Property initialize the schema for frozen Realms

How we initialize the schema results in RealmCoordinator's schema cache not
containing computed properties, so we need to call set_schema_subset() on
frozen Realms to populate them.

CHANGELOG.md

@@ -8,6 +8,9 @@ x.y.z Release notes (yyyy-MM-dd)
 * Freezing an object within the write transaction that the object was created
   in now throws an exception rather than crashing when the object is first
   used.
+* The schema for frozen Realms was not properly initialized, leading to crashes
+  when accessing a RLMLinkingObjects property.
+  ([#6568](https://github.com/realm/realm-cocoa/issues/6568), since 5.0.0).
 
 <!-- ### Breaking Changes - ONLY INCLUDE FOR NEW MAJOR version -->
 

Realm/RLMRealm.mm

@@ -1017,6 +1017,7 @@ - (RLMRealm *)frozenCopy {
     try {
         RLMRealm *realm = [[RLMRealm alloc] initPrivate];
         realm->_realm = _realm->freeze();
+        realm->_realm->set_schema_subset(_realm->schema());
         realm->_realm->read_group();
         realm->_dynamic = _dynamic;
         realm->_schema = _schema;

Realm/Tests/LinkingObjectsTests.mm

@@ -87,6 +87,39 @@ - (void)testLinkingObjectsOnUnmanagedObject {
                               @"Linking objects notifications are only supported on managed objects.");
 }
 
+- (void)testLinkingObjectsOnFrozenObject {
+    NSArray *(^asArray)(id) = ^(id arrayLike) {
+        return [arrayLike valueForKeyPath:@"self"];
+    };
+
+    RLMRealm *realm = [self realmWithTestPath];
+    [realm beginWriteTransaction];
+    PersonObject *hannah = [PersonObject createInRealm:realm withValue:@[@"Hannah", @0]];
+    PersonObject *mark   = [PersonObject createInRealm:realm withValue:@[@"Mark",  @30, @[hannah]]];
+    [realm commitWriteTransaction];
+
+    PersonObject *frozenHannah = hannah.freeze;
+    PersonObject *frozenMark = mark.freeze;
+    XCTAssertEqualObjects(asArray(frozenHannah.parents), (@[frozenMark]));
+
+    [realm beginWriteTransaction];
+    PersonObject *diane = [PersonObject createInRealm:realm withValue:@[@"Diane", @29, @[hannah]]];
+    [realm commitWriteTransaction];
+
+    PersonObject *frozenHannah2 = hannah.freeze;
+    PersonObject *frozenMark2 = mark.freeze;
+    PersonObject *frozenDiane = diane.freeze;
+    XCTAssertEqualObjects(asArray(frozenHannah.parents), (@[frozenMark]));
+    XCTAssertEqualObjects(asArray(frozenHannah2.parents), (@[frozenMark2, frozenDiane]));
+
+    [realm beginWriteTransaction];
+    [realm deleteObject:hannah];
+    [realm commitWriteTransaction];
+
+    XCTAssertEqualObjects(asArray(frozenHannah.parents), (@[frozenMark]));
+    XCTAssertEqualObjects(asArray(frozenHannah2.parents), (@[frozenMark2, frozenDiane]));
+}
+
 - (void)testFilteredLinkingObjects {
     NSArray *(^asArray)(id) = ^(id arrayLike) {
         return [arrayLike valueForKeyPath:@"self"];