Fix Relationship.instances cache.

This PR aims to fix several issues with Relationship cache:

1) It's not threadsafe, so I propose to use a TLS variable for this.
2) Memory obtained by cache remains non-freed before the next run of `serialize`. I think it should be freed immediately.
3) Memory should be freed in `ensure` block to prevent memory bloating in case of exception.

*There are only two hard things in Computer Science: cache invalidation and naming things.*

1 files changed, 8 insertions, 0 deletions

diff --git a/test/rels/tc_relationship.rb b/test/rels/tc_relationship.rb
index add1654f..845f789e 100644
--- a/test/rels/tc_relationship.rb
+++ b/test/rels/tc_relationship.rb
@@ -13,6 +13,14 @@ class TestRelationships < Test::Unit::TestCase
     instance = Axlsx::Relationship.new(source_obj, Axlsx::WORKSHEET_R, 'target')
     assert_equal instance.Id, Axlsx::Relationship.new(source_obj, Axlsx::WORKSHEET_R, 'target').Id
   end
+
+  def test_instances_cache_is_thread_safe
+    cache1, cache2 = nil
+    t1 = Thread.new { cache1 = Axlsx::Relationship.instances }
+    t2 = Thread.new { cache2 = Axlsx::Relationship.instances }
+    [t1, t2].each(&:join)
+    assert_not_same(cache1, cache2)
+  end
   
   def test_target_is_only_considered_for_same_attributes_check_if_target_mode_is_external
     source_obj = Object.new