YJIT: Deallocate `struct Block` to plug memory leaks

Previously we essentially never freed block even after invalidation.
Their reference count never reached zero for a couple of reasons:
1. `Branch::block` formed a cycle with the block holding the branch
2. Strong count on a branch that has ever contained a stub never
   reached 0 because we increment the `.clone()` call for
   `BranchRef::into_raw()` didn't have a matching decrement.

It's not safe to immediately deallocate blocks during
invalidation since `branch_stub_hit()` can end up
running with a branch pointer from an invalidated branch.
To plug the leaks, we wait until code GC or global invalidation and
deallocate the blocks for iseqs that are definitely not running.

1 files changed, 13 insertions, 0 deletions

diff --git a/bootstraptest/test_yjit.rb b/bootstraptest/test_yjit.rb
index 175f5bd86c..25085f0e8d 100644
--- a/bootstraptest/test_yjit.rb
+++ b/bootstraptest/test_yjit.rb
@@ -3423,3 +3423,16 @@ assert_equal '1', %q{
   bar { }
   bar { }
 }
+
+# test for return stub lifetime issue
+assert_equal '1', %q{
+  def foo(n)
+    if n == 2
+      return 1.times { Object.define_method(:foo) {} }
+    end
+
+    foo(n + 1)
+  end
+
+  foo(1)
+}