Fix various autoload race conditions. (#5898)

* Add RUBY_VM_CRITICAL_SECTION for detecting unexpected context switch.

* Prevent race between GC mark and autoload setup.

* Protect race on autoload state.

* Avoid potential race condition when allocating `autoload_featuremap`.

* Add NEWS entry for autoload fixes.

1 files changed, 4 insertions, 0 deletions

diff --git a/vm.c b/vm.c
index 8b172b5a39..c0309c7f61 100644
--- a/vm.c
+++ b/vm.c
@@ -48,6 +48,10 @@
 #endif
 #include "probes_helper.h"
 
+#ifdef RUBY_VM_CRITICAL_SECTION
+int rb_vm_critical_section_entered = 0;
+#endif
+
 VALUE rb_str_concat_literals(size_t, const VALUE*);
 
 /* :FIXME: This #ifdef is because we build pch in case of mswin and