Prevent unloading methods used in root_fiber while calling another Fiber (#2939)

Fixing SEGVs like:
http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2744905
http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2744420
http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2741400
[Bug #16664]

(cherry picked from commit adcf0316d1ecedae2a9157ad941550e0c0fb510b)

1 files changed, 18 insertions, 3 deletions

diff --git a/cont.c b/cont.c
index 793bce018b..a8321abcca 100644
--- a/cont.c
+++ b/cont.c
@@ -1097,6 +1097,15 @@ cont_save_thread(rb_context_t *cont, rb_thread_t *th)
 }
 
 static void
+cont_init_mjit_cont(rb_context_t *cont)
+{
+    VM_ASSERT(cont->mjit_cont == NULL);
+    if (mjit_enabled) {
+        cont->mjit_cont = mjit_cont_new(&(cont->saved_ec));
+    }
+}
+
+static void
 cont_init(rb_context_t *cont, rb_thread_t *th)
 {
     /* save thread context */
@@ -1105,9 +1114,7 @@ cont_init(rb_context_t *cont, rb_thread_t *th)
     cont->saved_ec.local_storage = NULL;
     cont->saved_ec.local_storage_recursive_hash = Qnil;
     cont->saved_ec.local_storage_recursive_hash_for_trace = Qnil;
-    if (mjit_enabled) {
-        cont->mjit_cont = mjit_cont_new(&cont->saved_ec);
-    }
+    cont_init_mjit_cont(cont);
 }
 
 static rb_context_t *
@@ -1124,6 +1131,14 @@ cont_new(VALUE klass)
     return cont;
 }
 
+void
+rb_fiber_init_mjit_cont(struct rb_fiber_struct *fiber)
+{
+    // Currently this function is meant for root_fiber. Others go through cont_new.
+    // XXX: Is this mjit_cont `mjit_cont_free`d?
+    cont_init_mjit_cont(&fiber->cont);
+}
+
 #if 0
 void
 show_vm_stack(const rb_execution_context_t *ec)