prevent GC from mjit worker.

ALLOC_N() can causes GC. Sometimes `mjit_copy_job_handler()`
can be called by mjit_worker thread which is not a Ruby thread,
so we need to prevent GC in this function. This patch has some
issues, but I introduce it to pass the tests.

mjit.c

@@ -54,13 +54,13 @@ mjit_copy_job_handler(void *data)
     }
 
     const struct rb_iseq_constant_body *body = job->iseq->body;
-    unsigned int ci_size = body->ci_size;
+    const unsigned int ci_size = body->ci_size;
     if (ci_size > 0) {
-        const struct rb_callcache **cc_entries = ALLOC_N(const struct rb_callcache *, ci_size);
-        if (body->jit_unit == NULL) {
-            create_unit(job->iseq);
-        }
-        body->jit_unit->cc_entries = cc_entries;
+        VM_ASSERT(body->jit_unit != NULL);
+        VM_ASSERT(body->jit_unit->cc_entries != NULL);
+
+        const struct rb_callcache **cc_entries = body->jit_unit->cc_entries;
+
         for (unsigned int i=0; i<ci_size; i++) {
             cc_entries[i] = body->call_data[i].cc;
         }
@@ -294,6 +294,9 @@ create_unit(const rb_iseq_t *iseq)
 
     unit->id = current_unit_num++;
     unit->iseq = (rb_iseq_t *)iseq;
+    if (iseq->body->ci_size > 0) {
+        unit->cc_entries = ALLOC_N(const struct rb_callcache *, iseq->body->ci_size);
+    }
     iseq->body->jit_unit = unit;
 }
 

mjit_worker.c

@@ -1131,6 +1131,20 @@ mjit_copy_cache_from_main_thread(const rb_iseq_t *iseq, union iseq_inline_storag
     CRITICAL_SECTION_FINISH(3, "in mjit_copy_cache_from_main_thread");
 
     if (UNLIKELY(mjit_opts.wait)) {
+        // setup pseudo jit_unit
+        if (iseq->body->jit_unit == NULL) {
+            // This function is invoked in mjit worker thread, so GC should not be invoked.
+            // To prevent GC with xmalloc(), use malloc() directly here.
+            // However, mixing xmalloc() and malloc() will cause another issue.
+            // TODO: fix this allocation code.
+            iseq->body->jit_unit = (struct rb_mjit_unit *)malloc(sizeof(struct rb_mjit_unit));
+            if (iseq->body->jit_unit == NULL) rb_fatal("malloc failed");
+            if (iseq->body->ci_size > 0) {
+                iseq->body->jit_unit->cc_entries =
+                  (const struct rb_callcache **)malloc(sizeof(const struct rb_callcache *) * iseq->body->ci_size);
+                if (iseq->body->jit_unit->cc_entries == NULL) rb_fatal("malloc failed");
+            }
+        }
         mjit_copy_job_handler((void *)job);
     }
     else if (rb_workqueue_register(0, mjit_copy_job_handler, (void *)job)) {