Fix data race in inlining extop fixup.

This switches to use the fixed size allocator for this memory, which
provides a free-at-safepoint function. Previously, the realloc could
lead to a thread reading memory that was freed by the realloc.

src/6model/reprs/MVMCompUnit.c

@@ -87,7 +87,10 @@ static void gc_free(MVMThreadContext *tc, MVMObject *obj) {
     MVM_free(body->inline_tweak_mutex);
     MVM_free(body->coderefs);
     MVM_free(body->callsites);
-    MVM_free(body->extops);
+    if (body->extops)
+        MVM_fixed_size_free(tc, tc->instance->fsa,
+            body->num_extops * sizeof(MVMExtOpRecord),
+            body->extops);
     MVM_free(body->strings);
     MVM_free(body->scs);
     MVM_free(body->scs_to_resolve);

src/core/bytecode.c

@@ -313,7 +313,8 @@ static MVMExtOpRecord * deserialize_extop_records(MVMThreadContext *tc, MVMCompU
     if (num == 0)
         return NULL;
 
-    extops = MVM_calloc(num, sizeof *extops);
+    extops = MVM_fixed_size_alloc_zeroed(tc, tc->instance->fsa,
+        num * sizeof(MVMExtOpRecord));
 
     pos = rs->extop_seg;
     for (i = 0; i < num; i++) {

src/spesh/inline.c

@@ -22,12 +22,15 @@ static void demand_extop(MVMThreadContext *tc, MVMCompUnit *target_cu, MVMCompUn
     num_extops = source_cu->body.num_extops;
     for (i = 0; i < num_extops; i++) {
         if (extops[i].info == info) {
-            MVMuint32 size = (target_cu->body.num_extops + 1) * sizeof(MVMExtOpRecord);
-            target_cu->body.extops = target_cu->body.extops
-                ? MVM_realloc(target_cu->body.extops, size)
-                : MVM_malloc(size);
-            memcpy(&target_cu->body.extops[target_cu->body.num_extops],
-                &extops[i], sizeof(MVMExtOpRecord));
+            MVMuint32 orig_size = target_cu->body.num_extops * sizeof(MVMExtOpRecord);
+            MVMuint32 new_size = (target_cu->body.num_extops + 1) * sizeof(MVMExtOpRecord);
+            MVMExtOpRecord *new_extops = MVM_fixed_size_alloc(tc,
+                tc->instance->fsa, new_size);
+            memcpy(new_extops, target_cu->body.extops, orig_size);
+            memcpy(&new_extops[target_cu->body.num_extops], &extops[i], sizeof(MVMExtOpRecord));
+            MVM_fixed_size_free_at_safepoint(tc, tc->instance->fsa, orig_size,
+               target_cu->body.extops);
+            target_cu->body.extops = new_extops;
             target_cu->body.num_extops++;
             uv_mutex_unlock(target_cu->body.inline_tweak_mutex);
             return;