gistfile1.txt

diff --git a/src/6model/6model.h b/src/6model/6model.h
index 85b20fe..adc6dfc 100644
--- a/src/6model/6model.h
+++ b/src/6model/6model.h
@@ -131,7 +131,13 @@ typedef enum {
     /* Note: if you're hunting for a flag, some day in the future when we
      * have used them all, this one is easy enough to eliminate by having the
      * tiny number of objects marked this way in a remembered set. */
-    MVM_CF_NEVER_REPOSSESS = 1024
+    MVM_CF_NEVER_REPOSSESS = 1024,
+
+    /* Is the object still in the process of being deserialized? This flag is
+     * set if so. It prevents multi-threaded programs grabbing a partially
+     * deserialized object while another thread is in the middle of doing its
+     * deserialization. */
+    MVM_CF_DESERIALIZING = 2048
 } MVMCollectableFlags;
 
 #ifdef MVM_USE_OVERFLOW_SERIALIZATION_INDEX
diff --git a/src/6model/sc.c b/src/6model/sc.c
index 37864ad..076ee95 100644
--- a/src/6model/sc.c
+++ b/src/6model/sc.c
@@ -153,12 +153,17 @@ MVMSerializationContext * MVM_sc_get_sc(MVMThreadContext *tc, MVMCompUnit *cu, M
 MVMObject * MVM_sc_get_object(MVMThreadContext *tc, MVMSerializationContext *sc, MVMint64 idx) {
     MVMObject **roots = sc->body->root_objects;
     MVMint64    count = sc->body->num_objects;
-    if (idx >= 0 && idx < count)
-        return roots[idx] ? roots[idx] : MVM_serialization_demand_object(tc, sc, idx);
-    else
+    if (idx >= 0 && idx < count) {
+        MVMObject *result = roots[idx];
+        return result && !MVM_DESERIALIZING_TEST(tc, &(result->header))
+            ? result
+            : MVM_serialization_demand_object(tc, sc, idx);
+    }
+    else {
         MVM_exception_throw_adhoc(tc,
             "Probable version skew in pre-compiled '%s' (cause: no object at index %"PRId64")",
             MVM_string_utf8_encode_C_string(tc, sc->body->description), idx);
+    }
 }
 
 MVMObject * MVM_sc_get_sc_object(MVMThreadContext *tc, MVMCompUnit *cu,
@@ -213,7 +218,9 @@ void MVM_sc_set_object(MVMThreadContext *tc, MVMSerializationContext *sc, MVMint
 MVMSTable * MVM_sc_get_stable(MVMThreadContext *tc, MVMSerializationContext *sc, MVMint64 idx) {
     if (idx >= 0 && idx < sc->body->num_stables) {
         MVMSTable *got = sc->body->root_stables[idx];
-        return got ? got : MVM_serialization_demand_stable(tc, sc, idx);
+        return got && !MVM_DESERIALIZING_TEST(tc, &(got->header))
+            ? got
+            : MVM_serialization_demand_stable(tc, sc, idx);
     }
     else {
         MVM_exception_throw_adhoc(tc,
diff --git a/src/6model/sc.h b/src/6model/sc.h
index aafc82c..7970228 100644
--- a/src/6model/sc.h
+++ b/src/6model/sc.h
@@ -163,3 +163,14 @@ MVM_STATIC_INLINE void MVM_SC_WB_ST(MVMThreadContext *tc, MVMSTable *st) {
     if (MVM_get_idx_of_sc(&st->header) > 0)
         MVM_sc_wb_hit_st(tc, st);
 }
+
+/* Set/clear/test the "currently being deserialized" flag on object/STable. */
+MVM_STATIC_INLINE void MVM_DESERIALIZING_SET(MVMThreadContext *tc, MVMCollectable *c) {
+    c->flags |= MVM_CF_DESERIALIZING;
+}
+MVM_STATIC_INLINE void MVM_DESERIALIZING_CLEAR(MVMThreadContext *tc, MVMCollectable *c) {
+    c->flags ^= MVM_CF_DESERIALIZING;
+}
+MVM_STATIC_INLINE MVMint64 MVM_DESERIALIZING_TEST(MVMThreadContext *tc, MVMCollectable *c) {
+    return c->flags & MVM_CF_DESERIALIZING;
+}
diff --git a/src/6model/serialization.c b/src/6model/serialization.c
index a7b9d95..1df54df 100644
--- a/src/6model/serialization.c
+++ b/src/6model/serialization.c
@@ -1929,6 +1929,7 @@ static void stub_stable(MVMThreadContext *tc, MVMSerializationReader *reader, MV
 
         /* Allocate and store stub STable. */
         st = MVM_gc_allocate_stable(tc, repr, NULL);
+        //MVM_DESERIALIZING_SET(tc, &(st->header));
         MVM_sc_set_stable(tc, reader->root.sc, i, st);
     }
 
@@ -2003,6 +2004,7 @@ static void stub_object(MVMThreadContext *tc, MVMSerializationReader *reader, MV
         else
             obj = MVM_gc_allocate_type_object(tc, st);
         MVM_sc_set_object(tc, reader->root.sc, i, obj);
+        //MVM_DESERIALIZING_SET(tc, &(obj->header));
     }
 
     /* Set the object's SC. */
@@ -2312,6 +2314,9 @@ static void deserialize_stable(MVMThreadContext *tc, MVMSerializationReader *rea
     if (st->REPR->deserialize_repr_data)
         st->REPR->deserialize_repr_data(tc, st, reader);
 
+    /* Flag that we're done deserializing it. */
+    MVM_DESERIALIZING_CLEAR(tc, &(st->header));
+
     /* Restore original read positions. */
     reader->stables_data_offset = orig_stables_data_offset;
     reader->cur_read_buffer     = orig_read_buffer;
@@ -2346,6 +2351,9 @@ static void deserialize_object(MVMThreadContext *tc, MVMSerializationReader *rea
                 REPR(obj)->name);
         reader->current_object = NULL;
     }
+
+    /* Flag that we're done deserializing it. */
+    MVM_DESERIALIZING_CLEAR(tc, &(obj->header));
 }
 
 /* Worklist manipulation functions. */
@@ -2574,6 +2582,9 @@ static void repossess(MVMThreadContext *tc, MVMSerializationReader *reader, MVMi
             });
         }
 
+        /* Flag it as now being in the process of being deserialized. */
+        //MVM_DESERIALIZING_SET(tc, &(orig_obj->header));
+
         /* Put it into objects root set at the apporpriate slot. */
         slot = read_int32(table_row, 4);
         MVM_sc_set_object(tc, reader->root.sc, slot, orig_obj);
@@ -2604,6 +2615,9 @@ static void repossess(MVMThreadContext *tc, MVMSerializationReader *reader, MVMi
                 "STable conflict detected during deserialization.\n"
                 "(Probable attempt to load two modules that cannot be loaded together).");
 
+        /* Flag it as now being in the process of being deserialized. */
+        //MVM_DESERIALIZING_SET(tc, &(orig_st->header));
+
         /* Put it into STables root set at the apporpriate slot. */
         slot = read_int32(table_row, 4);
         MVM_sc_set_stable(tc, reader->root.sc, slot, orig_st);