Reload: Don't free the saved class table while another thread may be reading it.

runtime/vm/isolate_reload.cc

Index: runtime/vm/isolate_reload.cc
diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index 0957f6fa388e471ee16abfaef84d7da022d02434..81ba9a8009611d4f026b6d8673d657b4457171aa 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -924,9 +924,13 @@ void IsolateReloadContext::RollbackClasses() {
       class_table->SetAt(i, saved_class_table_[i]);
     }
   }
-  free(saved_class_table_);
+
+  RawClass** local_saved_class_table = saved_class_table_;
   saved_class_table_ = NULL;
-  saved_num_cids_ = 0;
+  // Can't free this table immediately as another thread (e.g., the sweeper) may
+  // be suspended between loading the table pointer and loading the table
+  // element. Table will be freed at the next major GC or isolate shutdown.
+  class_table->AddOldTable(local_saved_class_table);

[siva, 2016/08/12 21:10:45]

When does saved_num_cids_ get set to 0?

[rmacnak, 2016/08/12 21:15:29]

Never. We can't set it to zero here because another thread may be suspended
between reading the table pointer and asserting cid < saved_num_cids_.

 }