Enable export lookup by two threads (mutator and background compiler) by carying a copy of exports_…

runtime/vm/raw_object.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/atomic.h"
 #include "vm/globals.h"
@@ skipping 1005 matching lines @@
   RawString* name_;
   RawString* url_;
   RawScript* script_;
   RawString* private_key_;
   RawArray* dictionary_;         // Top-level names in this library.
   RawGrowableObjectArray* metadata_;  // Metadata on classes, methods etc.
   RawClass* toplevel_class_;  // Class containing top-level elements.
   RawGrowableObjectArray* patch_classes_;
   RawArray* imports_;            // List of Namespaces imported without prefix.
   RawArray* exports_;            // List of re-exported Namespaces.
+  RawArray* exports2_;           // Copy of exports_, used by background
+                                 // compiler to detect cycles without colliding
+                                 // with mutator thread lookups.
   RawInstance* load_error_;      // Error iff load_state_ == kLoadError.
   RawObject** to_snapshot() {
     return reinterpret_cast<RawObject**>(&ptr()->load_error_);
   }
   RawArray* resolved_names_;     // Cache of resolved names in library scope.
   RawArray* loaded_scripts_;     // Array of scripts loaded in this library.
   RawObject** to() {
     return reinterpret_cast<RawObject**>(&ptr()->loaded_scripts_);
   }
 
@@ skipping 1324 matching lines @@
   COMPILE_ASSERT(kExternalTypedDataInt8ArrayCid ==
                  kTypedDataInt8ArrayViewCid + 15);
   COMPILE_ASSERT(kByteBufferCid == kExternalTypedDataInt8ArrayCid + 14);
   COMPILE_ASSERT(kNullCid == kByteBufferCid + 1);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_