Bug 1308039 - Add AutoAssertOnBarrier (r=jonco)

js/public/GCAPI.h

@@ -520,6 +520,20 @@ class JS_PUBLIC_API(AutoAssertNoAlloc)
 #endif
 };
 
+/**
+ * Assert if a GC barrier is invoked while this class is live. This class does
+ * not disable the static rooting hazard analysis.
+ */
+class JS_PUBLIC_API(AutoAssertOnBarrier)
+{
+    JSContext* context;
+    bool prev;
+
+  public:
+    explicit AutoAssertOnBarrier(JSContext* cx);
+    ~AutoAssertOnBarrier();
+};
+
 /**
  * Disable the static rooting hazard analysis in the live region and assert if
  * any allocation that could potentially trigger a GC occurs while this guard
@@ -609,6 +623,7 @@ ExposeGCThingToActiveJS(JS::GCCellPtr thing)
     if (IsInsideNursery(thing.asCell()))
         return;
     JS::shadow::Runtime* rt = detail::GetGCThingRuntime(thing.unsafeAsUIntPtr());
+    MOZ_DIAGNOSTIC_ASSERT(rt->allowGCBarriers());
     if (IsIncrementalBarrierNeededOnTenuredGCThing(rt, thing))
         JS::IncrementalReferenceBarrier(thing);
     else if (JS::GCThingIsMarkedGray(thing))
@@ -619,6 +634,7 @@ static MOZ_ALWAYS_INLINE void
 MarkGCThingAsLive(JSRuntime* aRt, JS::GCCellPtr thing)
 {
     JS::shadow::Runtime* rt = JS::shadow::Runtime::asShadowRuntime(aRt);
+    MOZ_DIAGNOSTIC_ASSERT(rt->allowGCBarriers());
     /*
      * Any object in the nursery will not be freed during any GC running at that time.
      */

js/src/jsgc.cpp

@@ -7013,6 +7013,20 @@ JS::AutoAssertOnGC::~AutoAssertOnGC()
     }
 }
 
+JS::AutoAssertOnBarrier::AutoAssertOnBarrier(JSContext* cx)
+  : context(cx),
+    prev(cx->runtime()->allowGCBarriers())
+{
+    context->runtime()->allowGCBarriers_ = false;
+}
+
+JS::AutoAssertOnBarrier::~AutoAssertOnBarrier()
+{
+    MOZ_ASSERT(!context->runtime()->allowGCBarriers_);
+    context->runtime()->allowGCBarriers_ = prev;
+}
+
+#ifdef DEBUG
 JS::AutoAssertNoAlloc::AutoAssertNoAlloc(JSContext* cx)
   : gc(nullptr)
 {

js/src/jspubtd.h

@@ -131,6 +131,7 @@ class StoreBuffer;
 namespace JS {
 
 class JS_PUBLIC_API(AutoEnterCycleCollection);
+class JS_PUBLIC_API(AutoAssertOnBarrier);
 struct PropertyDescriptor;
 
 typedef void (*OffThreadCompileCallback)(void* token, void* callbackData);
@@ -153,11 +154,17 @@ struct Runtime
     friend class JS::AutoEnterCycleCollection;
     JS::HeapState heapState_;
 
+    // In some cases, invoking GC barriers (incremental or otherwise) will break
+    // things. These barriers assert if this flag is set.
+    bool allowGCBarriers_;
+    friend class JS::AutoAssertOnBarrier;
+
     js::gc::StoreBuffer* gcStoreBufferPtr_;
 
   public:
     Runtime()
       : heapState_(JS::HeapState::Idle)
+      , allowGCBarriers_(true)
       , gcStoreBufferPtr_(nullptr)
     {}
 
@@ -169,6 +176,8 @@ struct Runtime
         return heapState_ == JS::HeapState::CycleCollecting;
     }
 
+    bool allowGCBarriers() const { return allowGCBarriers_; }
+
     js::gc::StoreBuffer* gcStoreBufferPtr() { return gcStoreBufferPtr_; }
 
     static JS::shadow::Runtime* asShadowRuntime(JSRuntime* rt) {