Expect null referent in DequeuePendingReference().

Following up on CL 170735.

It's possible that the referent may potentially be cleared which would
cause a check failure. Avoid that.

Bug: 12687968
Bug: 23896462
Change-Id: I8ccc5936b61ceacf250624681e65307f23ce0405

1 files changed, 6 insertions, 4 deletions

diff --git a/runtime/gc/reference_queue.cc b/runtime/gc/reference_queue.cc
index 0c0ab6d212..56957baebf 100644
--- a/runtime/gc/reference_queue.cc
+++ b/runtime/gc/reference_queue.cc
@@ -115,10 +115,12 @@ mirror::Reference* ReferenceQueue::DequeuePendingReference() {
             << "ref=" << ref << " rb_ptr=" << ref->GetReadBarrierPointer();
       }
       mirror::Object* referent = ref->GetReferent<kWithoutReadBarrier>();
-      CHECK(referent != nullptr) << "Reference should not have been enqueued if referent is null";
-      CHECK(concurrent_copying->IsInToSpace(referent))
-          << "ref=" << ref << " rb_ptr=" << ref->GetReadBarrierPointer()
-          << " referent=" << referent;
+      // The referent could be null if it's cleared by a mutator (Reference.clear()).
+      if (referent != nullptr) {
+        CHECK(concurrent_copying->IsInToSpace(referent))
+            << "ref=" << ref << " rb_ptr=" << ref->GetReadBarrierPointer()
+            << " referent=" << referent;
+      }
     }
   }
   return ref;