VM: Fix race between background compiler and guarded cid update.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 7819cd780ac26f66bf7a1621d56cd8d5c8611bac..bd63ed5ea9ff023a5c58fb771a5fa148ea980a6a 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -161,12 +161,29 @@ void ParsedFunction::AddToGuardedFields(const Field* field) const {
       (field->guarded_cid() == kIllegalCid)) {
     return;
   }
+
   for (intptr_t j = 0; j < guarded_fields_->length(); j++) {
-    if ((*guarded_fields_)[j]->raw() == field->raw()) {
+    const Field* other = (*guarded_fields_)[j];
+    if (field->Original() == other->Original()) {
+      // Abort background compilation early if the guarded state of this field
+      // has changed during compilation. We will not be able to commit
+      // the resulting code anyway.
+      if (Compiler::IsBackgroundCompilation()) {
+        if (!other->IsConsistentWith(*field)) {
+          Compiler::AbortBackgroundCompilation(Thread::kNoDeoptId,
+              "Field's guarded state changed during compilation");
+        }
+      }
       return;
     }
   }
-  guarded_fields_->Add(&Field::ZoneHandle(Z, field->Original()));
+
+  // Note: the list of guarded fields must contain copies during background
+  // compilation because we will look at their guarded_cid when copying
+  // the array of guarded fields from callee into the caller during
+  // inlining.
+  ASSERT(!field->IsOriginal() || Thread::Current()->IsMutatorThread());
+  guarded_fields_->Add(&Field::ZoneHandle(Z, field->raw()));
 }