Bug 1620853 - RelazifyFunctions testing function should preserve DoNo…

…tRelazify. r=jandem

When using the `relazifyFunctions` testing method, we mark live scripts on
stack as DoNotRelazify. Previously we would clear this flag after the
operation was done, but this may conflict with other uses of the flag, such
as the debugger. Only rare test-only code is affected.

Differential Revision: https://phabricator.services.mozilla.com/D65904

js/src/builtin/TestingFunctions.cpp

@@ -672,28 +672,26 @@ static bool GCParameter(JSContext* cx, unsigned argc, Value* vp) {
   return true;
 }
 
-static void SetAllowRelazification(JSContext* cx, bool allow) {
-  JSRuntime* rt = cx->runtime();
-  MOZ_ASSERT(rt->allowRelazificationForTesting != allow);
-  rt->allowRelazificationForTesting = allow;
-
-  for (AllScriptFramesIter i(cx); !i.done(); ++i) {
-    i.script()->setDoNotRelazify(allow);
-  }
-}
-
 static bool RelazifyFunctions(JSContext* cx, unsigned argc, Value* vp) {
   // Relazifying functions on GC is usually only done for compartments that are
   // not active. To aid fuzzing, this testing function allows us to relazify
   // even if the compartment is active.
 
   CallArgs args = CallArgsFromVp(argc, vp);
-  SetAllowRelazification(cx, true);
+
+  // Disable relazification of all scripts on stack. It is a pervasive
+  // assumption in the engine that running scripts still have bytecode.
+  for (AllScriptFramesIter i(cx); !i.done(); ++i) {
+    i.script()->setDoNotRelazify();
+  }
+
+  cx->runtime()->allowRelazificationForTesting = true;
 
   JS::PrepareForFullGC(cx);
   JS::NonIncrementalGC(cx, GC_SHRINK, JS::GCReason::API);
 
-  SetAllowRelazification(cx, false);
+  cx->runtime()->allowRelazificationForTesting = false;
+
   args.rval().setUndefined();
   return true;
 }