Issue #2686 - Don't rely on return value of ExecutionContext::Compile when the context may forbid running scripts

This is relevant if scripting is terminated due to execution timeout but there are setTimeout/Interval callbacks pending
or if scripting is disabled altogether.

dom/base/nsGlobalWindow.cpp

@@ -12941,10 +12941,11 @@ nsGlobalWindow::RunTimeoutHandler(Timeout* aTimeout,
          nsJSUtils::ExecutionContext exec(aes.cx(), global);
          rv = exec.Compile(options, handler->GetHandlerText());
 
-         if (rv == NS_OK) {
+         JS::Rooted<JSScript*> script(aes.cx(), exec.GetScript());
+         if (script) {
            LoadedScript* initiatingScript = handler->GetInitiatingScript();
            if (initiatingScript) {
-             initiatingScript->AssociateWithScript(exec.GetScript());
+             initiatingScript->AssociateWithScript(script);
            }
 
            rv = exec.ExecScript();

dom/base/nsJSUtils.cpp

@@ -315,8 +315,11 @@ nsJSUtils::ExecutionContext::JoinDecode(void **aOffThreadToken)
 }
 
 JSScript* nsJSUtils::ExecutionContext::GetScript() {
+  if (mSkip) {
+    return nullptr;
+  }
+
 #ifdef DEBUG
-  MOZ_ASSERT(!mSkip);
   MOZ_ASSERT(mScript);
   mScriptUsed = true;
 #endif

dom/base/nsJSUtils.h

@@ -163,7 +163,10 @@ public:
     // thread.
     nsresult JoinDecode(void** aOffThreadToken);
 
-    // Get a successfully compiled script.
+    // Get the compiled script if present, or nullptr.
+    //
+    // Compilation may succeed without producing a script when scripting is disabled for the global. Causes for this can
+    // be the global pref or dynamic change when script execution in a global is terminated due to max_script_run_time.
     JSScript* GetScript();
 
     // Execute the compiled script and ignore the return value.

dom/script/ScriptLoader.cpp

@@ -2377,10 +2377,7 @@ ScriptLoader::EvaluateScript(ScriptLoadRequest* aRequest)
             rv = exec.Compile(options, srcBuf);
           }
           if (rv == NS_OK) {
-             JS::Rooted<JSScript*> script(cx);
+             JS::Rooted<JSScript*> script(cx, exec.GetScript());
-             script = exec.GetScript();
-
-             // With scripts disabled GetScript() will return nullptr
              if (script) {
                  // Create a ClassicScript object and associate it with the
                  // JSScript.