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LibWeb: Do not run microtasks when the event loop is paused

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For example, running `alert(1)` will pause the event loop, during which
time no JavaScript should execute. This patch extends this disruption to
microtasks. This avoids a crash inside the microtask executor, which
asserts the JS execution context stack is empty.

This makes us behave the same as Firefox in the following page:

    <script>
        queueMicrotask(() => {
            console.log("inside microtask");
        });

        alert("hi");
    </script>

Before the aforementioned assertion was added, we would execute that
microtask before showing the alert. Firefox does not do this, and now
we don't either.
This commit is contained in:
Timothy Flynn 2025-01-19 13:13:59 -05:00 • committed by Tim Ledbetter
parent 34bf833a0a
commit 43dc0f52a6
Notes: github-actions[bot] 2025-01-19 20:48:47 +00:00
4 changed files with 42 additions and 21 deletions
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6
Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp
View file

@ -503,6 +503,9 @@ void perform_a_microtask_checkpoint()
// https://html.spec.whatwg.org/#perform-a-microtask-checkpoint
void EventLoop::perform_a_microtask_checkpoint()
{
if (execution_paused())
return;
// NOTE: This assertion is per requirement 9.5 of the ECMA-262 spec, see: https://tc39.es/ecma262/#sec-jobs
// > At some future point in time, when there is no running context in the agent for which the job is scheduled and that agent's execution context stack is empty...
VERIFY(vm().execution_context_stack().is_empty());
@ -653,6 +656,8 @@ EventLoop::PauseHandle::~PauseHandle()
// https://html.spec.whatwg.org/multipage/webappapis.html#pause
EventLoop::PauseHandle EventLoop::pause()
{
m_execution_paused = true;
// 1. Let global be the current global object.
auto& global = current_principal_global_object();
@ -667,7 +672,6 @@ EventLoop::PauseHandle EventLoop::pause()
// not run further tasks, and any script in the currently running task must block. User agents should remain
// responsive to user input while paused, however, albeit in a reduced capacity since the event loop will not be
// doing anything.
m_execution_paused = true;
return PauseHandle { *this, global, time_before_pause };
}