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LibWeb: Perform a microtask checkpoint - VERIFY after reentrancy return

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The HTML event loop spec explicitly provides guidance for reentrancy in
the "perform a microtask checkpoint" algorithm, so we cannot VERIFY
for empty execution context before this early exit (as much as we'd
like to).

https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint

See the microtask-checkpoint-reentrancy-via-responsexml-script test for
an example of how this can happen from user scripts.
This commit is contained in:
Jonathan Gamble
2026-02-16 18:41:55 -06:00
committed by Shannon Booth
parent a19db375b5
commit d065f6bf00
Notes: github-actions[bot] 2026-02-19 07:13:44 +00:00 
Author: https://github.com/jonbgamble
Commit: https://github.com/LadybirdBrowser/ladybird/commit/d065f6bf00f
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/7991
Reviewed-by: https://github.com/shannonbooth ✅
3 changed files with 29 additions and 4 deletions
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8
Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp
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@@ -610,14 +610,14 @@ 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());
// 1. If the event loop's performing a microtask checkpoint is true, then return.
if (m_performing_a_microtask_checkpoint)
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());
// 2. Set the event loop's performing a microtask checkpoint to true.
m_performing_a_microtask_checkpoint = true;