1312592 - While paused in the middle of a JS event loop, the debugger allows other event loops to run. Can't trust that JavaScript execution is actually paused.

Status: RESOLVED DUPLICATE of bug 1044074

(DevTools :: Debugger, defect)

Description

[chuck_services]

Attachment: debug_the_debugger.html

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12) AppleWebKit/602.1.50 (KHTML, like Gecko) Version/10.0 Safari/602.1.50

Steps to reproduce:

Write some JavaScript that adds an event listener to (at least) the scroll wheel ('wheel') and window resize ('resize'). Then, write some other JS that includes a debugger; statement.

Run the code with the debugger;. Once you're paused in the developer tools, scroll and/or resize the window. Those event listeners run to completion despite that JavaScript execution is supposed to be paused.


Actual results:

The event listeners fire while the debugger is paused.

I know this occurs with at least mouse wheeling and window resizing; it may also occur in other circumstances as well.

The event loops invoked by the wheeling and resizing run concurrently with the event loop that is currently paused by the debugger. Two JavaScript stacks are alive at once.

I tested in 49 and 51; it reproduces in both.


Expected results:

When you're paused in the debugger, all JavaScript execution should be completely paused. No event loop should go until after the current one terminates; the debug tools allowing the user to make that take an arbitrarily long time should not change that.

Multiple JS execution contexts should never be allowed to run concurrently against the same window.

Comment 1

[J. Ryan Stinnett [:jryans] (Use needinfo, replies may be slow)]

Thanks for the report!  This is a long-standing known issue with the debugger.

Comment 2

[Nick Fitzgerald [:fitzgen] [⏰PST; UTC-8]]

We have this bug on file as bug 1044074. Thanks for filing!