1728096 - Debugger performance issue with obfuscated javascript (debugger statements in loop)

Status: NEW

(DevTools :: Debugger, defect, P3)

Description

[Julian Descottes [:jdescottes]]

Extracted from a real live example, the following test page can hang the DevTools UI when you open the toolbox: https://prevent-debugger-example.glitch.me/

The page contains a simple script which will run a debugger statement in a setTimout 100.

The "prettified" script is

(function() {
  (function a() {
    try {
      (function b(i) {
        if (("" + i / i).length !== 1 || i % 20 === 0) {
          (function() {}.constructor("debugger")());
        } else {
          debugger;
        }
        b(++i);
      })(0);
    } catch (e) {
      setTimeout(a, 100);
    }
  })();
})();

The HTML page simple imports the script.

STRs:

• open https://prevent-debugger-example.glitch.me/
• open devtools
• wait until the debugger UI is correctly rendered (ie shows the breakpoint etc...) and you can interact with the rest of devtools UI

ER: Should be fast, there is just one source file
AR: The performance issue can go from "taking a few seconds to open" to "fully blocking the main process indefinitely". Recording a profile seems to make it even slower, although considering that the performance is different every time I try this, it might be a red herring.

Profile performance at https://share.firefox.dev/2WCl7Qj

We seem to perform a lot of calls to addSources. Considering the breakpoint is added via (function() {}.constructor("debugger")());, it creates a new dynamic source everytime. I assume that during the debugger startup, we must be picking up a lot of sources that way, before we settle on stopping on one of the debugger statements?

Comment 1

[Julian Descottes [:jdescottes]]

Note: reducing the timeout delay from 100ms to 10ms makes the issue much more visible. You can try the modified version at 10ms at https://spam-debugger-example.glitch.me/

Comment 2

[Julian Descottes [:jdescottes]]

Changing severity to S3 because performance edge case