Friday, 20 November 2009

Replay Debugging mochitest failures with VMWare Workstation 7

Ever since my last escapades with Replay Debugging in VMWare Workstation 6.5, I've been looking forward to improvements in this awesome technology. Thankfully the guys at VMWare have been hard at work, and now VMWare Workstation 7 now boasts improved Replay Debugging. I've found it much more robust and reliable, and Roc and I have already used it to debug some random orange bugs.

I've documented how to produce a Replay Debugging setup for debugging intermittent test failures in Mozilla mochitests, and put it up on MDC:

Now anyone can setup a machine to record and replay debug intermittent mochitests! A word of warning: you need a modern CPU in order to get good performance. I had poor performance when running on my two-year-old Core2Duo laptop, but replay performance is almost at real-time speeds on my shiny new Intel i7 950 box.

I still have two patches that need to be refined and then checked in, to facilitate replay debugging. The first enables the mochitest harness to loop forever on a test directory. The second enables you to set break points on specific JavaScript dump() calls, so you can break during replay close to where the action is.

We're far from having a fully automated record and replay setup, but we've made a start!