Record and replay debugging Firefox

This guide details how to setup VMWare Workstation 7 to use Record and Replay Debugging on Firefox on Windows. It's possible to replay debug with gdb on Linux --- but only with extraordinary pain, and only on hosts with gdb >= 7.0 (see and for more details).

This feature was removed from VMWare Workstation 8 (released 2011/09/14). For similar functionality check out the rr project (see also Recording Firefox).

With replay-debugging you can exactly record the behavior of a program and replay it in a debugger. During replay you can step through code, just as if you were debugging natively. This is invaluable when debugging non-deterministic bugs, like random test failures, as if you catch a random failure in a recording, you can reproduce it deterministically in a debugger, and debug forwards and backwards in the recording.

This guide outlines the steps required to setup replay debugging to debug random mochitest failures, but you can use it to setup replay-debugging on Firefox in general as well.

See also the official VMWare documentation

Hardware Requirements

You need a modern multi-core CPU with VT-x support to get adequate performance when replay debugging. We recommend a quad core i7 or Xeon chip. We also recommend at least 8 GB RAM and an SSD with at least 256 GB space.

Setting up the Host computer

The computer in which you run VMWare Workstation is known as the Host computer. The virtual machine in which you'll be recording playback is known as the Guest.

Install Visual Studio Professional on the Host. We've had success with VS2005 and VS2010. This must be installed before VMWare Workstation. You cannot use Visual Studio Express.

Note: Theoretically, all this works on VS2008 as well, but many VS2008 users get an error message "VMware: A valid executable name has not been specified in Debugger settings. You can change this in Project > Properties > Debugging." when they try to actually do replay debugging. The only known solution at this time is to use VS2005 or VS2010 instead.

Follow the steps in Windows Build Prerequisites to get a working build setup on the Host.

Install VMWare Workstation 7 for Windows 32-bit and 64-bit, Main Installation file with Tools. We've had success with version 7.1.3. This must be installed after Visual Studio so that Workstation's Virtual Integrated Debugger plugin gets installed into Visual Studio.

Setting up the Guest Virtual Machine

Create a Windows virtual machine in VMWare Workstation. This is known as the "Guest". Make your account username and password the same in the Guest as your account on the host. Give the virtual machine 512MB of memory and exactly one virtual CPU. (Record-and-replay does not work with multiple virtual CPUs due to fundamental limitations of the technology that can't really be fixed without elaborate hardware support. You can change the amount of memory in the VM, but giving it too much or too little will hurt performance during recording, snapshotting and/or debugging.) Once you've installed the Guest OS, shut down the Guest, and open the Virtual Machine Settings dialog. In the Hardware tab, change Processor > Virtualization Engine > Preferred Mode to Automatic with Replay. In the Options tab set Replay > Snapshot Frequency to 5 min. This helps to improve performance when replaying in reverse.

In the Host, create a directory somewhere. This will be your GuestDLLs directory. All DLLs used by the debug-target in the Guest must be in your GuestDLLs directory on the Host. Copy the *.DLL and *.DRV files from the Guest's C:\Windows\System32 directory to your GuestDLLs directory on the Host. You probably also need the Host's MSVC Runtime DLLs in the GuestDLLs folder. These are in C:\Program Files (x86)\Microsoft Visual Studio 8\VC\redist\Debug_NonRedist\x86\Microsoft.VC80.DebugCRT\

Install MozillaBuild in the Guest.

Install Visual Studio in the Guest. This isn't strictly necessary, but the MozillaBuild shell start batch files won't startup unless you've got Visual Studio installed (though you should be able to use start-shell-l10n.bat instead -- could someone confirm this?). If you want to do remote debugging in the Guest, you need to either have Visual Studio Pro installed in the Guest, so that the remote debug monitor is installed, or copy over the remote debug monitor from the host (find it in C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE on both Windows x86 and x64). You can also use Visual Studio's debugger in the Guest to determine what DLLs are loaded by the debug-target, so that you know what DLLs you need to copy over to the Host.

In the Guest, install Debugging Tools for Windows. Then run C:\Program Files\Debugging Tools for Windows (x86)\gflags.exe. From the "System Registry" tab, ensure that "Disable paging of kernel mode stacks" is turned on (checked). Press OK to close gflags.exe. You must reboot the Guest for this to take effect. This makes the call stacks reported by the debugger more accurate.

You may want to turn off screen saver and monitor power off in the Guest, to help preserve your sanity.

Turn off "Take snapshots in background" in Workstation > Edit > Preferences > Priority. This makes snapshots faster, but blocks the VM while taking them.

Record and Replay of Nightly Builds

Because nightly builds have debug information available from the Mozilla symbol server, it is relatively easy to use them for record and replay debugging.

  • Create a shared directory which is available on both the host and guest. Install Firefox to this location.
  • If debugging a test failure, also download and unpack the test package created with the nightly build.
  • In the guest, create a recording of the issue.
  • In MSVC on the host, Configure Project > Properties > Debugging and enter the Command as the path to firefox.exe on the host.
  • Replay! When prompted, you may have to inform MSVC of the location of guestdlls and installdir/components.

Building Firefox for Record Replay

Checkout a mozilla-central repository to your Host. Create a MOZCONFIG, specifying an object dir, and the following options:

mk_add_options MOZ_OBJDIR=/path/to/your/objdir/
ac_add_options --enable-libxul
ac_add_options --enable-debug --disable-optimize
ac_add_options --enable-tests

You must use --enable-libxul. This reduces the number of DLLs Firefox builds. Visual Studio has a limited number of breakpoints, and must reserve some of them for itself. The more DLLs the debug-target has, the more breakpoints MSVC seems to reserve, and so the fewer breakpoints you'll have for your own use. (We have seen MSVC reserve all the breakpoints so we couldn't set any of our own.)

Build your mozilla tree on the Host, and package it up along with mochitests:
$ cd /path/to/your/mozilla-central
$ MOZCONFIG=$PATH_TO_YOUR_CONFIG make -C /path/to/your/objdir/ package
$ MOZCONFIG=$PATH_TO_YOUR_CONFIG make -C /path/to/your/objdir/ package-tests

Copy $objdir/dist/ and $objdir/dist/ to the Guest.

You probably need to copy the MSVC C Runtime DLLs over to the Guest (unless you have the exact same version of MSVC on both the Host and the Guest). If you get errors from the Msys shell saying something like "bad file number", or "the application isn't configured correctly", you need the MSVC runtime DLLs in the Guest.

$ zip -9 -j "$VCINSTALLDIR"/redist/Debug_NonRedist/x86/Microsoft.VC80.DebugCRT/*

Copy to the Guest.

In the Guest, extract the Firefox build:

$ mkdir recordable
$ cd recordable
$ unzip ../
$ unzip ../

$ cp -r bin/* firefox/
$ cd firefox
$ unzip ../../
$ cd ..

You can run now mochitests with:

$ python mochitest/ --appname firefox/firefox.exe --utility-path=firefox --certificate-path=certs --autorun --close-when-done --console-level=INFO --log-file=./mochitest-plain.log --file-level=INFO

Configuring Visual Studio for Replay Debugging

Create a new project in Visual Studio. You can't just create a project by opening an EXE file, the VMware menu is greyed out if you do this. You must create a new project file using the File > New > Project > Win32 > Win32 Console Application. You can opt to create an empty project, and that works fine for our purposes.

Configure Project > Properties > Debugging and enter the Command as the path to firefox.exe on the Host.

Open VMWare > Options > Replay Debugging in VM > General. Set the Guest Command to be the path to the firefox.exe in the guest. This can be different from the path in the host.

Set the "Host Executable Search Path" to be the path to your GuestDLLs directory. Also add $objdir/firefox/components. The list is semicolon delimited, e.g. mine is C:\cpearce\vm\WinXPSP3\guestdlls;C:\cpearce\objdirs\red\dist\firefox\components.

Set VMWare > Options > Replay Debugging in VM > Advanced > Process Instance to Debug to n=2. This is because Firefox starts up twice during a mochitest run, and you only want to debug the second instance.

Setup symbols. Tools > Options > Debugging > Symbols, add as a symbol location. If you run Firefox from MSVC in the Host, and the symbols will be downloaded immediately.

Creating a Recording

Once you've got a build setup in the Guest and mochitests running, you want to try reproducing and recording failures. It's a good idea to take a snapshot before starting a recording, so that you have a known state to which you can return to after replaying the recording.

We can't initiate a recording from Visual Studio, as the Firefox executable must be wrapped by the mochitest script. So instead you must start the recording from Workstation, and then start the test run with (as above). Once you've recorded a test failure, you can shut down Firefox and stop the recording.

You'll want to enable a lot of logging in the modules you're debugging, redirect it to a log file, and copy the log file out to your Host after you've recorded a test failure. If you don't copy the log file out to the Host, you can't view the log file while replaying.

Replaying a Recording

You probably want to take a snapshot before you start replay debugging, so that you can return your system to the same state after you've replayed.

Open VMWare > Options > Replay Debugging in VM, and set "Virtual Machine" to point to your Guest's VMX file. Select the recording to replay. Ensure "Local or Remote" is Local.

To replay debug a mochitest run, Select VMWare > Start Replay Debugging in VM. This will suspend your existing VM (if it's running) and replay the recording. You should be able to open up a Firefox source file in Visual Studio, and set and hit break points.

If Visual Studio prompts you with errors when you start to replay debug saying that it can't find a DLL, start Firefox in the Guest, and attach Visual Studio, and check the "Modules" debug pane. This will tell you the path to all the DLLs that the process has loaded. Make sure you've got a copy of every DLL loaded in the guest in your GuestDLLs directory on the host. There's probably a DLL in the Guest's C:\Windows\SxS directory that you need in the Host's GuestDLL folder.


We're still working out a good workflow and what tools we require to make replay debugging the most effective.

Our current approach is to edit dom/base/nsGlobalWindow::Dump() so that it increments and prints a counter every time it's called. This means whenever Javascript calls dump() to log a message (in particular a test pass/fail message) we increment and print a counter as part of that message. You can then review the console log and set a conditional breakpoint in the nsGlobalWindow::dump() to break based on the value of the counter variable. You can use this to set a breakpoint on the message which comes before the first test failure. Once you hit that breakpoint, you then set other breakpoints in relevant code paths, and debug forwards (and backwards!) from there.

We also have a patch to make mochitest to loop forever on a directory, which still needs cleaning up but hopefully will land shortly...

When you hit a breakpoint, you can save a snapshot of the VM. You can then resume playback from that snapshot (rather than from the beginning of the recording) using the VMWare > Attach to process in Recording. This means you won't have to sit through the playback of your entire recording before getting to the interesting bits. Creating snapshots is usually very fast (a few seconds) so it's worth creating one at every interesting point during debugging, just in case you need to get back there later. As noted above, automatically taking snapshots every five minutes during recording (the minimum inter-snapshot delay) is also highly recommended.