Reproducers

As unbelievable as it may sound, the debugger has bugs. These bugs might manifest themselves as errors, missing results or even a crash. Quite often these bugs don’t reproduce in simple, isolated scenarios. The debugger deals with a lot of moving parts and subtle differences can easily add up.

Reproducers in LLDB improve the experience for both the users encountering bugs and the developers working on resolving them. The general idea consists of capturing all the information necessary to later replay a debug session while debugging the debugger.

Usage

Reproducers are a generic concept in LLDB and are not inherently coupled with the command line driver. The functionality can be used for anything that uses the SB API and the driver is just one example. However, because it’s probably the most common way users interact with lldb, that’s the workflow described in this section.

Capture

Until reproducer capture is enabled by default, you need to launch LLDB in capture mode. For the command line driver, this means passing --capture. You cannot enable reproducer capture from within LLDB, as this would be too late to capture initialization of the debugger.

> lldb --capture

In capture mode, LLDB will keep track of all the information it needs to replay the current debug session. Most data is captured lazily to limit the impact on performance. To create the reproducer, use the reproducer generate sub-command. It’s always possible to check the status of the reproducers with the reproducer status sub-command. Note that generating the reproducer terminates the debug session.

(lldb) reproducer status
Reproducer is in capture mode.
(lldb) reproducer generate
Reproducer written to '/path/to/reproducer'
Please have a look at the directory to assess if you're willing to share the contained information.

The resulting reproducer is a directory. It was a conscious decision to not compress and archive it automatically. The reproducer can contain potentially sensitive information like object and symbol files, their paths on disk, debug information, memory excerpts of the inferior process, etc.

Replay

It is strongly recommended to replay the reproducer locally to ensure it actually reproduces the expected behavior. If the reproducer doesn’t behave correctly locally, it means there’s a bug in the reproducer implementation that should be addressed.

To replay a reproducer, simply pass its path to LLDB through the --replay flag. It is unnecessary to pass any other command line flags. The flags that were passed to LLDB during capture are already part of the reproducer.

> lldb --replay /path/to/reproducer

During replay LLDB will behave similar to batch mode. The session should be identical to the recorded debug session. The only expected differences are that the binary being debugged doesn’t actually run during replay. That means that you won’t see any of its side effects, like things being printed to the terminal. Another expected difference is the behavior of the reproducer generate command, which becomes a NOOP during replay.

Augmenting a Bug Report with a Reproducer

A reproducer can significantly improve a bug report, but it in itself is not sufficient. Always describe the expected and unexpected behavior. Just like the debugger can have bugs, the reproducer can have bugs too.

Design

Coming soon.

Knows Issues

The reproducers are still a work in progress. Here’s a non-exhaustive list of outstanding work, limitations and known issues.

  • The VFS cannot deal with more than one current working directory. Changing the current working directory during the debug session will break relative paths.
  • Not all SB APIs are properly instrumented. We need customer serialization for APIs that take buffers and lengths.
  • We leak memory during replay because the reproducer doesn’t capture the end of an object’s life time. We need to add instrumentation to the destructor of SB API objects.
  • The reproducer includes every file opened by LLDB. This is overkill. For example we do not need to capture source files for code listings. There’s currently no way to say that some file shouldn’t be included in the reproducer.
  • We do not yet automatically generate a reproducer on a crash. The reason is that generating the reproducer is too expensive to do in a signal handler. We should re-invoke lldb after a crash and do the heavy lifting.