[Docs] Add reproducer documentation.

[JDevlieghere]

Summary

This adds a page about LLDB reproducers. It describes how to use the reproducers on the command line and lists some of the known issues/limitations.

[vsk]

I like it!

lldb/docs/resources/reproducers.rst

10

s/are an attempt//

74

Is it valid to type: lldb --capture --replay /path/to/reproducer? This matters for users who might alias lldb='lldb --capture' in their shell rcs.

[davide]

Thanks for writing this up!

[JDevlieghere]

lldb/docs/resources/reproducers.rst

74

Yes, replay will take precedence over all the other flags. Do you think it's worth including this scenario?

lldb/docs/index.rst

.. toctree::
    :hidden:
    :maxdepth: 1
    :caption: Development
 
    resources/contributing
    resources/build
    resources/test
    resources/bots
+   resources/reproducers
    resources/sbapi
    resources/caveats
 
 .. toctree::
    :hidden:
    :maxdepth: 1
    :caption: API Documentation
 

lldb/docs/resources/reproducers.rst

+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.
+
+.. contents::
+   :local:
+
+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.
+
+.. code-block:: bash
+
+  > 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.
+
+.. code-block:: none
+
+  (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.
+
+.. code-block:: bash
+
+ > 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.