Scripting Bridge API
====================

The SB APIs constitute the stable C++ API that lldb presents to external
clients, and which get processed by SWIG to produce the Python bindings to
lldb. As such it is important that they not suffer from the binary
incompatibilities that C++ is so susceptible to. We've established a few rules
to ensure that this happens.

Extending the SB API
--------------------

The classes in the SB API's are all called SB<SomeName>, where SomeName is in
CamelCase starting with an upper case letter. The method names are all
CamelCase with initial capital letter as well.

All the SB API classes are non-virtual, single inheritance classes. They should
only include SBDefines.h or other SB headers as needed. There should be no
inlined method implementations in the header files, they should all be in the
implementation files. And there should be no direct ivar access.

You also need to choose the ivars for the class with care, since you can't add
or remove ivars without breaking binary compatibility. In some cases, the SB
class is a thin wrapper around an internal lldb_private object. In that case,
the class can have a single ivar, which is either a pointer, shared_ptr or
unique_ptr to the object in the lldb_private API. All the lldb_private classes
that get used this way are declared as opaque classes in lldb-forward.h, which
is included in SBDefines.h. So if you need an SB class to wrap an lldb_private
class that isn't in lldb-forward.h, add it there rather than making a direct
opaque declaration in the SB classes .h file.

If the SB Class needs some state of its own, as well as the backing object,
don't include that as a direct ivar in the SB Class. Instead, make an Impl
class in the SB's .cpp file, and then make the SB object hold a shared or
unique pointer to the Impl object. The theory behind this is that if you need
more state in the SB object, those needs are likely to change over time, and
this way the Impl class can pick up members without changing the size of the
object. An example of this is the SBValue class. Please note that you should
not put this Impl class in the lldb namespace. Failure to do so leads to
leakage of weak-linked symbols in the SBAPI.

In order to fit into the Python API's, we need to be able to default construct
all the SB objects. Since the ivars of the classes are all pointers of one sort
or other, this can easily be done, but it means all the methods must be
prepared to handle their opaque implementation pointer being empty, and doing
something reasonable. We also always have an "IsValid" method on all the SB
classes to report whether the object is empty or not.

.. note::
  The implication of an object being "empty" can vary by class.

  For most classes, the lack of anything backing the class means that it
  would not be valid to interact with it by calling any other methods
  on it.

  One exception to this is ``SBError``, which can provide valid
  information even when empty. This is because it does not need an
  underlying object to be able to represent a success state.

Another piece of the SB API infrastructure is the Python (or other script
interpreter) customization. SWIG allows you to add property access, iterators
and documentation to classes. We place the property accessors and iterators in
a file dedicated to extensions to existing SB classes at
"bindings/interface/SB<ClassName>Extensions.i". The documentation is similarly
located at "bindings/interface/SB<ClassName>Docstrings.i". These two files, in
addition to the actual header SB<ClassName>.h, forms the interface that lldb
exposes to users through the scripting languages.

There are some situations where you may want to add functionality to the SB API
only for use in C++. To prevent SWIG from generating bindings to these
functions, you can use a C macro guard, like so:

::

  #ifndef SWIG
  int GetResourceCPPOnly() const;
  #endif

In this case, ``GetResourceCPPOnly`` will not be generated for Python or other
scripting languages. If you wanted to add a resource specifically only for the
SWIG case, you can invert the condition and use ``#ifdef SWIG`` instead. When
building the LLDB framework for macOS, the headers are processed with
``unifdef`` prior to being copied into the framework bundle to remove macros
involving SWIG.

Another good principle when adding SB API methods is: if you find yourself
implementing a significant algorithm in the SB API method, you should not do
that, but instead look for and then add it - if not found - as a method in the
underlying lldb_private class, and then call that from your SB API method.
If it was a useful algorithm, it's very likely it already exists
because the lldb_private code also needed to do it.  And if it doesn't at
present, if it was a useful thing to do, it's likely someone will later need
it in lldb_private and then we end up with two implementations of the same
algorithm.  If we keep the SB API code to just what's needed to manage the SB
objects and requests, we won't get into this situation.

Lifetime
--------
Many SB API methods will return strings in the form of ``const char *`` values.
Once created, these strings are guaranteed to live until the end of the
debugging session. LLDB owns these strings, clients should not attempt to free
them. Doing so may cause LLDB to crash.
Note that this only affects the C++ API as scripting languages usually
will usually create native string types from the ``const char *`` value.

API Instrumentation
-------------------

The reproducer infrastructure requires API methods to be instrumented so that
they can be captured and replayed. Instrumentation consists of two macros,
``LLDB_REGISTER`` and ``LLDB_RECORD``. Both can be automatically generated with
the ``lldb-instr`` utility.

To add instrumentation for a given file, pass it to the ``lldb-instr`` tool.
Like other clang-based tools it requires a compilation database
(``compile_commands.json``) to be present in the current working directory.

::

   $ ./bin/lldb-instr /path/to/lldb/source/API/SBDebugger.cpp


The tool will automatically insert ``LLDB_RECORD`` macros inline, however you
will need to run ``clang-format`` over the processed file, as the tool
(intentionally) makes no attempt to get that right.

The ``LLDB_REGISTER`` macros are printed to standard out between curly braces.
You'll have to copy-paste those into the corresponding ``RegisterMethods``
function in the implementation file. This function is fully specialized in the
corresponding type.

::

  template <> void RegisterMethods<SBDebugger>(Registry &R) {
    ...
  }


When adding a new class, you'll also have to add a call to ``RegisterMethods``
in the ``SBRegistry`` constructor.

The tool can be used incrementally. However, it will ignore existing macros
even if their signature is wrong. It will only generate a ``LLDB_REGISTER`` if
it emitted a corresponding ``LLDB_RECORD`` macro.
