[lldb-dev] ScriptInterpreterPython initialization

Zachary Turner zturner at google.com
Fri Mar 20 13:53:09 PDT 2015

>From the Python API perspective, is there any way to use the SB APIs
without first creating an SBDebugger() by calling SBDebugger.Create()?  If
the answer is no, then could we have the C++ implementation of
SBDebugger.Create() call SBDebugger::Initialize?  I haven't actually tried
to do that and see if it causes other issues, but it seems like a
reasonable approach if it works.

On Fri, Mar 20, 2015 at 1:32 PM Greg Clayton <gclayton at apple.com> wrote:

> > On Mar 20, 2015, at 1:06 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > Hi Greg,
> >
> > I found a problem with my initialization refactor, so I want to get your
> advice before I commit it.  The issue is related to the fact that
> modify-python-lldb.py inserts some code at the end to automatically call
> SBDebugger.Initialize() the first time anyone runs "import lldb".
> >
> > When using lldb from python.  When using lldb from python, this means
> the user doesn't have to call SBDebugger.Initialize() because "import lldb"
> will do it automatically.  When using lldb the executable, we call
> SBDebugger::Initialize() explicitly, which results in a call to this line
> in ScriptInterpreterPython:
> >
> >     int old_count = Debugger::TestDebuggerRefCount ();
> >
> >     PyRun_SimpleString ("sys.dont_write_bytecode = 1; import
> lldb.embedded_interpreter; from lldb.embedded_interpreter import
> run_python_interpreter; from lldb.embedded_interpreter import
> run_one_line");
> >
> >     int new_count = Debugger::TestDebuggerRefCount ();
> >
> >     if (new_count > old_count)
> >         Debugger::Terminate ();
> >
> > Since this is the first time lldb is imported, it results in a call to
> SBDebugger::Initialize() again.  This recursive call doesn't do anything
> because it sees that g_initialized has been set to true already.  But when
> it returns control back to the line after the PyRun_SimpleString(), it
> tests the new ref count against the old ref count and calls
> Debugger::Terminate.  But, the ref count was previously 1, so this results
> in the debugger refcount being 0.  In practice it turns out this doesn't
> matter, because all Debugger::Terminate() does is clear an already clear
> debugger list.  And actually I can fix it by changing
> SystemLifetimeManager::AddRef() to put the call to Debugger::Initialize()
> before the call to m_initializer->Initialize().
> >
> > But all of this is very fragile and confusing.  I have an idea for
> fixing this a "better" way, but I would like your opinion.
> >
> > If we remove the call to SBDebugger.Initialize() from __init__.py (by
> changing modify-python-lldb.py to not insert these lines) the problems
> disappear.  Now, when we write PyRun_SimpleString("import lldb") we don't
> get a recursive call to SBDebugger::Initialize().  We would need to update
> the test suite to explicitly call this.
> >
> > I think this makes for a more clear usage pattern.  implicitly calling
> SBDebugger.Initialize() just seems like a bad idea.  By making this change
> we would be able to remove all of the debugger ref count testing and
> conditionally calling Debugger::Terminate.
> >
> > Thoughts?
> My main concern is "things should just work". Python expects functions to
> be ready to be called after importing so I would like to not require people
> to call SBDebugger::Initialize() first after a "import lldb". What ever we
> have to do to make this work, we should do that.
> I don't know of _any_ other python module that requires you to call some
> function after importing the module, so it makes sense from a C++ API
> perspective, but not for python.
> So I would vote to do what you need to do to make the following two things
> happen:
> - from python when it loads LLDB "import lldb" should be enough
> - in the embedded interpreter we should obviously not have to call it
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