[Python-Dev] Stackless Python - Pros and Cons (original) (raw)

Christian Tismer tismer@appliedbiometrics.com
Mon, 07 Aug 2000 12:48:19 +0200

Jeremy Hylton wrote:

If someone is going to write a PEP, I hope they will explain how the implementation deals with the various Python C API calls that can call back into Python.

He will.

In the stackless implementation, builtinapply is a thin wrapper around builtinapplynr. The wrapper checks the return value from builtinapplynr for PyUnwindToken. If PyUnwindToken is found, it calls PyEvalFrameDispatch. In this case, builtinapply returns whatever PyEvalFrameDispatch returns; the frame dispatcher just executes stack frames until it is ready to return.

Correct.

How does this control flow at the C level interact with a Python API call like PySequenceTuple or PyObjectCompare that can start executing Python code again? Say there is a Python function call which in turn calls PySequenceTuple, which in turn calls a getitem method on some Python object, which in turn uses a continuation to transfer control. After the continuation is called, the Python function will never return and the PySquenceTuple call is no longer necessary, but there is still a call to PySequenceTuple on the C stack. How does stackless deal with the return through this function?

Right. What you see here is the incompleteness of Stackless. In order to get this "right", I would have to change many parts of the implementation, in order to allow for continuations in every (probably even unwanted) place. I could not do this.

Instead, the situation of these still occouring recursions are handled differently. continuationmodule guarantees, that in the context of recursive interpreter calls, the given stack order of execution is obeyed. Violations of this cause simply an exception.

I expect that any C function that may cause Python code to be executed must be wrapped the way apply was wrapper. So in the example, PySequenceTuple may return PyUnwindToken. This adds an extra return condition that every caller of PySequenceTuple must check. Currently, the caller must check for NULL/exception in addition to a normal return. With stackless, I assume the caller would also need to check for "unwinding."

No, nobody else is allowed to return Py_UnwindToken but the few functions in the builtins implementation and in ceval. The continuationmodule may produce it since it knows the context where it is called. eval_code is supposed to be the main instance who checks for this special value.

As said, allowing this in any context would have been a huge change to the whole implementation, and would probably also have broken existing extensions which do not expect that a standard function wants to do a callback.

Is this analysis correct? Or is there something I'm missing?

I see that the current source release of stackless does not do anything special to deal with C API calls that execute Python code. For example, PyDictGetItem calls PyObjectHash, which could in theory lead to a call on a continuation, but neither caller nor callee does anything special to account for the possibility. Is there some other part of the implementation that prevents this from being a problem?

This problem is no problem for itself, since inside the stackless modification for Python, there are no places where unexpected Py_UnwindTokens or continuations are produced. This is a closed system insofar. But with the continuation extension, it is of course a major problem.

The final solution to the recursive interpreter/continuation problem was found long after my paper was presented. The idea is simple, solves everything, and shortened my implementation substantially:

Whenever a recursive interpreter call takes place, the calling frame gets a lock flag set. This flag says "this frame is wrapped in a suspended eval_code call and cannot be a continuation". continuationmodule always obeys this flag and prevends the creation of continuations for such frames by raising an exception. In other words: Stack-like behavior is enforced in situations where the C stack is involved.

So, a builtin or an extension can call a continuation, but finally, it will have to come back to the calling point. If not, then one of the locked frames will be touched, finally, in the wrong C stack order. But by reference counting, this touching will cause the attempt to create a continuation, and what I said above will raise an exception.

Probably the wrong place to explain this in more detail here, but it doesn't apply to the stackless core at all which is just responsible for the necessary support machinery.

ciao - chris

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